Tutorial #: Circle correct course: PHYS 1P21 or PHYS 1P91 Name: Student #: BROCK UNIVERSITY Test 7: November 2015 Number of pages: 5 Course: PHYS 1P21/1P91 Number of students: 218 Examination date: 17 November 2015 Time of Examination: 12:00 12:50 Instructor: S. D Agostino A non-graphics, non-programmable electronic calculator is allowed. A formula sheet is attached. No other aids are allowed. Solve all problems in the space provided. Total number of points: 25 For the multiple-choice questions, circle the label for the best response in each case. 1. [2 points] A satellite S orbits planet X in a circular orbit at a constant speed. The net force acting on the satellite S is (a) zero, because the satellite S moves at a constant speed. (b) zero, because a body in motion tends to stay in motion, according to Newton s first law of motion. (c) zero, because the momentum of the satellite S is conserved. (d) [All of the above.] (e) [None of the above.] 2. [2 points] Two identical cars are travelling at the same speed. One is heading due east and the other due north. Which statement is true regarding the kinetic energies and momenta of the cars? (a) They have different kinetic energies and different momenta. (b) They have the same kinetic energies, but different momenta. (c) They have the same kinetic energies and the same momenta. (d) They have different kinetic energies, but the same momenta. 3. [2 points] A high-jumper, having just cleared the bar, lands on an air mattress and comes to rest. Had she landed directly on the hard ground, her stopping time would have been much shorter. Using the impulse-momentum theorem as your guide, determine which one of the following statements is correct. (a) The air mattress exerts the same impulse, but a smaller net average force, on the (b) The air mattress exerts a greater impulse, but a smaller net average force, on the
(c) The air mattress exerts the same impulse, but a greater net average force, on the (d) The air mattress exerts a smaller impulse, and a smaller net average force, on the (e) The air mattress exerts a greater impulse, and a greater net average force, on the 4. [2 points] Two balls of putty slide on a frictionless table and collide, sticking together, so the collision is inelastic. For the two balls in the collision, (a) momentum is not conserved, because some of the initial momentum is dissipated by friction or by other means, but kinetic energy is conserved. (b) momentum is conserved, but kinetic energy is not conserved. (c) both momentum and kinetic energy are conserved. (d) neither momentum nor kinetic energy are conserved. 5. [4 points] The alarm at a fire station rings and an 86.5-kg fire-fighter, starting from rest, slides down a pole to the floor below (a distance of 4.78 m). Just before landing, his speed is 1.10 m/s. What is the magnitude of the kinetic frictional force exerted on the fire-fighter as he slides down the pole?
6. [4 points] Two disks with masses m 1 and m 2 are moving horizontally to the right at a speed of v 0. They are on an air hockey table, which supports them with an essentially frictionless cushion of air. They move as a unit, with a compressed spring between them, which has a negligible mass. Then the spring is released and allowed to push the disks apart. Consider the situation where Disk 1 comes to a momentary halt shortly after the spring is released. Assuming that m 1 = 1.1 kg, m 2 = 2.4 kg, and v 0 = +5.0 m/s, find the velocity of Disk 2 at that moment.
7. [4 points] A ball is attached to one end of a string, the other end being fastened to the ceiling. The string is held horizontal, and the ball is released from rest. It swings downward and strikes a block initially at rest on a horizontal frictionless surface. Air resistance is negligible, and the collision is elastic. The masses of the ball and block are, respectively, 1.6 kg and 2.5 kg, and the length of the string is 1.15 m. Find the velocity (magnitude and direction) of the ball (a) just before the collision, and (b) just after the collision.
8. [5 points] The drawing shows a collision between two pucks on an air-hockey table. Puck A has a mass of 0.0240 kg and is moving along the x-axis with a velocity of +6.37 m/s. It makes a collision with Puck B, which has a mass of 0.0480 kg and is initially at rest. The collision is not head-on. After the collision, the two pucks fly apart with the angles shown in the drawing. Find the speeds after the collision of (a) Puck A and (b) Puck B.