# Physics - Explaining Motion (P4)

Revision cards for P4 (21st Century OCR)

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• Created by: Sabiha
• Created on: 31-10-12 21:26

## Calculating speed

The speed of something tells us how far it will travel in a certain time,

The velocity of something is its speed in a certain direction.

• Speed (m/s) = distance (m)   time (s)
• Velocity (m/s) = displacement (m)   time (s)

If you measure average speed overy a very year short time interval you get very close to a value for instantaneous speed.

Displacement: the distance for an object from its starting point in a straight line.

Displacement and velocity are both vector quantities because they use size (magnitude) and direction.

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## Graphs - Distance-time & Displacement

Distance-time graphs are used to visualise a journey.

• The time for the journey is plotted on the x-axis.
• The distance travelled for the journey is plotted on the y-axis.
• A straight line means that the vehicle is travelling at a constant speed.
• A horizontal line means that the vehicle is stationary (speed is 0).
• A curved line means that the speed is changing.
• If the curved line is getting steeper, it means the speed is increasing.
• The gradient of the line is the speed.
• The steeper the line is, the faster the speed.

Displacement graphs are used to visualise return journeys.

When the vehicle returns to its starting point, the displacement is zero.

• The displacement is negative if the vehicle stops behind its starting point.
• There is displacement if the vehicle moves past its starting point.
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## Accleration

Acceleration is the rate at which the speed increases. It is measured in m/s

Acceleration (m/s ) = change in speed (m/s)   time taken (s)

For example:

• If a car speeds up from rest to 25m/s in 5 seconds, the acceleration is:

(25 - 0)   5 = 5m/s

When something slows down it has a negative acceleration (deceleration).

There has to be a net force acting on an object for it to accelerate or decelerate.

When the net force or overall force is zero, the acceleration is also zero.

A vehicle travelling at a constant speed around a corner is changing its velocity.

So you use this equation to work out acceleration when using velocity:

Acceleration (m/s ) = change in velocity (m/s)   time taken (s)

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## Graphs - Speed-time & Velocity-time

Speed-time graphs are used to show the changes in speed during a journey.

• Speed is plotted on the y-axis.
• Time is plotted on the x-axis.
• Horizontal line means speed of jobject is constant/steady.
• If horizontal line is along the x-axis, speed = zero (stationary).

A straight line going up shows acceleration.

A straight line going down shows deceleration.

STEEPER THE LINE, THE GREATER THE SIZE OF ACCELERATION.

Instantaneous velocity = the instantaneous speed of a vehicle in a certain direction.

Velocity-time graphs are used to show direction in which an object is travelling.

• Positive velocity = object is travelling in a certain direction.
• Negative velocity = object is travelling in the opposite direction.

Gradient of velocity-time graph = acceleration.

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## Forces

Force = push/pull which acts between 2 objects. Forces act in pairs.

Forces are vector quantities because they have a magnitude (size) & a direction.

Repulsive force = pushes objects apart.

Attractive force = pulls objects towards eachother.

Reaction force = reaction on the surface.

The reaction force increases when you add more force on one object than the other. For example, if you're jumping upwards, you need to push harder on the floor - the reaction force increases. (Forces are unbalanced).

Force is represented by an arrow on diagrams.

On a scale diagram, length of arrow represents magnitude (size) and direction of the arrow shows which way the force acts.

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## Friction

Friction: force that acts between 2 surfaces.

Size of friction depends on:

• Roughness of surfaces (the rougher, the more friction there is).
• How hard the surfaces are pushed together (heavier objects = more friction).

When you try to push an object along a surface, friction will be equal so the object will not move.

Increase applied force = friction increases too.

Limiting friction: when the friction reaches a maximum and the object starts to move.

You need friction to walk. The resultant force (overall force) acting between your foot & the floor when walking is a combination of friction and the reaction of the surace.

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## Adding the effects of forces

Resultant force = overall/total force acting on an object.

When adding forces, make sure you add the size and direction of the force.

When the resultant force is zero, the forces are balanced and the acceleration is zero.

In a space with no friction, once the object starts moving it should keep moving at the same speed.

When the forces are unbalanced, there is a net force on the object and it will speed up, slow down or change direction.

Example:

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## Terminal Velocity

Terminal velocity: the constant maximum speed reached by a falling object.

EXAMPLE:

If an object was falling and the drag became equal to gravity, there's no acceleration so the object carries on dropping at terminal velocity.

Air resistance/drag: an upward force the atmosphere creates that slows down falling objects.

Drag acts in the opposite direction to the speed/velocity of the object.

Drag increases as the speed of the object increases.

Larger surface area of the object = larger the drag.

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## Momentum & Collisions

When 2 objects collide, large forces are exerted. The size of the force depends on:

• The mass of the object (heaver mass = larger force).
• The speed/velocity of the object (faster object = larger force)
• The duration of the collision (longer time to stop = lower force)
• Momentum (kg m/s) = mass (kg) x velocity (m/s)

Resultant force changes object's momentum. (larger force exerted = larger change in momentum).

• Change of momentum (kg m/s) = resultant force (N) x duration of impact (s)
• Force (N) = change in momentum (kg m/s)   time taken (s)

Force is equal to the rate of the change of momentum. For example:

A car of mass 1200kg craches at the speed of 20m/s. The collision stops the car in a time of 1.5seconds. SO:

Change in momentum = (1200 x 20) - 0 = 24000kg m/s

Force = 24000   1.5 = 16000N

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## Work

Work done: the energy used by the movement of a force,

Energy: the ability to do work.

Energy and work done are both measured in joules (J).

Work done (J) = Force (N) x distance moved in direction of the force (m)

When work is done on an object, the energy is transferred to it.

When work is done by an object, the energy is transferred from it to something else.

Amount of energy transferred (J) = Work done (J)

• All forms of energy have the potential to do work.
• Not all energy is transferred as work - some is always dissipated as heat. (wasted)
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## Potential Energy

When you lift an object, you do work against gravity.

1 joule of work will lift 1 Newton with a distance of 1 metre.

The work is transferred to GPE of the object. (gravitational potential energy).

• Object raised = GPE increases.
• Object falls = GPE decreases.

Change in GPE (J) = weight (N) x vertical height difference (m)

FOR EXAMPLE:

If the weight is 700N and you climb a tress with a height of 3m:

Gain in GPE = weight x height = 700N x 3m = 2100J

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## Kinetic Energy

When you push an object for it to move (increase its velocity), you do work. The work is transferred to the moving object as kinetic energy (KE).

Greater mass of object & faster its speed = greater kinetic energy.

Kinetic energy (J) =   x mass (kg) x [velocity]   ([m/s] )

• For example, car with mass of 800kg travelling at 12m/s:

KE =   x mass x velocity =   x 800 x 12 = 400 x 144 = 57600J

The work done by applied force = same as change in KE of object

• For example, driving force of car is 8kN & it moves a distance of 7.2m:

Work done = force x distance moved = 8000 x 7.2 = 57600J (change in KE)

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## Energy transfers

As roller coaster travels round its track, it goes up and down. So energy changes from KE to GPE. Total energy = GPE + KE.

Conservation energy: when there are no resistive forces and the total energy remains constant.

ENERGY CANNOT BE CREATED OR DESTROYED. IT CAN ONLY TRANSFER BETWEEN OBJECTS OR CHANGE ITS FORM.

When an object falls, its potential energy is transferred to kinetic energy.

Loss in GPE = gain in KE.

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