# P4 - Explaining Motion - Physics

Hey people, here are some revision cards about P4 of the GCSE physics course. I hope they help you out and explain what you need to know.

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• Created by: Kathryn.C
• Created on: 08-11-09 12:35

## Force and Counter-force

Force - is the influence that produces a change in a physical quantity e.g. "force equals mass times acceleration." Counter-force - the reaction force that counteracts the influence that produces a change in a physical quantity.

When a force is applied to an object that object will move - it will gain momentum - but when the force is applied a counter-force is produced which, depending on whether or not the object moves, is either equal to, less than or more than the applied force.

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## Examples of Force and Counter-force

+5 force applied right object remains still +5 counter-force left

_____________________________________________________________________

The force and counter-force are equal meaning that the object remains stationary as a result.

+10 force applied right object moves right +5 counter-force left _____________________________________________________________________

The force is stronger than the counter-force meaning that the object moves towards the right.

+5 force applied right object moves left +10 counter-force left _____________________________________________________________________

The counter-force is stronger than the applied force meaning that the object moves towards the left.

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## Motion Graphs and Momentum

A motion graph shows the velocity of an object - or the momentum of an object.

Momentum (mass in motion) of a moving object depends on two things;

1. The mass of the object - the bigger the mass, the bigger the momentum.

2. The velocity of the object - the faster the object is moving the more momentum it has.

The momentum of an object can be calculated using this equation;

momentum (kg m/s) = mass (kg) x velocity/ speed (m/s)

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## Friction

Friction - is the resistance that is encountered when an object is moved in contact with another.

Example - A work man pushing a box.

1. Jeff (our workman) pushes the box with a force of 25N (newtons) to try to slide it along. It does not move. The friction force exerted on the box is 25N. This exactly balances Jeff's push.

25N right (force applied by Jeff on box) & 25N left (friction of box on ground)

Counter-act each other. Box does not move.

2. Jeff then pushes harder, with a force of 50N. The box still does not move. The friction force exerted by the floor on the box is now 50N. Again, this balances Jeff's push.

50N right (force applied by Jeff on box) & 50N left (friction of box on ground)

Counter-act each other. Box does not move.

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## Continued example of Friction

3. Jeff pushes harder still, exerting a force of 70N. The box starts to move. It keeps speeding up while Jeff pushes. 70N is bigger than the maximum possible friction force for this box and floor surface.

70N right (force applied by Jeff on box) & friction at its maximum left (less than 70N)

Force applied by Jeff moves the box right.

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## Speed and Velocity

Speed: the distance travelled in a specific time. Is found by dividing distance travelled by time taken.

Velocity: speed of an object in a particular direction.

The Speed Equation

distance travelled (metres)

time taken (seconds)

Instantaneous speed is the speed at any given time - at a certain point and uses the same equation as average speed does.

distance travelled (metres)

time taken (seconds)

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## Resultant Force

What is a resultant force?

A resultant force is the force that is produced when two forces interact.

Example,

16N left & 7N right = resultant force of -9N left. An object would move left.

When applied to a parachutist;

1. Jump out of plan - sudden acceleration (downwards force is greater than air resistance)

2. Downwards force and air resistance equalise - parachutist has a steady speed.

3. Parachutist pulls chord for parachute - rapid deceleration (air resistance is now greater than downwards force)

4. Air resistance and downwards force equalise - but slower than before and parachutist has a steady speed.

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