1. GCSE PHYSICS: MOMENTUM
By the end of this presentation you should be able to…
State what momentum is.
Recall and use momentum = mass x velocity.
Recall the law of conservation of momentum

2. What is momentum?
“Momentum” is a very useful term that was devised to help us to control our environment and to predict how collisions will turn out.
If you think of something being thrown at a pane of glass, the outcome will depend on what is being thrown (its mass) and on how fast it is travelling (its velocity).
Very simply then: Momentum = mass x velocity
The unit of momentum does not have a name after a particular scientist, so can be remembered from the way in which it is calculated. It is : kg m/s

3. EXAMPLE
Think of a bowling ball (2kg) moving towards a stationary penguin (1kg) at a high speed.
Now think of a tennis ball (60g) kicked very gently towards the same penguin.
The effect the different amounts of momentum (contained by each object) has on the penguin is immediately obvious!

4. Worked Examples: 1
What is the momentum of a trolley that has a mass of 3kg if its moving at a constant speed of 2.5m/s?
Momentum = mass x velocity = 3 x 2.5= 7.5 kg m/s
Consider a trolley that has a mass of 4kg, moving at a constant speed of 2m/s towards a second trolley of identical mass. If this trolley is moving in the same direction at 1m/s, what is the total momentum of this system?
Total Momentum = (4 x 2) + (4 x 1) = 8 + 4= 12 kg m/s

5. The importance of DIRECTION
Would the outcome of a “head-on” collision between two cars be different to a collision involving one car being hit from behind by another faster car?

6. The importance of DIRECTION
Would the outcome of a “head-on” collision between two cars be different to a collision involving one car being hit from behind by another faster car?

7. The importance of DIRECTION
Would the outcome of a “head-on” collision between two cars be different to a collision involving one car being hit from behind by another faster car?
Yes, it would be very different. So in calculating the total momentum of a system with more than one moving object….
Velocities to the right are positive, and velocities to the left are negative.

8. The importance of DIRECTION
The two trolleys involved in a “head-on” collision have masses of 3kg each. The trolley moving to the right has a speed of 4m/s and the other has a speed of 2m/s. What is the total momentum of the system before they collide? (Consider no losses of energy due to friction or air resistance.)
Total momentum = [3 x(+4)] + [3 x (-2)] = 12 – 6 = 6kg m/s

9. The law of conservation of momentum
The ability to apply this situation in everyday situations in sport ,travel, construction and many other areas allows for increased effectiveness and safety in general.
The total momentum in a given direction is conserved in any collision, provided no external forces act on the system.
For mathematical applications at GCSE:
Total momentum before = Total momentum after
the collision the collision

10. EXPLOSIONS
Does this law hold when a bullet is fired from a gun? Or when a rocket is fired from a stationary position?
Once fired from the chamber, the 300g bullet can get up to 100m/s! What is its momentum?
The total momentum before the explosion must be zero, since neither gun nor bullet is moving.
0.3 x 100 = 30kg m/s

11. EXPLOSIONS The law of conservation of momentum is ALWAYS obeyed!
If the momentum of the bullet after being fired = 30kgm/s, then the momentum gained by the GUN must also be 30kgm/s, but acts in the opposite direction!
Determine the velocity of RECOIL of the gun if its mass is 5kg.
Momentum before explosion = momentum after explosion
0 = (0.3 x 100) + (5 x -v) , where –v is the velocity of the gun
0 = 30 – 5v, so 5v = 30 and v = 30/5 = 6m/s