Part 03 · Work, energy and momentum · Card 7 of 9
Momentum
Mass times velocity, p = mv. It has a direction, and the total in a closed system is conserved.
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Analogy
How hard something is to stop.
Example
Fast ball
p = m vp = 0.15 x 40p = 6 kg m/s
In practice
A 0.15 kg ball at 40 m/s has momentum 6 kg m/s.
Practice
Flashcards
- Is momentum a vector or a scalar?A vector, because it has direction.
- What is conserved in every collision?Total momentum, if no outside force acts.
Interview questions
- A 1500 kg car travelling at 15 m/s hits a stationary 1000 kg car and they lock together. Find their speed and the energy lost.
- Use momentum conservation
- Combined mass is 2500 kg
- Compare kinetic energy before and after
- Explain why airbags reduce injury using impulse.
- Same change in momentum
- Longer stopping time
- Smaller average force
Questions people ask
Why is momentum conserved?
It follows from Newton's third law. In a collision the two objects push each other with equal and opposite forces for the same time, so they exchange equal and opposite impulses. The total momentum before and after is the same unless an outside force acts.
Can two objects have the same momentum but different kinetic energy?
Yes. Momentum is mv while kinetic energy is p squared over 2m. At equal momentum, the lighter object has more kinetic energy. A 1 kg object at 2 m/s and a 2 kg object at 1 m/s both have p = 2 but energies of 2 J and 1 J.