p5 forces Flashcards
what has made the analysis of movement possible?
artificial limbs
what do scalar quantities have?
magnitude (size) only
what is a contact force?
a force acting on two objects that are physically touching
what do vector quantities have?
magnitude and direction
what and how can vectors be represented by?
an arrow
- the length represents the magnitude
- the direction also represents the direction of the vector
what is a non contact force?
a force acting on two objects that are not physically touching
examples of contact forces?
friction
air resistance
tension
normal contact force
examples of non contact forces?
gravitational force
electrostatic force
magnetic force
an example of forces acting on the earth and the sun?
they’re attracted to each other by a gravitational force, this is non contact as there is an equal and opposite force of attraction felt by both the earth and sun
an example on forces acting on a chair?
the chair exerts a downwards force onto earth called weight. while the earth exerts an equal and opposite force back onto the chair
what is weight?
the force acting on an object due to gravity. the force of gravity close to the earth is due to the gravitational field around the earth
what does the weight of an object depend on?
the gravitational field strength at the point where the object is
what is the ‘centre of mass’?
the weight of an object acting at a single point
what is the mass of an object directly proportional to?
the weight of an object
what is the weight of an object measured by?
newtonmeter
what is a resultant force?
a single force which replaces all of the forces acting on an object. this force has the same effect as all the forces that its replacing acting together
examples of forces acting on a moving car?
1) driving force, moving the car forwards
2) friction, dragging the car back
3) weight, acting downwards onto the ground
4) normal contact force, pushing the car up over the ground
examples of forces acting on a sky diver?
1) weight, pulling the diver down to the ground
2) drag, the air resistance pushing the diver upwards
what can a single force be resolved into?
two components acting at right angles to each other. the two component forces have the same effect as the single force
what causes an object to be at equilibrium?
when all the forces acting on an object combine to give a resultant force of 0
how can a scale diagram show that an object is at equilibrium?
the tip of the last force you draw should end where the tail of the fist force you draw begins. (eg three forces drawn should make a triangle)
how to find the missing force of an object which is at equilibrium?
join the two forces that you DO know tip to tail and join the end of the last force line to the start of the first one. this line is the missing force so you can measure it’s size and direction
how to split a single force into it’s horizontal and vertical components?
draw the force line on a scale grid. draw a horizontal arrow from the bottom end of the force and a vertical arrow to the top end of the force to form a right angled triangle
what causes work to be done on an object?
when a force causes an object to be moved through a distance. so force does work on an object when the force causes a displacement of the object
what is the relationship between work done and distance travelled?
one joule of work is done when a force of one newton causes a displacement of one metre.
therefore: one joule= one newton metre
describe how work is done
1) to make something move a force must be applied
2) the thing applying the force needs a source of energy (like fuel or food)
3) the force does ‘work’ to move the object and energy is transferred from one store to another
4) whether energy is transferred ‘usefully’ or ‘wasted’ , you can still say that ‘work is done’
what is ‘work done’ the same as?
energy transferred
describe the energy transfers when work is done
when pushing something along a rough surface, you are ‘doing work’ against frictional forces. energy is being transferred to the kinetic energy store of an object as the object is moving, but some is also transferred to thermal energy stores due to the friction.
what does work done against a frictional force resolve in?
an increase in temperature
what are some examples of forces that are involved in stretching, bending and compressing an object?
normal contact force and tension force
why does an object need two or more forces to be able to change shape
if only one force is applied, the object will only just move in the direction of the applied force
what is elastic deformation?
an object that has been elastically deformed can go back to it’s original shape and length after the force has been removed. these are called ELASTIC objects
what is inelastic deformation?
an object that has been inelastically deformed does not return to it’s original shape and length after the force has been removed
what two things are directly proportional to one another when stretching a spring?
the extension of a stretched spring is directly proportional to the force applied
what is a spring constant?
the measure of stiffness of a spring up until it’s limit of proportionality
when does a force ‘do work’ on a spring?
what energy type is stored in the spring?
what two things are the same in a spring?
a force that stretches or compresses a spring does work and elastic potential energy is stored in the spring. provided that the spring is not inelastically deformed, the WD on the spring and the elastic potential energy store are the same
describe the relationship between force and extension on a spring in a linear graph
the force and extension are directly proportional. therefore as more force is applied, the more the spring extends
describe the relationship between force and extension on a spring in a non-linear graph
the force and extension are not directly proportional. therefore the spring stretches more for each unit increase in force
what is the limit of proportionality?
the point at which hooke’s law is no longer true when stretching a material
what is a moment?
the turning effect of a force
how does force and distance affect a moment?
the force on a spanner causes a turning effect/ moment on the nut. a larger force or longer distance (of the spanner) would mean a larger moment
how can you get a maximum moment?
push at right angles to the spanner. pushing it at any other angle would mean that there is a smaller distance, so therefore a smaller moment
how can you tell that an object is balanced and wont turn?
if the total anticlockwise movement equals the total clockwise movement about a pivot
what can a simple lever or gear be used to do?
to transmit the rotational effects of a force
what do levers do?
they increase the distance from the pivot at which the force is applied. this means that levers make it easier to do work
examples of simple levers?
long sticks/ bars
wheelbarrows
what are gears and what do they do?
– they are circular discs with teeth around their edges
1) their teeth interlock with one another so that turning one causes another to turn in the opposite direction
2) they are used to transmit the rotational effect of a force in the opposite direction
how does the size of the gear affect the turning effect?
different sized gears can be used to change the moment of a force. a force transmitted to a larger gear will cause a bigger moment, as the distance of the pivot is bigger.
so the larger gear will turn slower than the smaller gear
what state of matters can a fluid be?
liquid or gas
what does the pressure of a fluid cause?
causes a force normal (at right angles) to any surface
what is a fluid?
substances that can flow because their particles are able to move around
how is pressure created?
1) particles from a fluid collide with the surfaces and other particles
2) the particles are light but they still have a mass and exert a force on the object they collide with.
what is pressure?
force per unit area
what is the pressure of a liquid dependent on?
the depth and density of a liquid
what is density
the ‘compactness’ of a substance. so how close together the particles are
in what state of matter would a substance have uniformed density?
for a liquid
in what state of matter would a substance not have uniformed density?
for a gas
why does pressure depend on density and depth
1) the more dense a liquid is, the more particles it has in a certain space. this means there are more frequent collisions, increasing pressure.
2) as the depth increases, the number of particles above that point increases . the weight of these particles add up and adds to the pressure felt at that point
what happens when an object is submerged in a fluid (either partially or completely)?
1) the pressure of the fluid exerts a force on it from every direction
2) pressure increases with depth, so the force exerted on the bottom of the object is larger than acting at the top
3) this causes a resultant force upwards (upthrust)
what is the upthrust equal to?
the weight of fluid that has been displaced by the object
when does an object float?
is its weight= upthrust
how does weight determine whether an object will sink or not?
1) if the upthrust on an object is equal to the object’s weight, then the forces balane and the object floats
2) if an object’s weight is more than the upthrust, the object sinks
how does density cause an object to float or sink?
1) an object that is less dense than the fluid it is placed in weights less than the equivalent volume of fluid. This means it displaces a volume of fluid that is equal to it’s weight before it is fully submerged
2) atp, the object’s weight is equal to the upthrust, so the object floats
3) an object that is denser than the fluid it is placed in is unable to displace enough fluid to equal it’s weight. therefore it’s weight is greater than the upthrust
what is the atmosphere?
a thin layer of air around the earth which gets less dense with increasing altitude
what creates atmospheric pressure?
air molecules colliding with a surface
what happens to the number of molecules/ weight of air as the height of a surface above ground level increase?
they/ it decreases
how does atmospheric pressure decrease?
as the height of a surface from ground level increases.
so pressure is inversely proportional to altitude
what is distance?
how far an object moves. does not involve direction so therefore it is scalar
what is displacement?
the measurement of distance from one point to another in a STRAIGHT line. it is a vector
what is speed and is it ever constant?
speed is how fast an object travels. it does not include direction so therefore it is scalar. its rarely ever constant
what are the factors that can affect speed at which an individual walks, runs or cycles?
age
fitness
terrain
distance travelled
what are the typical speeds of:
a) walking
b) running
c) cycling
d) cars
e) trains
f) planes
g) sound in air
Aa) 1.5 m/s
Ab) 3 m/s
Ac) 6 m/s
Ad) 25 m/s
Ae) 55 m/s
Af) 250 m/s
Ag) 330 m/s
what is velocity?
the speed of an object in a given direction
why does a car moving in a circular motion have a constant speed but changing velocity?
speed is scalar, therefore it does not take into consideration the direction that the car is moving in. however velocity does, therefore when moving in a circle, the direction is constantly changing, therefore so is the velocity
what can the distance of object that is moving in a straight line be represented as?
a distance- time graph
how can the speed be found from a distance-time graph?
calculating the gradient of the line
what do flat sections on a distance- time graph represent?
the object is stationary
what do straight uphill lines on a distance-time graph represent?
the object is travelling at a steady speed
what do curves on a distance-time graph represent?
acceleration or deceleration
how can you find an object’s speed at a certain point as it’s accelerating or decelerating in a distance-time graph?
draw a tangent to that curve and find the gradient of it
what does the gradient represent on a velocity- time graph and why?
acceleration, because acceleration= velocity // time
what do the flat sections mean on a velocity- time graph?
the object is travelling at a steady speed
what does the steepness mean on a velocity-time graph?
the steeper the graph, the greater the acceleration or deceleration
what do uphill and downhill sections mean on a velocity-time graph?
acceleration and deceleration
what do curves mean in a velocity-time graph?
change in acceleration and deceleration
what is area under any section of the graph in a velocity-time graph equal to?
the distance travelled in that time interval
how can you calculate the distance travelled with an irregular velocity-time graph?
counting the squares under the area and multiplying by the value of one square
what does friction do an in what way does it act?
friction will cause an object to slow down and stop when there is no force propelling it along.
friction always acts in the opposite direction to movement
what must be balanced in order for an object to travel at a steady speed?
the driving force and the frictional force
what is drag?
drag is the type of resistance you get in a fluid. for example, air resistance
whats the most important factor in decreasing drag?
keeping the shape of an object streamlined, this is where the object is allowed to allow fluid to flow past it easily, by doing this, less particles hit the object, reducing drag
what happens to the speed of an object whilst drag increases?
the speed decreases. for example, a car has much more friction to work against whilst travelling at 70 mph compared to 30 mph. so the engine has to work harder to maintain a steady speed at 70 mph
what happens to objects that fall through fluids? and what contributes to it?
the object reaches terminal velocity and its all about the balance of weight and air resistance
what is the process of reaching terminal velocity?
1) when falling objects first set off, the force of gravity is much more than the frictional force slowing them down, so they accelerate.
2) as the speed increases, the friction builds up.
3) this gradually reduces the acceleration until eventually the frictional force is equal to the accelerating force.
4) this means that the resultant force is 0
5) it would’ve reached its terminal velocity and fall at a steady speed
what does terminal velocity mean?
the maximum speed at which an object will fall through a fluid
what does terminal velocity depend on?
shape and area of an object
what is the accelerating force on all falling objects?
gravity, and it would make them all fall at the same rate if it wasnt for air resistance
what happens to falling objects on the moon where there is no air resistance?
all objects, regardless of their mass will simultaneously hit the ground together
what causes objects to fall at different speeds?
the air resistance
what is frictional force dependent on? and give a detailed example
the shape and area of an object.
for example a skydiver.
– without his parachute open, they have quite a small area and a force of ‘W= mg’ pulling him down.
– they then reach a terminal velocity of about 120 mph.
– but with the parachute open, theres much more air resistance and still only the same force of ‘W= mg’ pulling them down.
– this means their terminal velocity comes down to about 15 mph
what does newton’s first law state?
if the resultant force of an object is 0, then the object will remain stationary. if the resultant force on a moving object is 0, it’ll just carry on moving at the same velocity
when a vehicle is moving at a constant velocity, what forces must be balanced?
the resistive and driving forces must all be balances, as the velocity will only change if theres a non-zero resultant force acting on it
what will a non-zero resultant force always produce? and what 5 forms can this take place in?
acceleration or deceleration in the direction of the force.
1) starting
2) stopping
3) speeding up
4) slowing down
5) change in direction
what is newton’s second law?
the larger the resultant force on an object, the more the object accelerates. therefore, acceleration is inversely proportional to mass.
- the force and acceleration are directly proportional ( F= ma)
what is intertial mass?
the measure of how difficult it is to change the velocity of an object. it is defined as the ratio of force over acceleration
what is inertia?
the tendency to continue in the same state of motion
how can inertial mass be calculated?
rearranging the F=ma formula to
a= F / a
what is newtons third law?
when two objects interact, the forces they exert on each other are equal and opposite
because interacting forces have equal and opposite forces, how do resultant forces exist?
the two forces are acting on different objects, therefore the same force will have a different effect on a car compared to a truck
an example of newtons third law happening in an equilibrium situation?
- a man pushing against a wall.
1) as the man pushes the wall, there is a normal contact force being applied to the wall, this same force is being exerted back to him.
2) these two forces are the same size
when does newtons third law not apply?
when the forces exerted back on each other are not the same as as each other
what is stopping distance?
the distance it takes for a car to perform an emergency stop.
what two distances sum the stopping distance?
thinking distance + breaking distance
what is an emergency stop?
this is when maximum force is applied by the brakes in order to stop the car in the shortest possible distance. the longer it takes to perform an emergency stop, the higher the risk of crashing into whatevers infront of you
what is thinking distance?
how far the car travels during the driver’s reaction time. (the time between the driver seeing a hazard and applying the brakes)
what is breaking distance?
the distance taken to stop under the braking force (once tthe brakes are applied)
what factors affect thinking distance?
1) speed- the faster you’re going, the further you’ll travel during the time you take to react.
2) reaction time- the longer you take to react, the longer your thinking distance will be.
THIS CAN BE AFFECTED BY:
- tiredness
- drugs
- alcohol
- distractions can affect your ability to react
what factors affect braking distance?
1) speed- the faster a vehicle travels, the longer it takes for them to stop.
2) weather/ road surface- wet or icy surfaces mean there is less grip, therefore less friction between the road and vehicle’s tire, which can cause skidding.
3) the condition of your tires- if the tires are bald, they cannot get rid of water in wet conditions, leading them to skid over the water.
4- how good your brakes are- if the brakes are worn out or faulty, they wont be able to apply as much force as well-maintained tires, which could be dangerous when you brake hard.
why are speed limits important?
because speed affects the stopping distance so much
how can you measure reaction time?
1) computer based tests, eg clicking a mouse when the screen changes colour
2) ruler dropping test
how to conduct the reaction time ruler dropping test?
1) sit with ur arm resting on the edge of the table. ( stops ur arm from moving up or down).
2) someone else holds a ruler so that it hangs inbetween ur thumb and index finger, lined up with 0.
3) without giving any warning, the person holding the ruler should drop it, and the other person should pinch their 2 fingers to try and catch the ruler as quickly as possible.
4) the measurement on the ruler at the point where it was caught is how far the ruler
what does braking rely on?
the friction between the breaks and the wheels
how does breaking work?
1) when the break pedal is pushed, this causes brake pads to be pressed onto the wheels.
- this friction causes work to be done
2) the work done between the breaks and the wheels transfer energy from the kinetic energy stores of the wheels to the thermal energy stores of the breaks.
3) a larger breaking force means a larger deceleration which can be dangerous as they may cause the breaks to overheat or the vehicle to skid
typical stopping distances
70mph:
TD- 21m
SD- 75m
50mph:
TD- 15m
SD- 38m
30mph:
TD- 9m
SD- 14m
how do drivers avoid accidents?
leaving enough space infront of them so that if they had to stop suddenly, they can do so safely
what is ‘enough space’ ?
the stopping distance for whatever speed you’re going at
how does speed affect breaking distance more than thinking distance?
1) the speed of the car and the thinking distance are directly proportional.
- because thinking time stays constant, however as the speed increases, the more distance you cover in that time.
2) however, breaking distance increases faster the more you speed up.
3) the work done to stop the car is equal to the cars Ke store.
4) so as the speed doubles, the Ke store increases by a 4-fold and so does the WD to stop the car.
what is momentum?
mass x velocity. (all are directly proportional to momentum)
describe the conservation of momentum?
in a closed system, the total momentum before an event is the same as after the event.
examples of conservation of momentum in action?
1) snooker
2) a car collision
(page 121)
what can cause changes in momentum?
forces.
- for example, when a non–zero resultant force acts on a moving object, it causes its velocity to change
how is a force acting on a moving object directly proportional to the rate of change of momentum?
1) a larger force means a faster change of momentum.
2) likewise, if someones momentum changes very quickly, the forces on the body will be very large and more likely to cause injury
what are some safety features used in cars?
1) crumple zones- crumple on impact and increases the time taken for the car to stop
2) seat belts- stretch slightly, increasing the time taken for the wearer to stop.
3) air bags- inflate before you hit the dashboard of the car. the compressing air inside it slows you down gradually.
what are some safety features used for cycling?
helmets- contain a crushable layer of foam which helps to lengthen the time it takes for your head to stop in a crash and therefore reduces the impact on your brain
`what are some safety features used in gymnastics and playgrounds?
crash mats and cushioned flooring increases the time taken for you to stop if you fall on them as they are made from soft and compressible materials
