8.3 Conservation of momentum  (Page 5/9)

Which of the following is an example of an open system?

1. Two air cars colliding on a track elastically.
2. Two air cars colliding on a track and sticking together.
3. A bullet being fired into a hanging wooden block and becoming embedded in the block, with the system then acting as a ballistic pendulum.
4. A bullet being fired into a hillside and becoming buried in the earth.

A 40-kg girl runs across a mat with a speed of 5.0 m/s and jumps onto a 120-kg hanging platform initially at rest, causing the girl and platform to swing back and forth like a pendulum together after her jump. What is the combined velocity of the girl and platform after the jump? What is the combined momentum of the girl and platform both before and after the collision?

A 50-kg boy runs across a mat with a speed of 6.0 m/s and collides with a soft barrier on the wall, rebounding off the wall and falling to the ground. The boy is at rest after the collision. What is the momentum of the boy before and after the collision? Is momentum conserved in this collision? Explain. Which of these is an example of an open system and which is an example of a closed system? Explain your answer.

A student sets up an experiment to measure the momentum of a system of two air cars, A and B, of equal mass, moving on a linear, frictionless track. Before the collision, car A has a certain speed, and car B is at rest. Which of the following will be true about the total momentum of the two cars?

1. It will be greater before the collision.
2. It will be equal before and after the collision.
3. It will be greater after the collision.
4. The answer depends on whether the collision is elastic or inelastic.

A group of students has two carts, A and B , with wheels that turn with negligible friction. The carts can travel along a straight horizontal track. Cart A has known mass mA . The students are asked to use a one-dimensional collision between the carts to determine the mass of cart B . Before the collision, cart A travels to the right and cart B is initially at rest. After the collision, the carts stick together.

1. Describe an experimental procedure to determine the velocities of the carts before and after a collision, including all the additional equipment you would need. You may include a labeled diagram of your setup to help in your description. Indicate what measurements you would take and how you would take them. Include enough detail so that another student could carry out your procedure.
2. There will be sources of error in the measurements taken in the experiment, both before and after the collision. For your experimental procedure, will the uncertainty in the calculated value of the mass of cart B be affected more by the error in the measurements taken before the collision or by those taken after the collision, or will it be equally affected by both sets of measurements? Justify your answer.

A group of students took measurements for one collision. A graph of the students’ data is shown below.

1. Given m _A = 0.50 kg, use the graph to calculate the mass of cart B . Explicitly indicate the principles used in your calculations.
2. The students are now asked to Consider the kinetic energy changes in an inelastic collision, specifically whether the initial values of one of the physical quantities affect the fraction of mechanical energy dissipated in the collision. How could you modify the experiment to investigate this question? Be sure to explicitly describe the calculations you would make, specifying all equations you would use (but do not actually do any algebra or arithmetic).