Momentum Flashcards

All 27 cards in this deck

What is the equation for momentum?

$p = m\,v$ (momentum = mass × velocity)

What is the unit of momentum?

Kilogram metre per second, kg m/s

What are the steps to calculate the momentum of a moving object?

e.g. a 4 kg ball moving at 5 m/s

1. Use $p = m\,v$
2. Substitute: $p = 4 \times 5$
3. $p = 20$ kg m/s

What are the steps to find the mass of an object from its momentum and velocity?

e.g. momentum $24\,000$ kg m/s at $15$ m/s

1. Start from $p = m\,v$
2. Rearrange: $m = p \div v$
3. $m = 24\,000 \div 15 = 1600$ kg

True or false? A stationary object has zero momentum, whatever its mass.

True. $p = m\,v$, and $v = 0$ gives $p = 0$.

True or false? Momentum has a direction as well as a size.

True. It is a vector, so opposite directions are given opposite signs.

What is meant by 'momentum is conserved'?

Total momentum before an event = total momentum after the event (it stays the same).

What is meant by a 'closed system' in momentum questions?

A system on which the resultant external force is zero, so total momentum before = total momentum after.

True or false? Momentum is only conserved when the colliding objects stick together.

False. In a closed system momentum is conserved whether the objects stick together or bounce apart.

True or false? In a closed system, momentum is conserved even though kinetic energy may not be.

True. Total momentum stays the same; kinetic energy can be transferred to other stores.

What are the steps to find the velocity of two objects that move off together after a collision?

e.g. a 2 kg trolley at 3 m/s hits a stationary 4 kg trolley

1. Total momentum before: $2 \times 3 + 0 = 6$ kg m/s
2. Momentum after = 6 kg m/s, combined mass $= 2 + 4 = 6$ kg
3. $v = 6 \div 6 = 1$ m/s

What are the steps to find the velocity of a piece after an explosion or push-apart?

e.g. a stationary pair of trolleys spring apart; the 2 kg trolley moves right at 1 m/s, the other has mass 1 kg

1. Total momentum before $= 0$
2. Momentum after $= 0$: $(2 \times 1) + (1 \times v) = 0$
3. $v = -2$ m/s, i.e. 2 m/s to the left

True or false? When two objects moving in opposite directions collide, you add their momenta as two positive numbers.

False. One momentum must be taken as negative, so the total is the difference.

True or false? When two objects collide and move off together, you divide the total momentum by the combined mass to get their velocity.

True. The two objects now move as one object of mass $m_1 + m_2$.

What is the equation linking force, change in momentum and time?

$F = \dfrac{m\,\Delta v}{\Delta t}$

What does the equation $F = \dfrac{m\,\Delta v}{\Delta t}$ mean in words?

Force equals the rate of change of momentum.

What are the steps to calculate the change in momentum of an object?

e.g. a 1200 kg car slows from 20 m/s to 5 m/s

1. Find the change in velocity: $\Delta v = 20 - 5 = 15$ m/s
2. Use $\Delta p = m\,\Delta v$
3. $\Delta p = 1200 \times 15 = 18\,000$ kg m/s

What are the steps to calculate the average force during a collision from the change in momentum?

e.g. change in momentum 700 kg m/s in 0.28 s

1. Use $F = \dfrac{m\,\Delta v}{\Delta t}$
2. Substitute: $F = 700 \div 0.28$
3. $F = 2500$ N

In $F = \dfrac{m\,\Delta v}{\Delta t}$, what quantity does $\dfrac{\Delta v}{\Delta t}$ represent?

Acceleration — so the equation is equivalent to $F = m\,a$.

True or false? For the same change in momentum, a longer collision time means a smaller average force.

True. Force is the rate of change of momentum, so a bigger $\Delta t$ gives a smaller $F$.

True or false? For the same change in velocity in the same time, a heavier object needs a bigger force.

True. Its change in momentum $m\,\Delta v$ is larger, so the rate of change of momentum is larger.

What are the three steps in explaining how a safety feature reduces injury?

e.g. a cycle helmet in a crash

1. It increases the time taken to stop (the contact time)
2. So the rate of change of momentum decreases
3. So the force on the person is reduced

What effect do air bags and seat belts have on the time taken for a passenger's momentum to fall to zero?

They increase it, so the rate of change of momentum, and therefore the force, is smaller.

Why are playground surfaces and gymnasium crash mats cushioned?

They increase the time taken to stop a falling person, reducing the rate of change of momentum and so the force on them.

True or false? An air bag works by reducing the passenger's change in momentum.

False. The change in momentum is the same; the air bag increases the time for that change, reducing the force.

True or false? A seat belt that stretches slightly in a crash increases the force on the wearer.

False. Stretching increases the stopping time, so the force on the wearer is reduced.

True or false? Saying a crash mat 'slows down time' is an acceptable explanation of how it reduces injury.

False. You must say it increases the time taken to stop, reducing the rate of change of momentum and so the force.