# Forces and motion

This goes over the forces and motion from the double award exam.

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## Speed and velocity

Distance time graphs: If it is a straight line then the object is stationary and if it is a straight diagonal then it is going at a constant speed.

Speed describes the time taken for an object to cover a certain distance.

Velocity time graphs: If it is a straight line then it is going at a constant speed and if it is a straight diagonal line then it is accelerating.

Equation: speed (m/s) = distance (m)/time (s)

EXAM HINTS: Need to know the units, check the labels to see if it is a distance/time or velocity/time graph. The area under the line is the distance travelled.

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## Acceleration

Acceleration is speed is the change in velocity over time.

Equation: acceleration = final velocity – initial velocity/ time taken

(m/s2)

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## Forces

Newton’s laws of motion:

1.       Balanced forces mean no change in velocity

2.       A resultant force means acceleration

3.       Inertia – things like doing what they are already doing

Equation: Force (N) = mass (kg) x acceleration (m/s2)

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## Terminal velocity

When an object falls the two forces that affect it are gravity and air resistance.

NO RESULTANT FORCE EQUALS TO TERMINAL VELOCITY

At first the gravity force is greater than the air resistance so the object accelerates in a downwards direction.

As the object accelerates downwards (at 10 m/s2). Its velocity increases and as the velocity increases the air resistance force increases in the opposite direction.

Eventually the air resistance force and the force of gravity balances each other out and so there is zero resultant force on the object.

The object has now reached its TERMINAL VELOCITY.

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## Stopping distance

Equation: stopping distance = thinking distance + braking distance

Thinking distance is the reaction time covered from when you see the need to brake to when you start to push the pedal.

Thinking distance is increased by: alcohol, tiredness, drugs and distractions.

Thinking distance is decreased by: concentrating and caffeine.

Braking distance is increased by: poor tyre grip, ice, rain, badly maintained brakes and poor road surface.

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## Weight and mass

Weight is a force due to gravity.

Mass is how much stuff (matter) is something made of.

The more mass = the more inertia.

MASS stays the same wherever you are.

WEIGHT changes due to the force of gravity.

Equation: weight (N) =mass (kg) x gravitational field strength (10 kg m/s2)

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## Kinetic energy

The kinetic energy of an object is affected by mass and velocity.

Equation: kinetic energy (J) = ½ mass (kg) x velocity2

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