Paper 2 - P5 - Forces Flashcards
Vector quantity
> Vector quantities have magnitude and direction.
E.g. force, velocity, displacement, acceleration, momentum.
They can be represented by an arrow:
-The length of the arrow shows the magnitude and the direction of the arrow shows the direction of the quantity.
Scalar quantity
> Scalar quantities only have magnitude and NO direction.
>E.g. speed, distance, mass, temperature, time.
How does velocity (vector) follow on from speed (scalar)?
> Velocity is a vector, but speed is a scalar quantity.
Both bikes are travelling at the same speed but they have different velocities because they are travelling in different directions.
What two groups can forces be put into?
> All forces are either contact and non-contact.
Force - defintion
> A force is a push or a pull on an object that is caused by it interacting with something.
Contact forces
> When two objects have to be touching for a force to act, that force is called a contact force.
E.g. friction, air resistance, tension in ropes, normal contact force.
Non-contact forces
> If the objects do not need to be touching for the force to act, the force is non-contact.
E.g. magnetic force, gravitational force, electrostatic force.
What happens when forces interact?
> When two objects interact, there is a force produced on both objects.
An interaction pair.
This is basically Newton’s 3rd law.
E.g. the Sun and the Earth are attracted to each other by the gravitational force. This is a non-contact force. An equal but opposite force of attraction is felt by both the sun and the Earth.
E.g. a chair exerts a force on the ground, whilst the ground pushes back at the chair with the same force (the normal contact force). Equal but opposite forces are felt by both the chair and the ground.
Interaction pair - definition
> A pair of forces that are equal and opposite and act on two interacting objects.
Gravitational force
> Gravitational force is the force of attraction between masses.
Gravity attracts all masses, but you only notice it when one of the masses is really really big, e.g. a planet.
Anything near a planet or star is attracted to it very strongly.
It has 2 important effects.
The two important effects of gravity.
1) On the surface of a planet, it makes all things fall towards the ground.
2) It gives everything weight.
Mass - definition
> Mass is just the amount of ‘stuff’ in an object. For any given object this will have the same value anywhere in the universe.
Weight - definition
> Weight is the force acting on an object due to gravity (the pull of the gravitational force on the object).
Close to the Earth, this force is caused by the gravitational field strength around the Earth.
Gravitational field strength - info
> GFS varies with location.
>It’s stronger the closer you are to the mass causing the field, and stronger for larger masses.
Weight - info
> The weight of an object depends on the strength of the gravitational field at the location of the object. This means the weight of an object changes with its location.
E.g. an object has the same mass whether it’s on the Earth or Moon, but it’s weight will be less on the Moon as the GFS is weaker (1.6N/kg).
Weight is a force measured in newtons.
You can think of the force as acting from a single point on the object, called its centre of mass. For a uniform object this will be at the centre of the object.
Weight is measured using a calibrated spring balance (or newtonmeter).
Mass - info
> Mass is not a force.
>It’s measured in kilograms with a mass balance.
Relationship between mass and weight
> Mass and weight are directly proportional.
>Increasing the mass of a subject increases its weight, if you double the mass, you double the weight etc.
EQUATION - WEIGHT
Weight (N) = Mass (kg) x Gravitational Field Strength (N/kg).
What type of diagram show all the forces acting on an object?
> A free body diagram
Free Body Diagrams
> The size of the arrows show the relative magnitudes of the forces and the directions show the directions of the forces acting on the object.
What is the overall force on a point or object?
> Resultant force
Resultant Force
> The overall force on a point or an object.
If you have a number of forces acting at a single point, you can replace the single force (the resultant force).
If the forces all act along the same line, the overall effect is found by adding those going in the same direction and subtracting any going in the opposite direction.
Consider the horizontal and vertical directions separately. State the size and direction of the resultant force, e.g. 200N to the left.
Resultant Force Theory
> If a resultant force moves an object, work is done.
When a force moves an object through a distance, energy is transferred and work is done on the object.
1)To make something move, a force must be applied.
2) The thing applying the force must have 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) Work done and energy transferred are the same thing.
EQUATION - WORK DONE
> Work Done (J) = Force (N) x Distance (moved along the line of action of the force) (m).
> 1 Joule = 1Nm
Work done - explained example
> When you push something along a rough surface (like a carpet) you are doing work against frictional forces.
Energy is being transferred to the kinetic energy store of the object because it starts moving but some is being transferred to the thermal energy stores due to the friction.
This causes the overall temperature of the object to increase.
What do you use to find resultant forces?
> A scale drawing
Steps to a ‘scale drawing’
- Draw all the forces acting on an object, to scale, ‘tip-to-tail’.
2.Then draw a straight line from the start of the first force to the end of the last force - this is the resultant force. - Measure the length of the resultant force on the diagram to find the magnitude and angle to the direction of the force. Give as bearing.
>Basically pythag theorem.
Other than finding the resultant force what else can scale drawings be used for?
> You can use them to check to see if forces are balanced.
When is an object at equilibrium?
.If all forces on it are balanced.
Equilibrium and scale drdawings
> If all forces acting on an object combine to give a resultant force of zero, the object is in equilibrium.
On a scale drawing, this means that the tip of the last force you draw should end where the tail of the first force you drew begins. E.g. for 3 forces, the scale diagram will form a triangle.
Might be told forces acting on an object, and told to find a missing force, given that the object is in equilibrium. To do this, draw out the forces you do know (to scale and tip-to-tail), join the end of the last force to the start of the first force.
This line is the missing force so measure its size and direction.
Applying forces to a spring
> When you apply a force to an object you may cause it to stretch, compress or bend.
To do this, you need more than one force acting on an object - otherwise the object would simply move in the direction of the applied force instead of changing shape.
Work is done when a force stretches or compresses an object and causes energy to be transferred to the elastic potential energy store of the object.
If it i elastically deformed all this energy is transferred to the object’s elastic potential energy store.
Elastic deformation
> An object has been elastically deformed if it can go back to its original shape and length after the force has been removed.
Objects that can be elastically deformed are called elastic objects (e.g. a spring).
Inelastic deformation
> An object has been inelastically deformed if it doesn’t return to its original shape and length after the force has been removed.
The equation used with springs
> F (N) = k (N/m) x e (m)
Force = spring constant x extension
> The equation also works for compression.
What is extension directly proportional to?
> Force.
Spring constant
> N/m
>The spring constant depends on the material that you are stretching - a stiffer spring has a greater spring constant.
Limit of proportionality
> There is a maximum force above which the graph curves, showing that extension is no longer proportional to force.
The idea that there’s a limit to the amount of force you can apply to an object for the extension to keep increasing proportionally.
Investigating Springs Practical - equipment
>Clamp >Fixed ruler >Spring >Tape (to mark end of spring) >Hanging mass >Extra masses >Weighted stand
Investigating Springs Practical - pilot experiment
> Before starting you should do a quick pilot experiment to check your masses are an appropriate size for your investigation:
-Using an identical spring to the one you will be testing, load it with masses one at a time up to 5. Measure extension each time.
Work out increase in extension in the extension of spring with each mass, if its extension is bigger than previous mass then its passed the limit of proportionality and use smaller masses.
Investigating Springs Practical - checking whether deformation is elastic or inelastic?
> To check whether the deformation is elastic or inelastic, you can remove each mass temporarily and check to see if the spring goes back to the previous extension.
Investigating Springs Practical - method
- Measure the natural length of the spring with a millimetre ruler clamped to the stand. Make sure you take the reading at eye level and and add a marker to the bottom of the spring to make the reading more accurate.
- Add a mass to the spring and allow the spring to come to rest. Record the mass and measure the new length of the spring. The extension is the the change in length.
- Repeat process until you have enough measurements. (no fewer than 6).
Investigating Springs Practical - plotting results
> Once you’ve collected results, you can plot a force-extension graph of your results.
It will only start to curve once it has passed its’ limit of proportionality
Straight line = linear relationship between force and extension.
Bend + non-linear relationship, the spring stretches more for each unit increase in force.
Equation for work-done for linear relationship (springs)
> E(J) = 1/2 x k (N/m) x e^2 (m)
>Elastic potential energy = half x spring constant x extension squared.
What does the are under a force-extension graph show?
> The energy in the elastic potential energy store of a stretched spring.
Moment - defintion
> The turning effect of a force.
Moment equation
> M(Nm) = F(N) x d(m)
Moment of a force = force x distance
Distance is the perpendicular distance from the pivot to the line of action of the force.
Moments - key info
> If the total anticlockwise moment equals the total clockwise moment about a pivot, the object is balanced and won’t turn.
To get maximum moment you need to push at perpendicular to the spanner.
A larger force or longer distance would mean a larger moment.
Levers
> Levers make it easier for us to do work.
Levers increase the distance from the pivot at which the force is applied.
Since M=Fd this means less force is needed to get the same moment.
This means levers make it easier to do work, e.g. lift a load or turn a nut.
Examples of a simple levers
> Long sticks or bars
>Wheelbarrows.
Gears
> Gears transmit rotational effects.
Gears are circular discs with ‘teeth’ around their edges.
Their teeth interlock so that turning one causes another to turn in the opposite direction.
They are used to transmit the rotational effect of a force from one place to another.
Different sized gears can be used to change the moment of the force. A force transmitted to a larger gear will cause a bigger moment as the distance to the pivot is greater.
The larger gear will turn slower than the small gear.
Pressure definiton
> The force per unit area.
Fluid - definition
> A liquid or gas.
>Substances that can ‘flow’ because their particles are able to move around.
Fluid pressure
> As a fluids particles move around, they collide with the collide with surfaces and other particles.
Particles are light, but they still have a mass and exert a force on the object they collide with. Pressure is force per unit area, so this means the particles exert a pressure.
The pressure of a fluid means a force is exerted normal to any surfaces in contact with the fluid.
Equation to calculate the pressure at the surface of a fluid
> Pressure (Pa/) = force (N) x Area (m^2)
Pressure in pascals.
Force normal to a surface.
Area of that surface.
What does the pressure in a liquid depend on?
> Depth
>Density
Pressure of a liquid - density
> For a given liquid, the density is uniform (the same everywhere) and it doesn’t vary with shape or size.
The density of a gas can vary though.
The more dense a given liquid is, the more particles it has in a certain space. This means there are more particles that are able to collide so the pressure is higher.
Density - defintion
> Density is a measure of the ‘compactness’ of a substance, i.e. how close together the particles together the particles in a substance are.
Pressure of a liquid - depth
> As the depth of a liquid increases, the number of particles above that point increases.
The weight of these particles adds to the pressure felt at that point, so liquid pressure increases with depth.
You can calculate pressure at certain depth - pressure = height x gfs x density.
Equation for calculating the pressure of a liquid at a certain depth.
> Pressure (Pa) = height (m) x GFS (N/kg) x Density (kg/m^3)
Pressure in pascals.
Density of the liquid.
Height of the column of liquid (the depth).
Gravitational field strength. On earth = 9.8N/kg.
What does the force upthrust determine?
> Upthrust is a force that determines whether an object will sink or float.
Objects in a fluid and upthrust
> Objects in fluids experience upthrust.
When an object is submerged in a fluid, the pressure of the fluid exerts a force on it from every direction.
Pressure increases with depth, so the force exerted on the bottom of the object is larger than the force acting on the top of the object.
This causes a resultant force upwards, known as upthrust.
What is upthrust equal to?
> Upthrust is equal to the weight of fluid that has been displaced by the object.
E.g. the upthrust on a pineapple in water is equal to the weight of a pineapple-shaped volume of water.
When does an object float?
> If the upthrust on an object is equal to the object’s weight, then the forces balance and the object floats.
When does an object sink?
> An object sinks if its’ weight is more than the object.
Floating, sinking and density
> Whether an object will float or not depends on its density.
An object that is less dense than the fluid it is placed in weighs less than the equivalent volume of fluid. This means it displaces a volume of fluid that is equal to its weight before it is completely submerged.
At this point, the object’s weight is equal to the upthrust so the object floats.
An object that is denser than the fluid it is placed in is unable to displace enough fluid to equal its weight. This means that its weight is always larger than the upthrust, so it sinks.
Submarines and upthrust
> Submarines make use of upthrust.
To sink, large tanks are filled with water to increase the weight of the submarine so that it is more than the upthrust.
To rise to the surface the tanks are filled with compressed air to reduce the weight so that it’s less than the upthrust.
Atmosphere - definition
> A layer of air that surrounds Earth. It is thin compared to the size of the Earth.
Atmospheric pressure
> The pressure exerted by the air around you.
It is created on a surface by air molecules colliding with the surface.
As the altitude (height above the Earth) increases, atmospheric pressure decreases.
Why does atmospheric pressure decrease with height?
> Because as the altitude 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. This means that the weight of the air above it, which contributes to atmospheric pressure, decreases with altitude.
Distance
> How far an object has moved.
>It’s a scalar quantity so doesn’t involve direction.
Displacement
> It’s a vector quantity.
It measures the distance and direction in a straight line from an object’s starting and finishing point - e.g. the plane flew 5 metres north.
The direction could be a relative point, e.g. towards the school, or a bearing (a 3-digit angle from north, e.g. 035 degrees).
Distance vs displacement
> Distance is scalar, displacement is a vector.
>If you walk 5m north then 5m south your displacement is 0m but the distance is 10m.
Speed vs velocity
> Speed and velocity both measure how fast you are going but speed is scalar and velocity is a vector.
Speed is just how fast you’re going with no regard to the direction.
Velocity is speed in a given direction, e.g. 30mph north or 20m/s, 060 degrees.
This means you can have objects travelling at a constant speed with a changing velocity.
This happens when the object is changing direction whilst staying at the same speed.
How to measure the speed of an object that’s moving with a constant speed
> Time how long it takes the object to travel a certain distance, e.g. using a ruler and a stopwatch.
Then calculate using:
Distance (m) = speed (m/s) x time (s)
Typical everyday speeds - a person walking
1.5m/s
Typical everyday speeds - a person running
3m/s
Typical everyday speeds - a person cycling
6m/s
Typical everyday speeds - a car
25m/s
Typical everyday speeds - a train
55m/s
Typical everyday speeds - a plane
250m/s
Speed of sound
330m/s in air
Why does speed of sound change?
> The speed of sound changes depending on what the sound waves are travelling through.
Why does speed of wind change?
> Wind speed can be affected by things like temperature, atmospheric pressure and if there are any large buildings or structures nearby (e.g. forests reduce the speed of the air travelling through them).
Acceleration - definition
> The change in velocity in a certain amount of time.
>How quickly you’re speeding up.
Acceleration equation
> Acceleration (m/s^2) = change in velocity (m/s) divided by time (s).
Decelleration
> Just negative acceleration.
>If something slows down, the change in velocity is negative.
What is constant acceleration also known as?
> Uniform acceleration.
Uniform acceleration.
> Constant acceleration.
Acceleration due to gravity is uniform for objects in free fall.
It’s roughly 9.8m/s^2 near the Earth’s surface and has the same value as GFS.
Uniform acceleration - definition
> Final velocity (m/s) squared - initial velocity (m/s) squared = 2 x acceleration (m/s^2) x distance (m).
Initial velocity - definition
> The starting velocity of an object.
When do you use a distance-time graph
> If an object moves in a straight line.
Time (s) on x-axis.
Distance (m) on y-axis.
Distance-time graph: gradient
> The gradient tells you the speed.
>Steeper, the faster.
Distance-time graph: flat sections
> Flat sections are where the object’s stationary.
Distance-time graph: straight-up hill sections
> Straight up-hill sections mean that it’s travelling at a steady speed.
Distance-time graph: curves
> Curves represent acceleration or deceleration.
Distance-time graph: steepening curve
> An objects speeding up
Distance-time graph: levelling off curve
> Slowing down.
When do you use a velocity time graph?
> To show how an object’s velocity changes as it travels.
Velocity-time graph: gradient
> The gradient is the acceleration.
Velocity-time graph: flat sections
> Flat sections represent travelling at a steady speed.
Velocity-time graph: steeper graph
> The steeper the graph, the greater the acceleration or deceleration.
Velocity-time graph: uphill secctions
> Uphill sections are acceleration. Downhill sections are deceleration.
Velocity-time graph: curve
> Changing acceleration.
Velocity-time graph: area under graph
> The area under the graph is the distance.
Friction
> If an object has no force propelling it along it will always slow down and stop because of friction (unless you’re in space where there’s nothing to rub against).
Friction always acts in the opposite direction to movement.
To travel at a steady speed the driving force needs to balance the frictional forces.
You get friction between to surfaces in contact, or when an object passes through a fluid (drag).
Drag
> Drag is the resistance you get in a fluid (a gas or liquid).
Air resistance is a type of drag - it’s the frictional force produced by the air acting on a moving object.
The most important factor by far in reducing drag is keeping the shape of the object streamlined. This where the object is designed to allow fluid to flow easily across it, reducing drag.
Parachutes work in the opposite way - they want as much drag as they can get.
Speed’s influence on forces
> Frictional forces from fluids (drag) increase with speed.
A car has much more friction to work against when travelling at 70mph compared to 30mph.
So at 70mph the engine has to work much harder just to maintain a steady speed.
Terminal velocity
> Objects falling through fluids reach a terminal velocity.
- When falling objects first set off, the force of gravity is much more than the frictional force slowing them down so they accelerate.
- As the speed increases, the friction builds up.
- This gradually reduces the acceleration until eventually the frictional force is equal to the accelerating force (so the resultant force is 0).
- It will have reached its maximum speed or terminal velocity and will fall at a steady speed.
Terminal velocity - definition
> The constant speed that a freely falling object eventually reaches when the resistance of the medium through which it is falling prevents further acceleration.
Factors affecting terminal velocity
> Terminal velocity depends on shape and area.
Explanation of why shape and area affect terminal velocity
> The accelerating force acting on all falling objects is gravity and it would make them all fall at the same rate, if it wasn’t for air resistance.
This means that on the Moon, where there’s no air, hammers and feathers dropped simultaneously will hit the ground together.
However, on the Earth, air resistance causes things to fall at different speeds, and the terminal velocity of any object is determined by its drag in comparison to its weight.
The frictional force depends on its shape and area.
Terminal velocity example - parachutist
> The human skydiver, without his parachute open has quite a small area and a force of ‘W=mg’ pulling him down.
He reaches a terminal velocity of about 120mph.
But with his parachute open, there’s much more air resistance and still only the same force ‘W=mg’ pulling him down.
This means his terminal velocity comes right down to about 15mph, which is a safe speed to hit the ground at.
Newton’s First Law - key
> A resultant force is needed to make something start moving, speed up or slow down.
If the resultant force on a stationary object is zero, it’ll just carry on moving at the same velocity.
Newton’s First Law - info
> If something is moving at a constant velocity, the resistive and driving forces on it must a;; be balanced.
The velocity will only change if there’s a non-zero resultant force acting on the object.
1. A non-zero resultant force will always produce acceleration/deceleration in the direction of the force.
2. This ‘acceleration’ can take 5 different forms: starting, stopping, speeding up, slowing down, changing direction.
On a free body diagram the arrows will be equal.
Newton’s 2nd law - key
> The force and acceleration are directly proportional.
>Acceleration is proportional to the resultant force.
Newton’s 2nd law - info
> The larger the resultant force acting on an object, the more the object accelerates.
Acceleration is also inversely proportional to the mass of the object - so an object with a larger mass will accelerate less than one with a smaller mass (for a fixed resultant force).
Resultant Force (N) = mass (kg) x acceleration (m/s^2).
Inertia - defintion
> it is the tendency to continue in the same state of motion.
>Tendency for motion to remain unchanged.
Inertia
> Until acted upon by a resultant force, objects at rest stay at rest and objects moving at a steady speed will stay moving at that speed.
This tendency to continue at the same state of motion is called inertia.
An object’s inertial mass measures how difficult it is to change the velocity of an object.
Inertial mass can be found using Newton’s 2nd law of F=ma meaning m=F divided by a so inertial mass is just the ratio of force over acceleration.
Newton’s 3rd Law - key
> When two objects interact, the forces they exert on each other are equal and opposite.
Newton’s 3rd law - info
> If you push something, say a shopping trolley, the trolley will push back against you, just as hard.
And as soon as you stop pushing so does the trolley.
hard thing to get head around = if the forces are always equal, how does anything ever go anywhere?
Important thing to remember is that the 2 forces are acting on different objects.
Newton’s 3rd law - ice skating example
> If skater a pushes on skater b, she feels an equal and opposite force from skater b’s hand (the normal contact force).
Both skaters feel the same sized force in opposite directions, and so accelerate away from each other.
Skater a will be accelerated more than skater b if she has a smaller mass.
Newton’s 3rd law - man pushing wall
> An example of law in an equilibrium situation is a man pushing against a wall.
As the man pushes the wall, there is a normal contact force acting back on him.
These 2 forces are the same size.
As the man applies a force and pushes the wall, the wall pushes back on him with an equal force.
Investigating motion
> You can investigate how mass and force affect acceleration.
>Tests Newton’s 2nd law, F=ma.
Investigating motion - steps
- Set up trolley, so it holds a piece of card with gap in the middle that will interrupt the signal on the light gate twice. If you measure the length of each bit of card that will pass through the light gate and input this into the software, the light gate can measure the velocity for each bit of card. It can use this to work out the acceleration of the trolley.
- Connect the trolley to a piece of string that goes over a pulley and is connected on the other side to a hook (that you know mass of and can add more mass to).
- The weight of the hook and masses attached to it will provide the accelerating force, equal to the mass of the hook (m) x acceleration due to gravity (g).
- The weight of the hook and masses accelerates both the trolley and the masses, so you are investigating the acceleration of the system (masses and trolley together).
- Mark a starting line on the table the trolley is on, so that the trolley always travels the same distance to the light gate.
- Place the trolley on the starting line and hold it in place. You should let the hook and any masses on the hook hangs so the string is taut. Then , release the trolley.
- Record the acceleration measured by the light gate as the trolley passes through it. This is the acceleration of the whole system.
- Repeat this twice more to get a average acceleration.
Investigating motion - varying mass
> To investigate the effect of mass, add masses to the trolley, one at a time, to increase the mass of the system.
Don’t add masses to the hook, or you’ll change the force.
Record the average acceleration for each mass.
Investigating motion - varying force
> To investigate the effect of force, you need to keep the total mass of the system the same, but change the mass on the hook.
To do this, start with all the masses loaded onto the trolley and transfer the masses to the hook one at a time, to increase the accelerating force.
The mass of the system stays the same as you’re only transferring masses from one part of the system to the other.
Record the average acceleration for each force.
Investigating motion - explanation
> Newton’s 2nd Law can be written as F=ma. Here F=weight of the hanging masses, m+mass of the whole system and a = acceleration of the system.
By adding masses to the trolley, the mass of the whole system increases, but the force applied to the system stays the same. This should lead to a decrease in the acceleration of the trolley, as a=F divided by m.
By transferring masses to the hook you’re increasing the accelerating force without changing the mass of the whole system. So increasing the force should lead to an increase in the acceleration of the trolley.
Stopping distance
> Stopping distance= thinking distance+braking distance.
>The longer it takes to perform an emergency stop, the higher the risk of crashing.
Thinking distance
> How far the car travels during the driver’s reaction time (time between driver seeing hazard and applying the brakes.)
Braking distance
> The distance taken to stop under the brakes force (once brakes are applied).
Factors affecting thinking distance
> Your speed - the faster you’re going the further you’ll travel during the time you take to react.
Your reaction time - the longer your reaction time, the longer your thinking distance.This can be affected by tiredness, drugs or alcohol. Distractions can affect your ability to react.
Factors affecting braking distance
> Your speed - for a given braking force, the faster a vehicle travels, the longer it takes to stop.
The weather or road surface - if it is icy or wet, or there are leaves or oil on the road, there’s less grip and so less friction between a vehicle’s tyres and the road, which can cause tyres to skid.
The condition of your tyres - if the tyres of a vehicle are bald then they cannot get rid of water in wet conditions. This leads to them skidding on top of the water.
How good your brakes are - if brakes are worn or faulty, they won’t be able to apply as much force as well-maintained brakes, which could be dangerous when you need to brake hard.
Why are speed limits important?
Describing the factors affecting stopping distance and how this affects safety.
> The longer your stopping distance, the more space you need to leave in front in order to stop safely.
Icy conditions - skid - so driving too close to other cars in icy conditions is unsafe.
Investigating reaction times - steps
> Ruler drop test.
- ruler hanging between thumb and forefinger.
- finger in line with 0.
- ruler is dropped without warning.
- ruler caught between thumb and finger.
- see distance fallen.
Investigating reaction times - calculations
> You can calculate how long the ruler falls for because acceleration due to gravity is constant (9.8m/s^2).
v^2 - u^2 = 2as.
To find reaction time: a = change in v divided by t.
Investigating reaction times - faults
> Hard to do accurately so should do lots of repeats.
Better if ruler falls straight down - you might want to add a blob of modelling clay to bottom to stop it waving about.
Fair test - same ruler, same person dropping it.
Find mean reaction times.
Investigating reaction times - factors
> Could investigate factors affecting reaction time.
>Distractions and without distraction.
What dies braking rely on?
> Braking relies on friction between the brakes and wheels.
When brake pedal is pushed, it causes brake pads to be pressed onto the wheels. This contact causes friction, which causes work to be done.
The work done between the brakes and the wheels transfers energy from the kinetic energy stores of the wheels to the thermal energy stores of the brakes. The brakes increase in temp.
The faster a vehicle is going, the more energy it has in its kinetic energy store, so the more work needs to be done to stop it. This means that a greater braking force is needed to make it stop within a certain distance.
A larger braking force means a larger deceleration. very large decelerations can be dangerous because they may cause brakes to overheat or could cause the vehicle to skid.
You can estimate braking force required to stop using v^2 - u^2 = 2as.
Speed affects braking distance more than thinking distance.
> As car speeds up, the thinking distance increases at the same rate as speed. The graph is linear.
Braking distance however, increases faster the more you speed up. The work done to stop the car is equal to the energy in the car’s kinetic energy store (1/2mv^2). So as speed doubles, the kinetic energy store increases 4-fold, and so the work done to stop the car increases 4-fold. Since W=Fs and the braking force is constant, the braking distance increases 4-fold.
Momentum
> How much ‘oomph’ and object has.
It’s a property all moving objects have.
p=mv.
momentum (kgm/s) = mass (kg) x velocity (m/s).
Momentum is a vector quantity - has size and direction.
Rule of momentume
> In a closed system, the total momentum before an event is the same as after the event. This is called the conservation of momentum.
Momentum before= momentum after.
E.g. a moving car hits into the back of a parked car. The crash causes the two cars to lock together, and they continue moving in the direction that the original moving car was travelling, but at lower velocity.
What can you use the conservation of momentum to calculate?
> Velocities or masses.
>One direction will be positive and the other negative.
Changes in momentum
> Forces cause a change in momentum.
Force (N) = change in momentum (kgm/s) divided by change in time (s).
Larger force means faster change in momentum.
The longer it takes for a change in momentum, the smaller the rate of change of momentum, and so the smaller the force. Smaller forces mean the injuries are likely to be less severe.
Car safety features
> Crumple zones crumple on impact, increasing the time takes for the car to stop.
Seat belts stretch slightly, increasing the time taken for the wearer to stop.
Air bags inflate before you hit the dashboard of a car. The compressing air inside slows you down more gadually than if you had just hit the hard dashboard.
Safety features
> Helmets, e.g. bike helmets contain a crushable layer of foam which helps to lengthen the time taken for your head to stop in crash. This reduces impact on your brain.
Crash mats and cushioned playground flooring increase the time taken for you to stop if you fall on them. This is because they are made from soft, compressible materials.
