Momentum
A short lesson with diagrams, a worked example, a problem to try, and a quick check. Then quiz yourself on 4 practice questions.
Momentum is mass times velocity: p = mv. Because velocity has a direction, so does momentum: it points the way the object moves. To find a system’s total momentum, add the momenta with their signs.
1. Momentum has a direction
2. Mass and speed both count
A slow, heavy object can have more momentum than a fast, light one.
A 2000 kg truck rolling at 1 m/s: p = 2000 kg·m/s. A 0.15 kg baseball at 40 m/s: p = 6 kg·m/s.
Momentum is not the same as kinetic energy. Kinetic energy is never negative and has no direction; momentum can be + or −, and two momenta can cancel.
3. Example, then try it
A 2 kg cart moves right at 3 m/s and a 1 kg cart moves right at 4 m/s. What is their total momentum?
Both point right, so add: (2)(3) + (1)(4) = 10 kg·m/s to the right.
A 1200 kg car drives east at 15 m/s. A 3000 kg truck drives west at 5.0 m/s. Which has more momentum, and what is the total momentum of the two?
Choose + : east.
Car: p = (1200)(+15) = +18,000 kg·m/s. Truck: p = (3000)(−5.0) = −15,000 kg·m/s.
Compare sizes: the car’s 18,000 is bigger than the truck’s 15,000, even though the truck is heavier.
Total: 18,000 + (−15,000) = +3000 kg·m/s, or 3000 kg·m/s east.
Answer: the car has more momentum; the total is 3000 kg·m/s east.
4. Common mistakes
A 0.10 kg ball moves at 20 m/s. A 2.0 kg ball moves at 1.0 m/s. Which has more momentum?
(0.10)(20) = 2.0 kg·m/s and (2.0)(1.0) = 2.0 kg·m/s. Same size of momentum.
Ready to practice?
4 questions on Momentum, each with a hint and an explanation for every wrong answer.