Momentum Flashcards

All 14 cards in this deck

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 an equation?

inertial mass (kg) = force (N) ÷ acceleration (m/s²)

True or false? An object with a larger inertial mass is easier to speed up or slow down with a given force.

False. A larger inertial mass means its velocity is harder to change, so the same force gives a smaller acceleration.

What is momentum, and what is the equation for it?

Momentum = mass × velocity; $p = m \times v$

What is the unit of momentum?

kg m/s (kilogram metre per second).

Momentum is a vector quantity — how do you find the total momentum of two objects moving in opposite directions?
e.g. 3 kg m/s to the right and 2 kg m/s to the left

Choose one direction as positive and make the opposite direction negative, then add: $+3 + (-2) = +1$ kg m/s to the right.

State the law of conservation of momentum.

The total momentum before a collision equals the total momentum after the collision, provided no external forces act.

What are the steps to explain, using Newton's third law, why momentum is conserved in a collision?
e.g. two trolleys colliding

1. The force of each trolley on the other is equal and opposite.
2. The time of the collision is the same for both, so the changes in momentum are equal and opposite.
3. So there is no overall change in momentum: momentum before = momentum after.

Two objects at rest are pushed apart in an explosion (e.g. a gun firing a bullet). What is their total momentum afterwards?

Zero — the two momenta are equal in size and opposite in direction, because the total momentum before was zero.

True or false? In a collision, momentum is conserved but kinetic energy need not be.

True. Kinetic energy can be transferred to thermal and sound stores while the total momentum stays the same.

What is Newton's second law written in terms of momentum?

Force = change in momentum ÷ time; $F = \frac{mv - mu}{t}$

What happens to the force on an object if the same change in momentum happens over a longer time?

The force is smaller, because $F = \frac{\Delta p}{t}$.

True or false? A ball that bounces straight back off a wall has a smaller change in momentum than an identical ball that hits the wall and stops.

False. Momentum is a vector, so reversing direction gives a larger change in momentum (and a larger force for the same contact time).

What is inertia?

An object's tendency to resist any change to its motion: it stays at rest, or at a constant velocity, until a resultant force acts on it.