WJEC GCSE Physics 3 Motion and Collision

Speed, Velocity and Motion:

Speed is simply how fast you’re going- distance/ time. This is usually measured in ms^-1 in physics

Velocity is the speed of an object, in a given direction. Therefore it includes speed, as above, as well as a direction. If the object is going forwards (usually to the right) then the velocity will be +ve, however, if it is going backwards (to the left), then the velocity will be –ve.

Motion is just another word for movement

SUVAT Equations:

s/x – distance                    m/ km/ miles

u- initial velocity               ms^-1/ kms^-1

v- final velocity                  ms^-1/ kms^-1

a- acceleration                  ms^-2/ kms^-2

t- time                                  s

Momentum:

This is the quantity of motion of a moving body- the speed and strength at which it’s moving. Momentum can be either positive or negative, and cannot be created (from nothing) or destroyed, only transferred from one object to another- during collision. It is measured in kgms^-1 as it takes into account both mass and velocity.

Momentum can be calculated with the following formula:

Momentum (p) = Mass (m) x Velocity (v)

If an object (such as a car) has a higher momentum during a collision, then the impact will be harder.

Change in momentum is related to the force and the time for which it acts; therefore, in a car crash, the force is drawn out over a longer period of time, as there are safety features, such as air bags and crumple zones, which increase the time of collision, thus decreasing the momentum.

The law of Conservation of Momentum-

Momentum before = Momentum after

This means that no momentum is used, created or destroyed, during a collision or explosion, only passed from one object to the next. Therefore, if an object hits a wall and stops, it will lose all of its momentum, this momentum will have been passed to the wall, the wall cannot move, and will therefore pass the momentum to whatever it can, e.g. the Earth.

To work out the total momentum before, you must work out the momentum (mass x velocity) of car/ object A and add it to the total momentum of object B. To work out the momentum afterwards, you either, work out A and add it to B, or work out their combined momentum (depending on whether it is an elastic or and inelastic collision.

Before an explosion, the total momentum equals 0, as nothing is moving. After the explosion, the total momentum will still equal 0. This is because for every particle moving in one direction, there must be an equal particle moving in the opposite direction.

Elastic Collision-

In an elastic collision, not only the momentum is conserved, but also the Kinetic Energy. The total kinetic energy of the two objects before the collision is equal to the total kinetic energy of the two objects after the collision. To work out whether a collision is elastic, you must work out the kinetic energy for both cars before the collision, and after the collision. If they are equal, the collision can be regarded as elastic. Work out kinetic energy with the following formula:

Kinetic energy = ½ x mass x (velocity) ²     /          K.E. = ½ mv²

Change in momentum-

This is related to the force and time according to this equation;

Force(Newtons) = Change in Momentum/ Time

Circular Motion:

Velocity is a vector quantity, therefore it takes into account both force (speed) and direction. Since it is velocity which is used to calculate acceleration, acceleration is also a vector. An object can, therefore, accelerate without changing its speed.

For example, a Ferris wheel spins at a constant speed, but the direction of each of the cars changes, therefore so does the velocity, therefore so does the acceleration.

The resultant force will balance out and acct towards the centre of the circle.