Topic 5 - Forces Flashcards
what do vector quantities have?
a magnitude and a direction
what are some examples of vector quantities?
force, velocity, displacement, acceleration, momentum, etc.
what do scalar quantities have?
only magnitude and no direction
what are some examples of scalar quantities?
speed, distance, mass, temperature, time, etc.
how are vectors usually represented?
by an arrow - the length shows magnitude and the direction shows the direction of the quantity
what is a force?
a push or a pull on an object caused by it interacting with something
what is a contact force?
when 2 objects have to be touching for a force to act. e.g. friction, air resistance, tension in ropes, normal contact force etc.
what is a non-contact force?
when an object does not need to be touching for the force to act.
e.g. magnetic force, gravitational force, electrostatic force, etc
what is an interaction pair?
a pair of forces that are equal and opposite and act on 2 interacting objects
what is an example of a non-contact force?
the sun and the earth are attracted to each other by the gravitational force
an equal but opposite force is felt by both
what is an example of a normal contact force?
a chair exerts a force on the ground, whilst the ground pushes back at the chair with the same force. equal but opposite forces are felt by both
what 2 important effects does gravity have on a planet
on the surface of a planet, it makes all things fall towards the ground
it gives everything a weight
what is mass?
the amount of matter in an object
what is weight?
the force acting on an object due to gravity.
what is weight caused by close to the earth?
the gravitational field around the earth
what determines the gravitational field strength?
the distance you are from the mass causing the field, and the size of the mass
what does weight depend on?
the strength of the gravitational field at the location of the object.
e.g. an object has the same mass whether on earth or the moon but will have a different weight
what is weight measured in?
weight is a force measured in newtons. its like the force is acting from a single point called the centre of mass
what is the centre of mass?
a point at which you assume the whole mass is concentrated
in a uniform shape this is at the centre
how is weight measured?
using a calibrated spring balance (or newton meter)
how is mass measured?
it is not a force. its measured in kilograms with a mass balance
how to calculate weight?
weight (N) = mass (kg) X gravitational field strength (N/kg)
what is the gravitational field strength usually for the earth?
9.8N/kg
what is the gravitational field strength usually for the moon?
1.6N/kg
what do free body diagrams show?
every force acting on the object or system but none of the forces the object or system exerts
what is a resultant force?
all of the overall forces acting at a single point.
what is work done?
when a force moves an object through a distance, energy is transferred
what is required for an object to move?
a force must be applied by something that has an energy source
what happens when work is done?
a force is used to move an object and energy is transferred from 1 store to another
what is the formula for work done?
W=Fs work done(J) = force(N) X distance (m)(moved along the line of action of the force)
what are Jules in Newton meters?
1J = 1Nm
how to use scale drawings to find resultant forces?
draw all the forces to scale then draw a straight line from the start of the 1st force to the end of the last. this is the resultant force the length is its magnitude and the angle between the start is its direction
when is an object in equilibrium?
when the resultant force is 0.
on a scale diagram the last force ends where the 1st one started
how can a force at an angle be drawn?
it is the same as the horizontal force and vertical force end to end at right-angles from each other
what 3 things could an object do when you apply a force to it?
it could stretch, compress or bend
a 2nd force is required or the object would just be moved
what does elastically deformed mean?
an object can go back to its original shape and length after the force has been removed. it is an elastic object
what does inelastically deformed mean?
if an object doesn’t return to its original shape and length after the force has been removed
what happens when work is done to stretch, compress or bend an object?
sometimes energy is transferred to the elastic potential energy store.
if it is elastically deformed all of the energy is transferred to this store
what is the equation for force for elastic object?
F=ke
Force(N) = spring constant (N/m) X extension (m)
what does spring constant depend on?
it depends on the material that you are stretching, a stiffer spring has a greater spring constant
how can force be worked out by compressing a string?
the extension is the difference between the natural and compressed lengths
what is the relationship between force and extension?
they are directly proportional to a point. a graph will show this as a curve at the end of a straight line where the limit of proportionality is
what can be done to test if a spring is fit for an investigation between force and extension?
add 5 weights to a spring attached to a clamp, alongside a ruler, then add weights and measure the extension if the extension is bigger each time the limit of proportionality as been reached and the spring is unfit as it won’t have enough measurements for the graph
what can be done to test if a spring is deformed elastically or inelastically using an investigation between force and extension?
a weight can be removed each time it is added and if it springs back it is elastic
how to conduct an investigation between force and extension?
measure the natural length of the spring with a millimetre ruler attached to a clamp. attach a piece of tape to the end of the spring to make it more accurate and read it at eye level.
as each mass is added measure the length of the spring. extension is change in length
repeat several times then plot a force extension graph with the results
how can a force extension graph be used to find spring constant?
when the line of best fit is a straight line there is a linear relationship, the gradient is the spring constant
how can the energy stored in a spring before its past its limit be worked out?
E=1/2ke^2
elastic potential energy (J) = 1/2 X spring constant (N/m) X extension^2 (m)
how can a force-extension graph be used to find the energy in the energy store of a stretched spring?
its the area under the graph up to that point
what is the equation for working out the size of a moment?
M=Fd
moment (Nm) = force (N) X distance (m) (perpendicular distance from pivot to line of action of force)
what is a moment?
the turning effect of a force.
what causes a larger moment?
a larger force or longer distance. e.g longer spanner.
push at right angles to the spanner
what do levers do?
they increase the distance from a pivot that the force is applied, so less force is needed for the same moment. less work is done
what are gears?
circular discs with “teeth” around their edges
what do gears do?
their teeth interlock so that turning 1 causes another to turn in the opposite direction
what are gears used for?
they are used to transmit the rotational effect of a force from 1 place to another. a force transmitted to a larger gear will cause a bigger moment as the distance to the pivot is greater but it will move slower than smaller gears
what are fluids?
substances that can flow because their particles are able to move around
how do particles exert a pressure?
as they move around they collide with surfaces and other particles. they have a small mass and exert a force on the object they collide with. pressure is force per unit area, so they exert a force
what does the pressure of a fluid mean?
a force is exerted normal (at right angles) to any surface in contact with the fluid
what is the equation for the pressure at the surface of a fluid?
p=F/A
pressure in pascals (Pa) = force normal to a surface (N) / area of that surface (m^2)
what is density?
a measure of how close together the particles in a substance are
what is density like for all liquids?
its uniform, and doesn’t vary with shape or size. the more dense a liquid, the more particles in a certain space, so more collisions and the pressure is higher. as depth increases so does pressure as the weight of the particles above that point adds to the pressure
what is the equation to calculate pressure?
p=hρg
pressure (Pa) = depth (m) (height of liquid) X density (kg/m^3) X gravitational field strength (N/kg)
what is upthrust?
a resultant upwards force caused when an object is submerged, water exerts a force on it, more force is exerted on the bottom due to pressure
what is upthrust equal to?
the weight of fluid that has been displaced by the object
what causes an object to float?
if the upthrust on an object is equal to the objects weight. if the object weighs more than the upthrust it will sink
what determines whether an object can float?
its density. if the object is less dense than the water then it displaces a volume of fluid that is equal to its weight before it is completely submerged
how do submarines use upthrust?
they fill large tanks with water to increase the weight of the submarine to sink. to rise the tanks are filled with compressed air to reduce the weight so that its less than the upthrust
what is the atmosphere?
a layer of air that surrounds earth. it is thin compared to the size of the earth
how is atmospheric pressure created?
by air molecules colliding with a surface. as altitude increases the pressure decreases
why does atmospheric pressure decrease as height increases?
the atmosphere gets less dense, so there are fewer air molecules that are able to collide with the surface. there are also fewer air molecules above a surface as the height increases so the weight of the air above it decreases with altitude
what is distance?
how far an object has moved
what is displacement?
its a vector quality and it measures the distance and direction in a straight line from an objects starting point to its finish
what is speed?
how fast you’re going with no regard for direction
what is velocity?
speed in a given direction
what is the formula to work out the speed of an object moving at a constant speed, or the average speed for an object with changing speed?
s=vt distance travelled (m) = speed (m/s) X time (s)
what is the typical speed for a person walking?
1.5 m/s
what is the typical speed for a person running?
3 m/s
what is the typical speed for a person cycling?
6 m/s
what is the typical speed for a car?
25 m/s
what is the typical speed for a train?
30 m/s
what is the typical speed for a plane?
250 m/s
what does someones speed depend on?
their fitness, age, distance travelled, the terrain they’re moving over and more
what is the speed of sound in air?
330 m/s
what can wind speed be affected by?
temperature, atmospheric pressure, and large structures near by
what is acceleration
the change in velocity in a certain amount of time
what is the formula for finding an objects average acceleration?
a=Δv/t
acceleration (m/s^2) = change in velocity (m/s) / time (s)
what is deceleration?
negative acceleration
what is constant acceleration?
uniform acceleration. e.g. acceleration due to gravity for objects in free fall (usually 9.8 m/s^2)
what is the equation for uniform acceleration?
v^2 - u^2 = 2as
final velocity^2 (m/s) - initial velocity^2 (m/s) = 2 X acceleration (m/s^2) X distance (m)
when can a journey be plotted on a distance time graph?
if an object moves in a straight line
on a distance - time graph what is the gradient?
the speed the object is moving
on a distance - time graph what are flat sections?
where the object is stationary
on a distance - time graph what is a straight positive line?
the object is travelling at a steady speed
on a distance - time graph what is a curve?
it represents acceleration or deceleration.
a steepening curve is acceleration, whereas levelling off means its slowing down
how to find speed on a distance - time graph when the object is changing speed?
find the gradient of the tangent of the curve
on a velocity-time graph what is the gradient?
the acceleration
on a velocity-time graph what are flat sections?
the object is travelling at a steady speed
on a velocity-time graph what are diagonal lines?
acceleration or deceleration. uphill is getting faster, downhill is getting slower
on a velocity-time graph what is a curve?
changing acceleration
on a velocity-time graph what is the area under the graph?
the distance travelled in that time period selected
when is friction created?
between 2 surfaces in contact or when an object passes through a fluid. it always acts in the opposite direction to movement
what is drag?
the resistance you get in a fluid e.g. air resistance
how is drag reduced?
by making the shape of the object streamlined so that fluid can flow easily across it
how does friction increase?
with speed. the faster an object is going, the more friction is working against it and the harder it has to work
how is terminal velocity reached when objects fall through a fluid?
when it first sets off the force of gravity is much higher than the frictional force so it accelerates, as speed increases friction builds up, this gradually reduce acceleration until it is equal to the accelerating force
what is the accelerating force acting on all falling objects?
gravity
what causes objects to fall at different speeds?
air resistance
what is the terminal velocity of a falling object determined by?
its drag in comparison to its weight. the frictional force depends on its shape and area
what is Newtons 1st law?
if the resultant force on a stationary object is zero, the object will remain stationary. if the resultant force on a moving object is zero, it’ll carry on moving at the same velocity
what can a non-zero resultant force mean?
starting, stopping, speeding up, slowing down or changing direction
what is the relationship between force and acceleration?
they are directly proportional. the larger the resultant force acting on an object the more the object accelerates
what is the relationship between acceleration and mass?
acceleration is inversely proportional to mass
what is newton’s 2nd law?
F=ma resultant force (N) = mass (kg) X acceleration (m/s^2)
what is inertia?
the tendency that objects have to continue in the same state of motion
what is an objects inertial mass?
how difficult it is to change the velocity of an object
how to find out inertial mass?
m=F/a
its the ratio of force over acceleration
what is newtons 3rd law?
when 2 objects interact, the forces they exert on each other are equal and opposite
what are some examples of newton’s 3rd law?
if skater pushes another one she will feel an equal force them and they will both accelerate in opposite directions.
if a man pushes against a wall normal contact force will act back on him with equal force
how to test newtons 2nd law?
put a trolley with s pieces of card on it on a track with string attached to weights going through a pulley attached.
if the lengths of each piece of card are entered into the light gate connected to a data logger then it can work out the velocity of each as it passes through.
the difference in these 2 speeds can then be used to calculate the acceleration of the trolley.
weights on the other side of the pulley will pull the trolley forward.
repeat to get an average acceleration
how can newtons 2nd law be tested on the effects of masses?
masses can be added to the trolley 1 at a time to increase the mass of the system
how can newtons 2nd law be tested on the effects of force?
keep the mass of the system the same but change the mass on the hook on the other side of the pulley. do this by starting with the masses on the trolley then transfer the masses to the hook 1 at a time.
what should the results to test for newtons 2nd law show?
by adding masses to the trolley, the mass of the system increases, the force applied is the same, so the acceleration should decrease
by increasing accelerating force, without changing the mass of the system, the acceleration should increase
what is the equation to work out stopping distance?
stopping distance = thinking distance + braking distance
what is an emergency stop?
the maximum force applied to the brakes in order to stop the car in the shortest possible distance
what is thinking distance?
how far the car travels during the driver’s reaction time (time between seeing hazard an applying brakes)
what is braking distance?
the distance taken to stop under the braking force
what are the typical car braking distances?
14m at 30mph
55m at 60mph
75m at 70mph
what is thinking distance affected by?
your speed
your reaction time
how does speed affect thinking distance?
the faster you’re going the further you’ll travel before reacting
how does reaction time affect thinking distance?
the longer the reaction time, the longer your thinking distance
what is braking distance affected by?
your speed
weather
condition of tyres
condition of brakes
how does speed affect braking distance?
the faster a vehicle, the longer it takes to stop
how does weather affect braking distance?
wet, icy, oily or a road covered in leaves will have less grip which can cause tyres to skid
how does the condition of your tyres affect braking distance?
if tyres have no tread left they can’t get rid of water in wet conditions and so they skid
how does the condition of your brakes affect braking distance?
if they are worn or faulty, they can’t apply as much force as well-maintained brakes
why are speed limits really important?
speed affects stopping distance a lot
how do brakes work?
when the brake pedal is pushed, brake pads are pressed onto the wheels, causing friction and work done. work done transfers energy from the kinetic energy stores of the wheels to the thermal energy stores of the brakes
what is typical reaction time?
between 0.2 and 0.9s
what factors affect reaction time?
tiredness
drugs
alcohol
distractions
how can reaction time be measured using a computer?
using a computer-based test, like clicking a mouse in response to a stimulus on the screen
how can reaction time be measured using a ruler?
1 person rests their arm on the desk, another person holds a ruler between their thumb and finger at 0cm.
they have to drop it without warning and the person has to catch it as fast possible
the longer the distance that it took the person to catch the ruler in the longer the reaction time
how can you make sure the ruler reaction time test is fair?
use the same ruler, have the same person dropping it. repeat several times
how can the danger of a long stopping distance be reduced?
leaving enough space between cars
what is the relationship between speed and stopping distance?
speed and thinking distance are directly proportional
braking distance increases faster and faster with speed
what is the formula for momentum?
p=mv
momentum (kg m/s) = mass (kg) X velocity (m/s)
what is the conservation of momentum?
in a closed system the total momentum before an event is the same as after the event
what is the equation that links force and the rate of change of momentum?
F=mΔv/Δt
force (N) = change in momentum (kg m/s) / change in time (s)
what are some safety features that cars have?
crumple zones - crumple on impact, increase time taken for the car to stop
seat belts - stretchy, increase time taken for wearer to stop
air bags - inflate before you hit the dashboards, compressed air slows you down slower than if you hit the dashboard
what are some safety features that bike helmets have?
they have a crushable layer of foam which helps lengthen the time taken for your head to stop in a crash. this reduces the impact on your brain
what are some safety features that crash mats have?
increase the time taken for you to stop if you fall on them, because they are made from soft, compressible materials
why are things like cars designed to make you take longer to stop in emergencies?
force causing a change = rate of change in momentum
larger force - faster change in momentum
if change in momentum is slowed down there will be a smaller force and less chance of injury
