mechanics Flashcards
what is the difference between scalar and vector quantities?
MECHANICS
scalars only have magnitude, but vectors have magnitude and direction
list common quantities that are either scalars or vectors
MECHANICS
how do you find the resultant vector?
MECHANICS
⋅ adding two or more vectors is called finding the resultant of them - whatever quantity it is (displacement, force, momentum), the procedure is the same
1) you should always start by drawing a diagram. draw the vectors ‘tip-to-tail’. if you’re doing a vector subtraction, draw the vector you’re subtracting with the same magnitude but pointing in the opposite direction
2) if the vectors are perpendicular to each other, then you can use pythagoras and trigonometry to find the resultant vector
3) if the vectors aren’t perpendicular, you may need to draw a scale diagram
how do you resolve vectors?
MECHANICS
resolving a vector is the opposite of finding the resultant vector - you start from the resultant vector and split it into two components perpendicular to each other (you’re basically working backward from finding the resultant)
why is it useful to resolve a vector?
MECHANICS
⋅ resolving a vector is very useful because the two components of vectors don’t affect each other - this means you can deal with two directions completely separately
(components perpendicular to each other don’t affect each other)
are speed, displacement, velocity and acceleration a scalar or a vector quantity?
MECHANICS
⋅ speed is a scalar quantity
⋅ displacement, velocity and acceleration are all vector quantities
what is the definition for speed?
MECHANICS
speed is how fast an object is moving, regardless of direction (i.e. the magnitude of velocity)
what is the definition for displacement?
MECHANICS
displacement (s) is how far an object has travelled from its starting point in a given direction
what is the definition for velocity?
MECHANICS
velocity (v) is the rate of change of an object’s displacement (an object’s speed in a given direction)
what is the definition for acceleration?
MECHANICS
acceleration (a) is the rate of change of an object’s velocity
what is the average speed?
MECHANICS
during a journey, the average speed is just the total distance covered over the total time elapsed
(average speed = total distance/total time elapsed)
what is the instantaneous speed?
MECHANICS
the speed of an object at any given point in time is known as its instantaneous speed
what is uniform acceleration?
MECHANICS
uniform acceleration is constant acceleration
(in mechanics, uniform = constant)
what could cause acceleration?
MECHANICS
acceleration can mean a change in speed, change in direction or both
what are the four main equations used to solve problems involving uniform acceleration?
MECHANICS
v = u + at
s = 0.5(u + v)t
s = ut + 0.5a(t^2)
v^2 = u^2 + 2as
[less important one: s = vt - 0.5a(t^2)]
how to derive v = u + at?
MECHANICS
how to derive s = 0.5(u + v)t?
MECHANICS
literally adding velocities together and dividing by the number of velocities there are (always just two because you just consider u and v) to find the average velocity (like finding any other average)
how to derive s = ut + 0.5a(t^2)?
MECHANICS
sub v = u + at into s = 0.5(u + v)t
how to derive v^2 = u^2 + 2as?
what is free fall?
MECHANICS
⋅ free fall is defined as the motion of an object undergoing an acceleration ‘g’
⋅ so basically the only force acting on an object is its weight and nothing else
what are 5 things you need to remember when looking at free fall?
MECHANICS
1) acceleration is a vector quantity
2) the magnitude of ‘g’ is usually taken as 9.81 m s^-2, though it varies slightly at different points above the earth’s surface
3) the only force acting on the object in free fall is its weight
4) all objects in free fall fall at the same rate
5) the objects can have an initial velocity in any direction and still undergo free fall as long as the force providing the initial velocity is no longer acting
how do you calculate g? (practical)
MECHANICS
1) set up the equipment as shown in the diagram:
2) measure the height h from the bottom of the ball bearing to the trapdoor
3) flick the switch to simultaneously start the timer and disconnect the electromagnet, releasing the ball bearing
4) the ball bearing falls, knocking the trapdoor down and breaking the circuit - which stops the timer. record the time t shown on the timer
5) repeat this experiment three times and average the time taken to fall from this height. do this for a range of different heights
6) you can then use these results to find g using a
s-(t^2) graph
how do you find g from the graph obtained in the practical? (practical)
MECHANICS
1) use your data from the experiment to plot a graph of height (s) against the time it takes for a ball to fall SQUARED (t^2). then draw a line of best fit
2) you know that with constant acceleration: s = ut + 0.5a(t^2)
3) if you drop the ball [so the ball moves from rest], initial speed u = 0, so s = 0.5a(t^2)
4) rearranging this gives 0.5a = s/(t^2), or 0.5g = s/(t^2)
5) gradient of line of best fit Δs/Δ(t^2) is equal to 0.5g, so: g = 2 x Δs/Δ(t^2)
⋅ eg using this graph) g = 2 x (0.44/0.09) = 9.8 m s^-2 (to 2 sf)
how do you increase the accuracy of the measurements in the practical for finding g?
MECHANICS
to increase the accuracy of the measurements you take, you can:
⋅ use a small and heavy ball bearing so you can assume that air resistance is so small that you can ignore it
⋅ use a ruler with smaller increments and pick a certain point on the ball bearing to measure from to reduce errors in measuring the height
how do you increase the accuracy of the method used in the practical for g?
MECHANICS
increasing accuracy in method:
⋅ in the experiment , using a computer to automatically release and time the ball bearing’s fall removes the random error that might arise if you timed a ball bearing manually by eye with a stopwatch, reducing the uncertainty in the time measurement
what could you use instead of an electromagnet and ball in the practical for finding g?
MECHANICS
you could do a similar experiment with light gates:
⋅ drop a ball bearing from a height h so it falls through the light gate. the light gate then automatically calculates the velocity of the falling object
⋅ you can then use (v^2) = (u^2) + 2as to calculate the acceleration due to gravity, g
why is using either a computer or light gate instead to find g better in the practical?
⋅ both methods give less uncertainty in g than measuring the time of the ball manually by the eye would
⋅ light gates can calculate the velocity of the ball automatically, instead of it being calculated from the time measurements, which could reduce systematic error
⋅ this means that the main uncertainty would now only be caused by measuring h (instead of t)
how can you use video and a ruler to find g? (practical)
MECHANICS
1) set up a video camera in front of a metre rule and record the ball as it is dropped from the top of the ruler
2) once the ball hits the floor, you can stop recording and analyse the video with video editing software
3) go through the video frame by frame, making note of the distance the ball has travelled every 0.1 seconds
4) create a table to calculate the ball’s velocity. you can then calculate the average value for g from the table or plot a graph of velocity against distance. the gradient of this graph will give acceleration due to gravity
how can you use video and strobe lights to find g?
you can use a regular camera in a dark room with a strobe light to find g
1) set the camera to take long exposure
2) while the camera is taking the photo, turn on the strobe light and drop the ball
3) as the ball falls, it will be lit up at regular intervals by the strobe light
4) this means the ball will appear multiple times in the photograph, in a different position each time
5) calculate the change in distance between each location of the ball and create a table (shown)
6) the frequency the strobe light flashes at gives you the time interval between distances
7) work out the velocity of the ball using the table and plot a graph of velocity against time - g will be the gradient
what are the main causes of uncertainty in the practical to find g (using video and strobe light) and how do you reduce them?
MECHANICS
1) the main cause of uncertainty in this experiment is in measuring the distance fallen by the ball - other sources of uncertainty are small bc you’re not timing the ball yourself (bc video is measuring time)
2) a parallax (systematic error due to looking at the ruler at an angle) will also affect your distance measurements, so make sure your camera is at a right angle to the ruler (and use multiple cameras if the ruler is large)
3) an uncertainty caused by the time interval between pictures can be reduced by either increasing the frequency of the strobe light, or using a camera with a higher frame rate
4) repeating the experiment and calculating the average value of g can also increase the accuracy of the experiment
5) measuring over a larger distance or using smaller time increments means you will average over more values, which is likely to give a more accurate value for g
what can you replace a (acceleration) with in equations of motion (suvat) when talking about projectiles?
MECHANICS
you can just replace a with g in equations of motion (suvat) when talking about projectiles
why can you use equations of motion (suvat) for projectiles?
MECHANICS
as g is a constant acceleration, you can use the equations of motion (suvat) in projectiles
what direction does g act in?
MECHANICS
⋅ g acts in the vertical plane (NOT THE HORIZONTAL PLANE) and acts downwards
⋅ g does NOT act in the horizontal plane
⋅ so also note that g doesn’t affect the horizontal component, only the vertical, because g acts vertically
how can you use resolving vectors to work out the time of flight of an object and how far an object moves horizontally/vertiaclly through the air (using projectiles)?
MECHANICS
1) resolve the initial velocity of the object into the horizontal and the vertical components
2) use the vertical component to work out how long an object is in the air and/or how high it goes
3) use the horizontal component to work out how far the object goes horizontally while it’s in the air
what does acceleration look like on a displacement-time graph?
MECHANICS
⋅ a graph of displacement against time for an accelerating object always produces a curve
⋅ if the object is accelerating at a uniform rate, then the rate of change of the gradient will be constant (the second derivative will be constant)
what does the gradient look like for different accelerations
⋅ different accelerations have different gradients
⋅ for example, when comparing the green example (the one on the front of the flashcard) and the pink examples (shown below), if the lion has a different acceleration it’ll change the gradient of the curve
⋅ decelerating has a decreasing gradient, instead of an increasing gradient like positive acceleration
how can you tell the difference between speed-time graphs and velocity-time graphs?
MECHANICS
speed-time graphs and velocity-time graphs are pretty similar - the big difference is that the velocity-time graphs can have a negative part to show negative velocity (negative velocity = something travelling in the opposite direction/backwards)
what does the gradient of a velocity-time graph tell you?
MECHANICS
the gradient of a velocity-time graph tells you the acceleration
what features of the acceleration can you recognise from the gradient of a velocity-time graph?
MECHANICS
1) uniform acceleration is always a straight line
2) the steeper the gradient, the greater the acceleration
since you know acceleration is just the gradient of a velocity-time graph, what suvat equation can you generate using this information?
MECHANICS
⋅ v = u + at
⋅ because… (look at image)
since the equation of a straight line (y = mx +c) and the suvat equation v = u + at look similar, what does this mean?
MECHANICS
⋅ the equation for a straight line is y = mx + c
⋅ you can rearrange the acceleration equation (v = u + at) into the same form, getting v = at + u
⋅ this means that on a linear v-t graph, acceleration (a) is the gradient (m) and initial speed (u) is the y-intercept (c)
how do you plot a v-t graph by calculating v as t increases?
MECHANICS
why does the area under a velocity-time graph give?
MECHANICS
⋅ the area under a velocity-time graph tells you the displacement of an object
⋅ areas under any negative parts of the graph count as negative areas, as they show the object is moving back toward its starting point
what does the area under a speed-time graph give?
MECHANICS
the area under a speed-time graph gives the total distance travelled
what does a non-uniform acceleration also look like on a velocity-time graph?
MECHANICS
⋅ non-uniform acceleration is a curve on a v-t graph
⋅ as acceleration is changing, the gradient of the v-t graph will also be changing (bc a = Δv/Δt) - so you won’t get a straight line
how do you know if the acceleration is increasing or decreasing from a velocity-time graph?
MECHANICS
1) an increasing acceleration is shown by an increasing gradient - like in the curve (1)
2) a decreasing acceleration is shown by a decreasing gradient - like in curve (2)
how do you estimate the displacement from a curved velocity-time graph?
⋅ as the v-t graph is no longer a simple straight line, you have to estimate the area under the curve (bc s = area)
⋅ if the graph is on squared paper, an easy way to do this is to just count the number of squares under the curve and multiply the area by what the area of one square represents (so eg, 2 m s^-1 x 1 s, so area of 1 square = 2 m)
⋅ another way is to split the curve up into the trapezia and calculate the area of each trapezium and add them all up
how do you investigate what affects the motion of a trolley? (practical)
MECHANICS
1) to investigate how the distance a trolley has travelled affects its speed, set up the experiment shown in the diagram
2) measure the length of the trolley
3) mark a start line on the ramp to make sure the trolley always starts from the same position
4) measure the angle of the ramp (°) and the distance from the start line to the light gate (d)
5) place the trolley on the ramp and line it up with the start line. let go of the trolley so its initial velocity, u, is 0
6) the data logger will record the time taken for the trolley to pass through the light gate and calculate the velocity of the trolley as it passes through the light gate
7) change the starting position of the trolley, so that d is varied
8) repeat this experiment 3 times and average the recorded velocities to reduce the error in your final result
how could you lower the uncertainty in the measurement for the velocity of the trolley?
MECHANICS
using a light gate gives a much lower uncertainty in the measurement than using a stopwatch and calculating the velocity manually
how can you investigate other factors using the trolley experiment?
MECHANICS
⋅ you can use the same setup as in the experiment to investigate other factors
⋅ eg) you could change the angle of the ramp or material it’s made from - or change the mass, the size or the shape of the trolley
what could you use instead of a data logger in the trolley experiment?
MECHANICS
⋅ you can use a ticker timer instead of a data logger
⋅ ticker timers create holes in (or make dots on) a long piece of paper (ticker tape) at regular time intervals
⋅ this is usually about every 50th of a second
how do you use the ticker timer instead of a data logger in the trolley experiment?
MECHANICS
1) you can calculate how long it takes the trolley to travel down the ramp by attaching the ticker tape to the back of the trolley and threading it through the timer
2) switch the ticker timer on when you release the trolley, and switch it off when the trolley reaches the end of the ramp
3) the time taken can then be calculated from the number of holes punched into the ticker tape (50 holes = 1 second)
what are some of the uncertainties in using the ticker timer instead of the data logger [for the trolley practical]
MECHANICS
1) ticker timers are able to measure the time very accurately but rely on manually turning the machine on and of, which will add uncertainty to measurements eg) human error/reaction time
2) uncertainty can also be introduced when having to count the total number of dots
what is the advantage of using a data logger over a ticker timer (in the trolley experiment)
MECHANICS
data loggers do not have this human error and can calculate the velocity and display it in real-time - saving time and allowing comparisons between experiments to easily be made
how can you investigate what happens to the motion of an air glider when two gliders collide? (practical)
MECHANICS
you can use the air glider experiment to measure how the mass and velocity of a glider just before the collision affects the velocities of both gliders after the collision
1) set up the experiment as shown in the diagram, with a video camera positioned side-on with the motion of the gliders
2) measure the length and the mass of both gliders
3) turn on the video camera and start recording
4) push one glider so it hits the second glider
5) when both gliders have come to a stop, stop recording
(if you use the velocities you can find to work out the total momentum before and after the collision, you should find it is the same)
how can you analyse the results from the air glider experiment? (practical)
MECHANICS
1) using the video analysis software, you can view your videos frame-by-frame. pick a point of reference on the metre rule and count how many frames it takes the slider to pass that point
2) by knowing how many frames per second the video is shot at (the frame rate of the video), you can calculate the time taken (t) for the whole glider to pass that point. you recorded the length (L) of each glider, and so you can calculate their velocities
what can you use iterative models for?
MECHANICS
⋅ using the equations of motion for constant acceleration, you can model how a uniformly accelerating body’s velocity and displacement vary over time
⋅ to do this, use an iterative method, where velocity and displacement of the body are calculated over lots of small time increments to model the body’s behaviour over a longer time period
how can you use a spreadsheet for an iterative method?
MECHANICS
1) create a spreadsheet [/table] like the one shown below (the one shown below models the motion for a body accelerating from rest at 4 m s^-1 over increments of 1 s)
2) for each row, the change in velocity (Δv) will be constant (as Δv = aΔt = 4 x 1 = 4 m s^-1)
3) add this Δv to the velocity (v) from the previous row, giving you the new velocity for each increment
4) to find the change in displacement (Δx) for each row use Δx = uΔt + 0.5a(Δt)^2 (where u is the value of v from the previous row). add this to the previous x value to get a new displacement at the end of each time interval
5) repeat these calculations to build up a full model of a body’s motion over time. if you enter the correct formulae into the spreadsheet, then it can automatically complete as many rows (iterations) as you want
6) you can then plot a graph if v against t (which should be a straight line through the origin with a gradient equal to the body’s acceleration) and x against t (which should be a curve)
how can you use vectors for iterative methods?
MECHANICS
1) consider an object fired horizontally, so that it begins to move with the projectile motion under the influence of gravity
2) initially, the object has the horizontal speed v0 and the vertical speed 0. following the first time increment, the object still has the horizontal speed v0, but its vertical speed has increased by Δv. this gives the resultant velocity of v1 as shown
3) following the second increment, the horizontal speed has remained constant (because gravity doesn’t act on the horizontal component of speed) and the vertical speed has again increased by Δv. the new resultant velocity is v1 + Δv = v2
4) the pattern continues for each time increment. the resultant velocity increases for each increment (the blue arrow gets longer each time), as the object is accelerating
what do free-body force diagrams show?
MECHANICS
1) free-body force diagrams show a single body on its own
2) the diagram should include all the forces that act on the body, but not the forces it exerts on the rest of the world
3) remember forces are vector quantities and so the arrow labels should show the magnitude and the direction of the forces
4) if the body is in equilibrium (i.e. not accelerating) the forces on it will be balanced
what does it mean if you resolve a force?
MECHANICS
resolving a force means splitting it into its components
why do you resolve a force?
MECHANICS
⋅ forces can be in any direction, so they’re not always perpendicular to each other
⋅ this is sometimes a bit awkward for calculations
⋅ to make an ‘awkward’ force easier to deal with, you can think of it as two separate, independent forces, acting at right angles to each other
do resolved forces have any effect on each other?
MECHANICS
resolved forces have no effect on each other as they are perpendicular (eg, the horizontal force will have no vertical effect, and vice versa)
how do you find the size of the component force in a particular direction?
⋅ to find the size of a component force in a particular direction
⋅ forces are vectors, so you treat them in the same way as a velocity or displacement - put them end to end (tip to tail) as shown in diagram
⋅ so using trigonometry you get:
F horz = Fcosθ
and
F vert = Fsinθ
⋅ remember that cos90° = 0 (so Fcos90° (which would be vert force in terms of cos) = 0 , so forces which act at 90° to each other are independent (i.e. they have no effect on each other)
how do you find the resultant force?
MECHANICS
1) if two forces act on an object, you find the resultant force by adding the vectors together and creating a closed triangle, with the resultant force represented by the third side
2) forces are vectors, so use vector addition - draw the forces as vector arrows ‘tip-to-tail’
3) then use trigonometry to find the angle and the length of the third side
what do you need to consider when resolving forces?
MECHANICS
⋅ choose the most sensible axes for resolving
⋅ use directions that make sense fo the situation you’re dealing with
⋅ if you have an object on a slope, choose your directions[/axes] along the slope and at right angles to it (not any other direction)
⋅ you can turn paper to an angle if that helps [make the diagram make more sense]
what do examiners like to call a slope?
MECHANICS
examiners like to call a slope an ‘inclined plane’
what is linear momentum?
MECHANICS
linear momentum is just the momentum in a straight line (not in a circle or anything complicated like that)
what two things does the momentum of an object depend on?
MECHANICS
the object’s mass and its velocity
what is the equation for momentum?
MECHANICS
momentum = mass x velocity
p = mv
is momentum conserved and what does this mean?
MECHANICS
⋅ assuming no external forces act, momentum is always conserved (this is the principle of the conservation of momentum)
⋅ this means the total momentum of two objects before they collide equals the total momentum after the collision
how can you use the principle of the conservation of momentum?
MECHANICS
the principle of the conservation of momentum can be handy for working out the velocity of objects after a collision:
can the conservation of momentum be applied in explosions?
MECHANICS
⋅ yes, the conservation of momentum can be applied in explosions
⋅ eg) if you fire an air rifle, the forward momentum gained by the pellet equals the backward momentum of the rifle, and you feel the rifle recoiling into your shoulder
what are the two types of collisions?
MECHANICS
collisions can be elastic or inelastic
what is an elastic collision?
MECHANICS
a perfectly elastic collision is one where the momentum is conserved and the kinetic energy is conserved - i.e. no energy is dissipated as heat, sound, etc
what does it mean if a collision is inelastic?
MECHANICS
if a collision is inelastic it means that some of the object’s kinetic energy is converted into other forms during a collision (but momentum is still conserved)
does the conservation of momentum differ whether it’s elastic or inelastic?
MECHANICS
momentum is always conserved, no matter if the collision is elastic or inelastic
what can the conservation of momentum be used for?
MECHANICS
you can use the principle of the conservation of momentum to predict the behaviour of real-world objects, eg) balls in sports games
what can you deduce from newton’s first law of motion?
MECHANICS
⋅ newton’s first law of motion state that the velocity of an object will not change unless a resultant[/external] force acts on it
⋅ this means the body will remain at rest or moving in a straight line at a constant speed[/velocity], unless acted on by a resultant force
considering newton’s first law, what happens if the forces on a body aren’t balanced
⋅ if any forces acting on a body aren’t balanced, the overall resultant force will cause the body to accelerate
⋅ if you gave the apple in the example a shove, there would be a resultant force acting on it and it would roll off the table
what is newton’s first law of motion?
MECHANICS
newton’s first law of motion states: “a body will remain stationary or move at a constant velocity unless acted upon by a resultant force”
what is newton’s second law of motion?
MECHANICS
⋅ the resultant force on an object is equal to its rate of change of momentum
⋅ F = ma
what does F = ma mean?
MECHANICS
1) the greater the net resultant force acting on a body of a certain mass, the greater the acceleration of the body (F ∝ a)
2) for a given [resultant] force, the greater the mass of the body it acts on, the less acceleration the body will experience (a = F/m)
what do you need to consider for F = ma?
MECHANICS
what is the definition of a newton?
MECHANICS
⋅ F = ma
⋅ 1 N = 1 kg m s^-2
⋅ 1 Newton is 1 kilogram metre per seconds squared
at what rate do objects fall (if you ignore air resistance) and why?
MECHANICS
⋅ all objects fall at the same rate (if you ignore air resistance)
⋅ on earth, the force that causes objects to accelerate toward the ground is the gravitational pull of the earth
⋅ the gravitational field strength on the earth (g) is pretty much constant - so all objects should accelerate toward the ground at the same rate, no matter what their mass is
how does netwon’s second law explain how objects fall at the same rate (if you ignore air resistance)?
MECHANICS
⋅ newton’s second law explains how all objects fall at the same rate (if you ignore air resistance)
⋅ consider two balls being dropped at the same time - ball 1 being heavy, and ball 2 being light
⋅ then use newton’s second law to find their acceleration (as balls are free-falling)
what is newton’s third law a consequence of?
MECHANICS
netwon’s third law is a consequence of the conservation of momentum
what is newton’s third law?
MECHANICS
newton’s third law states that: “if an object A exerts a force on object B, then object B exerts an equal but opposite force on object A”
OR
⋅ “if body A exerts a force on body B, then body B exerts an equal force back on body A but in the opposite direction”
what should you realise when using newton’s third law?
MECHANICS
the two forces mentioned in newton’s third law actually represent the same interaction, just seen from two different perspectives (not two forces being applied to one object)
what does netwon’s third law look like when you push a wall?
MECHANICS
if you (body A) push against a wall (body B), the wall will push back against you, just as hard (an equal force exerted back on body A in the opposite direction)
what does newton’s third law look like when pulling a cart?
MECHANICS
if you pull the cart, whatever force you exert on the rope, the rope exerts the exact pull on you
what does newton’s third law look like when you go swimming?
MECHANICS
⋅ when you go swimming, you push back against the water with your arms and legs, and the water pushes you forward with an equal-sized force
⋅ so the backward momentum of the water is equal to your forward momentum
in newton’s third law, are pairs forces always the same type?
MECHANICS
newton’s third law applies in all situations and to all types of forces - but the pairs of forces in question are always the same type, eg) they are both gravitational or both are electrical
what are the requirements for two forces to be a pair of forces (in newton’s third law)?
MECHANICS
how do you explain the fact that newton’s 3rd law is a consequence of the conservation of momentum?
MECHANICS
- the resultant force acting means a change in mass or acceleration (F = ma) - which means a change in momentum (as p = mv and p = Ft)
- momentum is always conserved when no external force acts, so whenever one object exerts a force on another (and changes its momentum), the second object must exert an equal-sized force back on the first object so that the overall change in momentum is zero
what is friction?
MECHANICS
friction is a force that opposes motion
what are the two types of friction?
MECHANICS
1) contact friction between solid surfaces (what we usually mean when we just used the word ‘friction’)
2) fluid friction (known as drag or fluid resistance or air resistance)
what is a fluid?
MECHANICS
a ‘fluid’ is a word that means either a liquid or a gas - something that can flow
what does fluid friction (aka the drag force) depend on?
MECHANICS
1) the force depends on the thickness [/viscosity] of the fluid
2) the drag force increases as the speed of the object increases (for simple situations, drag force is directly ∝ to the speed of the object)
3) the drag force also depends on the size and the shape of the object moving through the fluid - the larger the area pushing against the fluid, the greater the resistance force
what things do you need to remember about frictional forces?
MECHANICS
1) frictional forces always act in the opposite direction to the motion of the object
2) frictional forces can never speed things up or start something moving
3) frictional forces convert kinetic energy into heat
when does terminal velocity happen?
MECHANICS
- terminal velocity - when the frictional force equals the driving force
- you will reach terminal velocity at some point, if you have:
1) a driving force that stays the same all the time[/constant]
2) a frictional or a drag force (or collection of resistance forces) that increases with speed
what are the three main stages to reaching terminal velocity?
MECHANICS
what does the velocity-time graph look like for an object reaching terminal velocity?
MECHANICS
what does the acceleration-time graph look like for an object reaching terminal velocity?
MECHANICS
do things falling through the air or water (instead of moving along a plane) reach terminal velocity too?
MECHANICS
⋅ yes, things falling through the air or water reach terminal velocity too
EG)
⋅ when something is falling through the air, the weight of the object is a constant force accelerating through the air
⋅ in this case, air resistance is the frictional force opposing this motion, which increases with speed (for something falling through water, frictional force is water resistance)
what happens once a parachutist jumps out of a plane? and what happens once their parachute opens?
MECHANICS
what does the velocity-time graph look like for a parachutist?
MECHANICS
the v-t graph for the terminal velocity of a parachutist is a bit different from that of a regular object because you have a new terminal velocity being reached, after the parachute is opened, which you also have to calculate
how do you calculate the terminal velocity of a ball bearing (a little steel ball) in a viscous (thick) liquid, using the set up shown? (PRACTICAL)
MECHANICS
1) put elastic bands around tube of viscous liquid at fixed distances using ruler
2) drop ball bearing into tube, + use stopwatch to record time at which it reaches each band. record your results in table below:
3) repeat this few times to reduce effect of random errors on your results. you can use strong magnet to remove ball bearing from tube
4) calculate times taken by ball bearing to travel between consecutive elastic bands + calculate average for each reading. use average times + distance between bands to calculate average velocity between each pair of elastic bands
5) you should find that average velocity increases at first, then stays constant - this is ball bearing’s terminal velocity in viscous liquid used
what’s an alternative method to investigate terminal velocity than using a ball bearing and viscous fluid?
you could also investigate the terminal velocity by dropping paper cones through the air and using a light gate to get a more accurate velocity readings
how can you investigate what affects terminal velocity?
MECHANICS
⋅ you can change parts of your experiment to see what effect they have on terminal velocity and the time taken to reach the terminal velocity
⋅ you could EG)
1) change the liquid used in the experiment
-> the terminal velocity will be lower in more viscous (thicker) liquids because the drag force is greater
-> try mixing the water into wallpaper paste and see how much the terminal velocity increases when drag is lower
2) change the size of the ball
-> what happens if the ball is larger? or smaller?
3) change the shape of the thing you are dropping
-> the drag force will be greater
4) change the mass of the thing you are dropping, while keeping the size the same (by using more dense materials?)
-> you should find that heavier objects reach a faster terminal velocity because a greater drag force is needed to balance the extra weight (remember, objects with different masses only fall at the same rate if the drag is ignored)
how do you estimate the terminal velocity of the ball bearing from your data from the experiment? (practical)
MECHANICS
1) use your average velocity data from the experiment for investigating terminal velocity to plot a graph of velocity against time
2) draw a smooth curve (curved line of best fit) and use it to estimate the terminal velocity of the ball bearing
what should you remember if you are asked to draw force diagrams for a ball bearing as it falls?
MECHANICS
remember that the forces are balanced when the ball reaches terminal velocity
when is work done?
MECHANICS
⋅ work is done whenever energy is transferred
OR
⋅ work is done if a force is applied to an object and the object is displaced as a result
what are some examples of work done and the energy changes that can happen?
MECHANICS
what does ‘work’ mean in physics?
MECHANICS
‘work’ in physics means an amount of energy is transferred from one form to another when a force causes a movement of some sort
what happens when work is done on an object?
MECHANICS
1) usually you need the force to move a body of some kind because you’re having to overcome another force
2) the thing being moved has kinetic energy while it’s moving
3) the kinetic energy has been transferred to another form of energy when the movement stops
what is the equation to find how much work is done when a force is applied by body A to body B (usually with the purpose of moving body B?
MECHANICS
ΔE = FΔs
or
work done = force causing motion x displacement
what are some points to remember about work done?
MECHANICS
1) work is the energy that’s been changed from one form to another - it’s not necessarily the total energy
⋅ eg) moving a book from a lower shelf to a higher one will increase the book’s GPE, but it had some PE to start with -> here, work done would be the increase in the PE not the total PE of the book
2)
3) the force F will be a fixed value in any calculations, either because it’s constant or because it’s the average force
4) ΔE = FΔs assumes that the direction of the force is the same as the direction of the movement
5) ΔE = FΔs gives you the definition of the Joule (J):
what should you remember for work done calculations using ΔE = FΔs?
⋅ remember that the displacement needs to be in metres (when s is measured and for calculations)
⋅ if you have the displacement in centimetres or kilometres, you need to convert it to metres first
what definition of the joule does ΔE = FΔs give?
MECHANICS
ΔE = FΔs gives you this definition of the joule (J): “one joule is the work done when a force of 1 newton moves an object through a distance[/displacement] of 1 metre”
is the force doing work on an object always acting in the same direction as the displacement of the object?
MECHANICS
no, the force isn’t always in the same direction as the displacement of the object
how do you calculate the work done when the force isn’t in the same direction as the displacement (of the object)?
MECHANICS
1) to calculate the work done in a situation like the one shown on the front of the flashcard, you need to consider the horizontal and vertical components of the force
2) the only displacement is in the horizontal direction
⋅ this means that the vertical force is not causing any displacement (and hence not doing any work) - it’s just balancing out some of the object’s weight, meaning there’s a smaller reaction force
3) the horizontal force is causing the displacement - so to calculate the work done, this is the only force you need to consider. which means you get:
ΔE = FΔscosθ
⋅ where θ is the angle between the direction of the force and the direction of the displacement
what is power?
MECHANICS
⋅ power is the rate of work done
OR
⋅ power: the amount of energy transferred from one form to another per second
what is the equation for power?
MECHANICS
power = work done/time
OR
P = (ΔE)/t
what is the definition of the watt (W)?
MECHANICS
the definition of the watt (W) is defined as a rate of energy transfer equal to 1 joule per second (J s^-1)
