Physics - Explaining Motion

Additional Science OCR 21st Century CGP Book - P4 Notes

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- To find the Speed of an object you must remember this formula:

Speed = Distance Divided By Time

- The Speed Formula tells you what the average speed is.

- For example: A cat skulks 20 metres in 40 seconds. Find: a) Its speed b) How long it will take to skulk 75m

- Answer: Using the doemula trangle

a) s=d/t = 20/40 = 0.5m/s b) t=d/t = 75/0.5 = 150s = 2min 30 seconds

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Speed Continued:

- In real life its very rare for an object to go exactly the same speed for a long period of time

- It is more useful to know the average speed and that is why the skulking cat averaged at 0.57m/s over its journey

- Instantaneous Speed:

- Is still an average, just an average over a really short time period

- Speed cameras use lines painted on the road to measure the distance travelled by a vehicle in a set time

- The police want to know how fast you are going right now, not your average

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Distance - Time Graphs:


Flat sections are where it stops

The steeper the graph, the faster it is going

"Downhill" sections mean it is coming back towards its starting point

Curves represent acceleration & deceleration

A steeping curve means it is speeding up (increasing gradient), a levelling off curve means it is slowing down (decreasing gradient)

DIstances can be referred too as positive or negative - All this means is that the object is going in the opposite direction

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Velocity - Time Graphs:

Gradient = Acceleration

Flat sections represent a steady speed

The steeper the graph, the greater the accelration or deceleration

Uphill sections (/) are acceleration

Downhill sections (\) are deceleration

The area under any section of the graph (or all of it) is equal to the distance travelled in that time interval

A curve means changing acceleration

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The speed of an object is just how fast its going - the direction isn't important e.g. speed = 30mph

Velocity is sometimes a more useful measure of motion, because it describes both the speed & direction e.g. velocity = 30mph due north

A quantity like speed, that has only a size is called a scalar quantity; mass, temperature, time & length etc

A quantity like velocity, that has a direction as well, is a vector quantity; force acceleration, momentum etc

Velocity can be possitive or negative

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Just like negative directions, you'll also come across negative velocities

If you're car is travelling at 20 m/s but then you turn around & go the opposite way, the speed may still be the same but the velocity's changed from 20 m/s to -20 m/s

If two objects are travelling in opposite directions, you can say that one has a positive velocity & the other has a negative velocity


1) Two hamsters strap themselves to a rocket, one goes north, other goes south

2) Hamster 1 has a velocity of 200m/s due north whereas Hamster 2 has a velocity of 230 m/s due south

3) Another way of saying this: Hamster 1 is +200m/s & Hamster 2 is -230 m/s

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Forces & Friction:

Forces occur when two objects interact

When an object exerts a force on another object, it always experiences a force in return - these 2 forces are called an "interaction pair"

When you push against a wall, the wall will push back at you just as hard -  As soon as you stop pushing, so does the wall - There must be an opposing force otherwise if there wasn't then you (and the wall) would fall over

An object resting on a surface experiences a "reaction force"

If you put a book on a table, the weight of the book acts downwards on the table - and the table exerts an equal & opposite force upwards on the book

This force is called areaction force because it is the tables "response" to the weight of the book

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Forces & Friction:

Moving objects normally experience friction


When an objects is moving relative to another oe, both objects experience a force in the direction that opposes the movement

There are 3 types of friction:

a) Friction between solid surfaces which are gripping e.g. the earth's crust - two big section of rock sliding past each other  

b) Friction between solid surfaces which are sliding past each other but friction is making them stay put - unless the force gets too strong and they shift suddenly

c) Resistance or Drag from fluids, liquids or gases e.g. air e.g. molecules of air going over a streamlined sports car or an object passing through molecules of fluid

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Forces & Motion:

Arrows show direction of forces

The length of the arrow shows the size of the force

The direction of the arrow shows the direction of the force

If the forces are on opposite sides and they are both the same size, they are balanced

If an object's resting on a surface, its weight is pushing down on it because of gravity

This causes an equal reaction force from the surface pushing up on the object

If an object is moving at a steady speed, the forces must be in balance

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Forces & Motion:

Just because something is moving doesn't mean that there's an overall force acting on it - unless it's changed speed or direction, the overall force is zero

The resultant force is the overall force acting on an object - the force you get when you take in all the individual forces together & their directions

It is the force that decides the motion of the object - whether it will accelerate, decelerate or stay at a steady speed

"Accelerate" means change velocity

If there is a resultant force acting on an object, its speed or direction (or both) changes

Unless there's an overall force acting on an object, it won't accelerate

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Forces & Motion Continued:

If a car's engine exerts a bigger force (forwards) than the drag force (backwards), the car will accelerate

If the thrust arrow, going in the direction that the car is going, is bigger than the drag arrow which is going in the opposite direction, there is a resultant force in the forward direction

The bigger the resultant force, the greater the acceleration

Momentum (kg m/s) = Mass (kg) x Velocity (m/s)

Momentum is mainly about  how hard it is to stop an object moving

The heavier an object is, the faster its moving, the harder it is to stop

The greater the mass of an object and the greater it's velocity, the more momentum the object has

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Momentum is a vector quantity - it has a size and direction (like velocity but not speed)

Example: A 65kg kangaroo is moving in a straight line at 10 m/s. What is the momentum?

Answer: Momentum = Mass x Velocity

                                 = 65 x 10

                                 = 650 kg m/s

A resultant force of zero means that a stationary object will stay still

If the object was moving, it would stay at the same speed in the same direction

If the resultant force of an object was not zero, its momentum changes in the direction of the force

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Change In Momentum Depends on the Force:

When a resultant force acts on an object, it causes a change in momentum - the change it causes depends on the size of the force & the time it acts for

Change of Momentum (kg m/s)  = Resultant Force (N) x Time for which the force acts (s)

Example: A rock with a mass of 1kg is travelling through space at 15 m/s. A comment hits the rock, giving a resultant force of 2500 N for 0.7 seconds. Calculate the rock's initial momentum, then calculate the change in its momentum resulting from the impact with the comet.

Answer: Momentum = Mass x Velocity = 1 x 15 = 15 kg m/s                                           Change of Momentum = 2500 x 0.7 = 1750 kg m/s

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Car Safety Features Reduce Forces:

If your momentum changes slowly, like a nice controlled braking, the forces acting on your body are small and you're unlikely to get hurt.

The greater the time, the smaller the force

In collision you can't really effect the change in momentum - the car's mass & velocity stay the same - however, the average force on an object can be lowered by slowing the object down over a longer time

Safety features in a car increase collision time to reduce the forces on the passengers:

Crumple Zones: Crumple on impact, increasing the time taken for the car to stop

Airbags: Slow you down more gradually

Seat belts: Stretch slightly, increasing the time taken for the wearer to stop - reduces forces acting on the chest

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When a force moves an object, energy is transferred & work is done

When an object moves, it needs some effort to move it e.g. fuel, food or electricity - by doing this it transefrs the energy it recieves (as fuel) into other forms

When this energy is transferred "usefully" e.g. by lifting a load or is "wasted" e.g. lost as heat, you can still say that "work is done"

"Work Done" & "Energy Transferred" are "one and the same"

If energy is transferred, then the object doing the work loses energy:

Change in energy (J) = Work Done (J)

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Work Continued:

Example: If a man is sweeping, doing work on the rubbish, he loses ernergy - if he does 500 J of work, then he loses 500 J of energy - The rubbish is having work done so it gains energy (though not the full 500 J - some will be lost as noice & heat)

Work Done (J) = Force (N) x Distance (m)

This formula only works if the force is in exactly the same direction as the movement

To find out how much work has been done (in joules), you multiply the force in neutons by the distance moved in metres

Example: Some hooligan kids drag an old tractor tyre 5 m over flat ground - they pull with a total force of 340 N - find the work done

Answer: W = F x d = 340 x 5 = 1700 J

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Kinetic Energy:

Kinetic energy is the energy of movement

The kinetic energy of something depends both on its mass & speed - The greater its mass & the faster its going, the bigger its kinetic energy

Kinetic Energy (KE) = 1/2 mass x velocity squared

Example: A car of mass 2450 kg is travelling at 38 m/s - Calculate its kinetic energy

Answer: KE = 1/2mv squared = 1/2 x 2450 x 38 squared = 1768900 J (jouels becauses its energy)

To increase something's kinetic energy, you need to increase its speed - to increase speed you have to apply a force on it - if you are applying a force, you're doing work

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Kinetic Energy Continued:

Kinetic Energy is just the movement of energy, so if you do work on an object, it doesn't accelerate, then you have increased its KE

Increase in KE = Work Done

Energy is always conserved - you cannot destroy or create energy - it gets transformed from one kind of energy to another e.g. a light bulb transforms electrical energy into light and heat energy

If energy is conserved, then you'd expect the increase in an object's kinetic energy to be equal to the amount of work that's be done to it

The problem is, some of the energy gets "wasted" as heat because of friction & air resistance

The increase in an object's KE is normally a bit less than the amount of work done on it

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Gravitational Potential Energy:

G.P.E is "Height Energy"

G.P.E is the energy stored in an object when you raise it to a height against a force of gravity

You have to lift something to increase its gravitational potential energy & that energy is only released when the object falls (movement again)

Change in G.P.E (J) = Weight (N) x Change in Height (m)

Example: A 4000 N cow walks onto a geyser and it propelled 10 m upwards - Calculate its change in G.P.E

Answer: Weight x Change in height = 4000 x 10 = 40,000 J (or 40 kj)

In reality, energy will be lost due to friction, air resistance & even sound

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Converting G.P.E into K.E

When something falls, its G.P.E is converted into K.E - so the further it goes, the faster it goes

Some of the G.P.E will be dissipated as heat due to air resistance

Kinetic Energy gained = Potential Energy lost

if you ignore friction & air resistance (between the tracks & the wheels on a roller coaster), the amount of K.E it gains will be the same as the amount of G.P.E it loses

Roller coasters are constantly transferring between potential energy & kinetic energy

End of P4!

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Alexandra Miles


I never realised how much information you have to remember for p4 - explaining motion! Time to revise a lot I think!

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