Calculating momentum, conservation of momentum in collisions and explosions, and why a longer impact time reduces the force. Higher tier content.
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1.What is momentum?
A property of a moving object, equal to its mass multiplied by its velocity.
2.State the equation for momentum.
momentum = mass x velocity
3.What is the unit of momentum?
Kilogram metres per second.
4.Is momentum a scalar or a vector?
A vector, because velocity has direction.
5.A 2 kg ball moves at 5 m/s. Calculate its momentum.
2 x 5 = 10 kg m/s.
6.State the principle of conservation of momentum.
In a closed system, the total momentum before an event equals the total momentum after it.
7.What is a closed system in this context?
One where no external forces act, so nothing adds or removes momentum.
8.What is the total momentum before an explosion of a stationary object?
Zero, because nothing is moving.
9.What must the total momentum be after that explosion?
Also zero, so the pieces move in opposite directions with equal and opposite momenta.
10.Why does a gun recoil?
Momentum is conserved, so as the bullet gains forward momentum the gun gains equal backward momentum.
11.Why does the gun move much more slowly than the bullet?
The gun has a far greater mass, so the same magnitude of momentum gives it a much smaller velocity.
12.Two trolleys collide and stick together. What is conserved?
The total momentum, so the combined mass moves with a velocity that keeps the total the same.
13.A 4 kg trolley at 3 m/s hits a stationary 2 kg trolley and they stick. Find the final velocity.
Momentum before = 12 kg m/s. Total mass after = 6 kg. Velocity = 12 divided by 6 = 2 m/s.
14.Why do you have to be careful with direction in momentum problems?
Momentum is a vector, so motion in opposite directions must be given opposite signs.
15.State the equation for force in terms of change in momentum.
force = change in momentum divided by time taken
16.What does that equation tell you about impact time?
For the same change in momentum, a longer impact time means a smaller force.
17.How does an airbag reduce injury?
It increases the time over which the momentum of the occupant changes, which reduces the force on them.
18.How does a crumple zone work?
It deforms on impact, extending the time taken for the car to stop and so reducing the force.
19.How does a cushioned running shoe reduce impact?
It increases the time for your foot to come to rest, lowering the peak force on your joints.
20.Why do gymnasts land on thick mats?
The mat increases the time taken to stop, which reduces the force on their body.
21.Why does a longer stopping time reduce force but not momentum change?
The momentum change is fixed by the starting and finishing velocities. Only the rate at which it happens changes.
22.Give the general safety principle from momentum.
Anything that increases the time of an impact reduces the force experienced.
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