Momentum Flashcards

All 23 cards in this deck

Define momentum.

The product of an object's mass and its velocity. It is a vector quantity.

What is the equation for momentum?

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

What is the unit of momentum?

kilogram metre per second, $\mathrm{kg\ m/s}$

True or false? Momentum is a scalar quantity.

False. Momentum is a vector — it has a direction as well as a size.

How is momentum written for two objects moving in opposite directions?

e.g. one object moving right, one moving left

Choose one direction as positive: momentum to the right is $+$, momentum to the left is $-$.

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

e.g. momentum $20\ \mathrm{kg\ m/s}$, velocity $5\ \mathrm{m/s}$

1. Rearrange $p = m \times v$ to $m = p \div v$
2. Divide: $20 \div 5 = 4\ \mathrm{kg}$

State the law of conservation of momentum.

In a closed system (no external forces), the total momentum before an event equals the total momentum after it.

What are the steps to find the velocity of two objects that stick together in a collision?

e.g. a $2\ \mathrm{kg}$ trolley at $3\ \mathrm{m/s}$ hits a stationary $1\ \mathrm{kg}$ trolley and they join

1. Total momentum before: $2 \times 3 = 6\ \mathrm{kg\ m/s}$
2. Total mass after: $2 + 1 = 3\ \mathrm{kg}$
3. Velocity after $= 6 \div 3 = 2\ \mathrm{m/s}$

What are the steps to find a velocity after an explosion (two objects pushed apart from rest)?

e.g. a stationary $1\ \mathrm{kg}$ trolley and $2\ \mathrm{kg}$ trolley spring apart, the $1\ \mathrm{kg}$ one at $4\ \mathrm{m/s}$

1. Total momentum before $= 0$
2. So the two momenta after are equal and opposite: $1 \times 4 = 4\ \mathrm{kg\ m/s}$ one way, $4\ \mathrm{kg\ m/s}$ the other
3. Velocity of $2\ \mathrm{kg}$ trolley $= 4 \div 2 = 2\ \mathrm{m/s}$ in the opposite direction

In a trolley collision investigation, why is the runway set at a slight slope?

To compensate for friction, so the trolley travels at a constant speed.

What are the steps to investigate conservation of momentum using trolleys?

e.g. trolley A runs into stationary trolley B and they stick together

1. Measure the mass of each trolley on a balance.
2. Use light gates to measure A's velocity before and the combined velocity after.
3. Compare momentum before ($m_A u_A$) with momentum after ($(m_A + m_B)v$) — they should be equal.

True or false? When two objects collide and stick together, some momentum is lost.

False. Total momentum is conserved; it is kinetic energy that can be lost in the collision.

State Newton's second law in terms of momentum.

force = change in momentum ÷ time ($F = \dfrac{mv - mu}{t}$)

What are the steps to estimate the force on a car in a road collision?

e.g. a $1000\ \mathrm{kg}$ car travelling at $20\ \mathrm{m/s}$ stops in $0.5\ \mathrm{s}$

1. Change in momentum $= 1000 \times 20 = 20\,000\ \mathrm{kg\ m/s}$
2. Force $=$ change in momentum $\div$ time $= 20\,000 \div 0.5 = 40\,000\ \mathrm{N}$

What are the steps to find the force on a ball that bounces back off a wall?

e.g. a $0.5\ \mathrm{kg}$ ball hits at $4\ \mathrm{m/s}$ and rebounds at $4\ \mathrm{m/s}$ in $0.2\ \mathrm{s}$

1. Take towards the wall as positive: $u = +4\ \mathrm{m/s}$, $v = -4\ \mathrm{m/s}$
2. Change in momentum $= 0.5 \times (-4) - 0.5 \times (+4) = -4\ \mathrm{kg\ m/s}$
3. Force $= 4 \div 0.2 = 20\ \mathrm{N}$ away from the wall

How do crumple zones, airbags and seat belts reduce the force on a passenger in a crash?

They increase the time taken for the momentum to change, so the rate of change of momentum, and therefore the force, is smaller.

True or false? An airbag protects a passenger by reducing their change in momentum.

False. The change in momentum is the same; the airbag increases the time it takes, so the force is smaller.

Why are large decelerations dangerous to people in a vehicle?

A large change in momentum in a very short time means a very large force acts on the body, which can cause serious injury.

What is inertial mass?

A measure of how difficult it is to change the velocity of an object, including changing it from rest.

How is inertial mass defined as a ratio?

inertial mass = force ÷ acceleration

What are the steps to calculate the inertial mass of an object?

e.g. a force of $20\ \mathrm{N}$ gives an acceleration of $4\ \mathrm{m/s^2}$

1. Use inertial mass $=$ force $\div$ acceleration
2. Divide: $20 \div 4 = 5\ \mathrm{kg}$

True or false? An object with a larger inertial mass needs a larger force to give it the same acceleration.

True. Force = inertial mass × acceleration, so more mass needs more force for the same acceleration.

True or false? Inertial mass only describes how hard it is to speed an object up.

False. It describes how hard it is to make any change in velocity — speeding up, slowing down or changing direction.