Force & Momentum Flashcards

All 6 cards in this deck

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.