igcse pysics momentum
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Momentum
Momentum is a vector quantity given by the following formula:
To have momentum, an object needs to have mass and to be in motion.
momentum = mass × velocity
Which of these two vehicles do you think has a higher momentum?
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Calculating momentum
Momentum is a vector quantity. The van has a momentum of 1500 × 10 = 15000 kgm/s to the right. The car’s momentum is 15000 kgm/s to the left, or –15000 kgm/s to the right:
10 m/s 10 m/s
Two vehicles, each weighing 1.5 tonnes, are driving towards each other at 30 mph. If they collide, what will happen?
Both vehicles come to a halt. What has happened to their momentum?
initial momentum to the right = 15000 + –15000 = 0
final momentum to the right = 0
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Momentum calculations
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If the resultant force acting on an object is not zero, all the forces are said to be unbalanced.
This forms the basis of Newton’s second law of motion, which states:
What is Newton’s second law?
If the forces on an object are unbalanced, two things about the object can change:
the speed of the object may change – it may either increase or decrease
the direction of motion may change.
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What is impulse?
Newton’s second law in the form F = Δ(mv) / Δt can be rearranged to give: F Δt = Δ(mv)
The quantity F Δt is the impulse of the force, so it can be seen that
impulse = change in momentum
Impulse is a vector quantity with the same direction as the force. It is measured in newton seconds (N s).
The impulse of a force is defined as: impulse = F Δt
Note that N s are the same as kg m s–1 as expected since impulse = Δ(mv), but N s are usually used for impulse.
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Impulse and collisions
The size of force controls how much damage there is.
Look at the equation for impulse. What determines how much damage is done in a collision, and how can it be reduced?
F Δt = Δ(mv)
Momentum is conserved in a collision, so impulse is constant.
To reduce the force, the time over which the collision takes place should be increased.
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Conservation of momentum
initial momentum
final momentum
=
Momentum is always conserved in any event or interaction:
When a snooker ball collides with another, what happens to the momentum of each ball?
What happens when a car brakes and comes to a halt, or when a rock falls to the ground and stops?
Momentum is transferred to the Earth.
Momentum is not created or lost, but transferred from one object to the other.
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Explosions and recoil
Remember: momentum is always conserved.
What happens to the momentum in the following situations?
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Explosions and recoil
Remember: momentum is always conserved.
What happens to the momentum in the following situations?
Explain in terms of the momentum of the fuel being expelled from the rocket.
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Explosions and recoil
Remember: momentum is always conserved.
What happens to the momentum in the following situations?
the idea of recoil: the ground does not visibly move when the basketball player jumps, but it must carry an equal and opposite momentum.
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Explosions and recoil
Remember: momentum is always conserved.
What happens to the momentum in the following situations?
Direction in momentum calculations: all parts of the fireworks have much higher speeds after they explode, but their directions cancel out and momentum is conserved.
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Investigating momentum
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Multiple-choice quiz
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