Physical Science / Unit 7: Motion and Forces
7.6 Momentum
Notes
Momentum measures how hard it is to stop a moving object: $p = mv$. Units: kg·m/s.
- Example: A 1,000 kg car at 20 m/s has $p = 1000 \times 20 = 20{,}000$ kg·m/s.
- Conservation of momentum: in a closed system with no outside forces, total momentum before a collision equals total momentum after.
- Example: A 2 kg cart at 3 m/s hits a stationary 1 kg cart and they stick together. Total momentum $= 2 \times 3 = 6$. Combined mass 3 kg, so $\displaystyle v = \frac{6}{3} = 2$ m/s.
Practice
What is the momentum of a 0.5 kg ball moving at 10 m/s?
$p = 0.5 \times 10 = 5$.
Which has the greatest momentum?
Momenta: 6, 5, 5 and 16 kg·m/s.
A 3 kg cart moving at 4 m/s sticks to a stationary 1 kg cart. What is their speed after the collision?
Total momentum is 12 kg·m/s, shared by 4 kg: $\displaystyle v = \frac{12}{4} = 3$.
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