# Edexcel A2 Physics 4

Unit 4 for Edexcel Physics A2

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## Momentum

Momentum is a vector quantity, measured in Kgms-1.

Momentum = mass x velocity

p = mv

Newtons second law of motion - 'The acceleration of an object is directly proportional to the net force and inversely proportional to its mass'. This law can be written as F=ma, this can be changed to include momentum.

F=ma --> a = v - u / t

F= m (v - u / t) -------> mv - mu / t --------> change in momentum / time

F = change in momentum / change in time

F = delta (p) / delta (t)

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## Impulse

The product of a force applied for a certain time is known as Impulse.

Impulse = change in momentum = Ft

Impulse = force x time

Ft = mv

Impulse in measured in Kgms-1 or Ns.

To stop something moving we need to remove all of its momentum. This idea allows us to calculate the impulse needed to stop a moving object.

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## Collisions

Elastic Collisions - All momentum is conserved, Kinetic energyis conserved, relative speed of approach = relative speed of separation

Example of a perfectly elastic collision - a collision caused by non-contact forces, such as alpha particles being scattered by a nucleus.

Inelastic Collisions - All momentum is conserved, Kinetic energy is not conserved some is lost (usually as sound and heat), speed of separation has to be calculated.

In a perfectly inelastic collision the relative speed of separation is zero (objects stick together after collision and treated as one object)

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## Force - Time Graph

We can plot graphs of the force during a collision against time.

We can find the impulse, or change in the momentum, by calculating the area under the force-time graph.

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## Conservation of Linear Momentum

The principle of conservation of linear momentum can be used to predict the motion of objects after a collision.

The Principle of the Conservation of Momentum states: if objects collide, the total momentum before the collision is the same as the total momentum after the collision. (with the condition that no external forces act on the objects).

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