Momentum is a quantity that is conserved. This means that the total amount is constant throughout an interaction, such as a collision, provided that there are no external forces acting.
Since velocity is a vector quantity, so too is momentum, and so momenum in the reverse direction will be negative.
Key Concepts
By applying Newton's three laws to a collision between two balls, we can show that the total momentum for isolated bodies is constant. Momentum is conserved.
Identical bodies
An elastic collision means that total kinetic energy is constant. Recall that kinetic energy is a scalar (since the velocity term is squared) and so energies are added together no matter what the direction.
When two identical balls collide elastically they swap velocity, so if one was stationary before the other will be stationary after.
Non-identical bodies
The momentum before a collision always equals the momentum after, provided the bodies are isolated.
Use quizzes to practise application of theory.
START QUIZ!
Two balls collide as shown
Which of the following statements is true
The momentum of each ball is not conserved as they are not isolated
Consider the following collision.
If the mass of the red ball is 1 kg and the grey is 4 kg calculate the following values
Change of momentum of the red ball Ns
Change of momentum of the grey ball Ns
Change in momentum of the two balls Ns
Change in momentum = mass x change in velocity
A footballer kicks a ball. Which of the following statements is true.
The footballer and ball interact with the ground.
There following diagram represents the before position and 4 possible after positions for a collision
Which are possible
C implies zero momentum
D implies negative momentum
The diagram shows two possible outcomes for a collision between 2 balls of equal mass
The velocity of the balls in each case is:
A: Red ball Blue ball
B: Red ball Blue ball
1 ball hits 4 identical balls as shown
The velocity of the 4 balls after the collision = ms-1
momentum before = 5m
momentum after = 4mv -m = 5m
4v = 6
2 boxes hit 4 identical boxes as shown
The velocity after the collision is ms-1
2mx3 =6mv
v=6/6
Some blocks are pushed apart by a massless spring
The velocity of the right hand group = ms-1 (2 sf)
Initial momentum = 0
Final momentum = 10mv -16m = 0
v = 16/10
How much of Conservation of momentum have you understood?
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