## Impulse, Force and Momentum

Posted in AQA GCSE P2, P2: Momentum by Mr A on 7 Feb 2010

• Force and momentum
• Impulse = change in momentum
• Force and impulse
• Examples

Force and Momentum

Physicists often say “momentum is conserved”. However, this is not always the case. It is only true of there is no external force acting.

If there is an external force acting on an object, then the momentum of that object is not conserved, it is changing.

Impulse = Change in Momentum

The word impulse means the change in momentum. In Physics, we use the greek symbol Δ (“Delta”) to mean “change in”, so

Δp = “change in momentum” = impulse

Force and Impulse

The external force acting on an object is related to the impulse given to the object by:

$F = \frac{\Delta p}{t}$

where

• F = force (Newtons, N)
• Δp = impulse = change in momentum (kgm/s or Ns)
• t = time (seconds, s)

Worked Examples

1. A 2000kg car is travelling at 60mph (27m/s) crashes into a wall. The impact lasts 0.02s. What force does the wall exert on the car?
2. What force does a passenger experience if his mass is 70kg?
3. Suppose the car has a much better crumple zone, and the passenger wears his seatbelt, such that the impact time is reduced to 0.6s. What force would he experience then?

## Momentum and Collisions

Posted in AQA GCSE P2, P2: Momentum by Mr A on 2 Feb 2010

• Conservation of momentum
• Collisions
• Explosions

Conservation of momentum

The conservation of energy is a very important thing in Physics. However, energy is not the only thing that is conserved.

Unless a force is acting, momentum is also conserved. This means that the total momentum before something happens will be equal to the total momentum after that thing happens.

Collisions: example

A bus and a car are each travelling at 40mph, but in opposite directions. When they collide the car remains in contact with the bus, and they both continue at 15mph in the direction the bus was travelling. If the car’s mass is 2000kg, what is the mass of the bus?

Explosions: example

A cannon, initially stationary, fires a 15kg cannonball at 200m/s. If the cannon itself has a mass of 1500kg, with what speed does it recoil?

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## Momentum

Posted in AQA GCSE P2, P2: Momentum by Mr A on 31 Jan 2010
• What is momentum?
• Applying formula
• Examples

Momentum is a measure of how hard it is to stop something: if an object has a lot of momentum, it will be hard to stop it; if its momentum is low, it will be easy.

How much momentum do the following things have? “Loads”, or “not much”?

Momentum Equation

$p = mv$

• p = momentum, $kg ms^{-1}$ or $Ns$
• m = mass, $kg$
• v = velocity, $ms^{-1}$

Worked Examples

1. What is the momentum of a 70kg person running at 5m/s?
2. How fast is that same person going if they have a momentum of 210Ns?

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