8.1 Linear Momentum and Force

A boy standing on a frictionless ice rink is initially at rest. He throws a snowball in the +x-direction, and it travels on a ballistic trajectory, hitting the ground some distance away. Which of the following is true about the boy while he is in the act of throwing the snowball?

1. He feels an upward force to compensate for the downward trajectory of the snowball.
2. He feels a backward force exerted…

(a) Calculate the momentum of a 2000-kg elephant charging a hunter at a speed of $7 . \text{50 m}/\text{s}$. (b) Compare the elephant’s momentum with the momentum of a 0.0400-kg tranquilizer dart fired at a speed of $\text{600 m}/\text{s}$. (c) What is the momentum of the 90.0-kg hunter running at $7 . \text{40 m}/\text{s}$ after missing the elephant?

(a) At what speed would a $2 . \text{00} \times \text{10}^{4} -\text{kg}$ airplane have to fly to have a momentum of $1 . \text{60} \times \text{10}^{9} \text{kg} \cdot \text{m}/\text{s}$ (the same as the ship’s momentum in the problem above)? (b) What is the plane’s momentum when it is taking off at a speed of $\text{60}.\text{0 m}/\text{s}$? (c) If the ship is an aircraft carrier that launches these airplanes with a catapult, discuss the implications of your answer to (b) as it relates to recoil effects of the catapult on the ship.

A runaway train car that has a mass of 15,000 kg travels at a speed of $5 .\text{4 m}/\text{s}$ down a track. Compute the time required for a force of 1500 N to bring the car to rest.