When is momentum conserved?: If you are not sure how to answer these questions,

Question

When is momentum conserved?: If you are not sure how to answer these questions, you might want to try reading the chapter on momentum in a freshman physics textbook. In Halliday and Resnick that would be chapter 9 (a) (2 points) A 1000 kilogram train car is sitting still on a frictionless track, and a 100 kilogram person is inside sitting on the floor of the car. Then the person inside the car stands up and starts walking to the left at 1 meter per second. The train car starts moving to the right. What (if any) horizontal forces are acting in this system? (b) (2 points) What is the total momentum of the system after the guy starts walking? c) (2 points) I throw a rock up and to the right. If we ignore air resistance, does the horizontal component of momentum stay constant during the rock's flight? Does the vertical component stay constant? (d) (4 points) A catapult launches a cow toward King Arthur and his Knights of the Round Table. The cow starts traveling up and to the right, and the catapult recoils to the left. If the friction between the ground and catapult wheels is negligible, is the horizontal component of momentum conserved in the cow-catapult system? Why or why not? Is the vertical component of momentum conserved in the cow-catapult system? Why or why not?

Explanation / Answer

a) Because the train is on frictionless tracks, there is no external force acting on the system. However inside the system the man exerts force on the train to move forward and consequently train also moves back by exerting equal amount of force on the man.

b) Due to no external force, the momentum of the system is conserved. So after the guy starts walking the momentum will be zero because before he started walking it was zero.

c) Yes, the horizantal component stays constant because there is no horizantal force on the rock. and NO, the vertical component doesnot stay constant because the weight of the rock is acting downward which decreases the momentum.

d) Horizantal component of momentum is conserved becasue there are no horizantal forces due to lack of friction. But the vertical component of momentum is not conserved because the earth exerts force on both the catapult and cow.

Related Questions

Navigate

• Browse (All)
• Browse W
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.