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AP Physics 1 Unit 5: Momentum & Impulse

Unit 5 (12-18% exam weight): After scalar Energy, we return to vectors with Momentum. Explains collisions, explosions, airbags, and rocket propulsion.

Momentum helps us understand why falling on a pillow hurts less than concrete, and why airbags save lives.

1. What is Momentum ( $p$ )?

Newton defined momentum as the “quantity of motion.” It is simply mass × velocity.

$\vec{p} = m\vec{v}$

Units: kg⋅m/s (vector quantity)

Since velocity $\vec{v}$ has direction, momentum $\vec{p}$ is also a vector pointing in the direction of motion.

2. Impulse ( $J$ )

To change an object’s momentum (speed it up, slow it down, or turn it), you must apply a force for a specific amount of time. This is called Impulse.

$\vec{J} = \vec{F}_{avg} \Delta t = \Delta \vec{p}$

Impulse = Force × Time = Change in Momentum ( $mv_f - mv_i$ )

The Physics of Airbags (The Egg Drop)

This equation explains car safety. In a crash, the change in momentum ( $\Delta p$ ) is fixed—you are going to stop. However, you can choose how you stop.

Short Time: High Force (Ouch!)
Long Time: Low Force (Safe)

Physics infographic comparing an egg hitting a wall (short time) vs. a pillow (long time).

The Airbag Principle: To save the egg (or a passenger), you must extend the time of impact ( $\Delta t$ ). Since $\Delta p$ is fixed, increasing time decreases the force.