Momentum Flashcards – AQA A-level Physics (7408)

What is the equation for an object's linear momentum?

$p = mv$

What is the direction of an object's linear momentum?

Momentum is a vector in the direction of the object's velocity.

In one dimension, what determines the signs of velocity and momentum?

The chosen positive direction.

What are the units of linear momentum?

$\mathrm{kg\,m\,s^{-1}}$

State the law of conservation of linear momentum.

When the resultant external force on a system is zero, its total momentum remains constant. For a collision or explosion, total momentum before equals total momentum after.

What sign convention is used for momentum in one-dimensional motion?

Choose a positive direction consistently; momentum in the opposite direction is negative.

What is the total momentum after an initially stationary system explodes?

Zero, although its parts can have non-zero momenta in opposite directions.

What happens to the total kinetic energy of a system in an elastic collision?

It is conserved.

What happens to the total kinetic energy of a system in an inelastic collision?

It is not conserved: some is transferred to other forms, such as internal energy associated with deformation or heating.

Under what condition is total momentum conserved in an elastic or inelastic collision?

When the resultant external force on the system is zero.

How is resultant force related to momentum, and what is the equation for average resultant force over a time interval?

Resultant force is the rate of change of momentum. For an interval, $F_{\mathrm{avg}}=\frac{\Delta p}{\Delta t}$.

What is impulse?

The change in momentum.

For a constant resultant force, how are force, time interval and change in momentum related?

$F\Delta t=\Delta p=\Delta(mv)$

What does the signed area between a force–time graph and the time axis give, including when force varies?

The impulse, equal to the change in momentum.

What equation relates average impact force to the change in momentum and contact time?

$F_{\mathrm{avg}}=\frac{\Delta p}{\Delta t}$

For a given change in momentum, how does increasing contact time affect the magnitude of the average impact force?

It reduces the magnitude of the average force.

In ethical impact-protection design, what should be considered rather than assuming that momentum is eliminated?

The foreseeable effects of the protection on people affected by a collision.