113 Physics - Fall 2002 - workshop module 6 Force, potential, work, energy
1. You throw a ball from the ground to someone leaning out of a second story window. During the motion of the ball, does gravity do positive, negative or zero work on the ball? Explain how you arrive at your answer. During this same motion of the ball (from the ground to the window above), how does the kinetic energy of the ball change (increase, decrease, or stay the same)? What is the connection between this change in KE and the work done by gravity? The same ball is now dropped back to the ground. As the ball falls, does gravity do positive, negative or zero work on it? How does this affect the kinetic energy of the ball? 2. Consider a 2 kg mass stacked on top of at 7 kg mass as shown below. The two masses are attached to a spring (which is attached to the wall) and can move back and forth on a frictionless surface. The coefficient of static friction between the bottom surface of the top mass and the top surface of the bottom mass is 0.45. The spring constant is 200 N/m. a) What is the maximum amplitude of the simple harmonic motion (maximum amount the spring is compressed and stretched) of the system shown below such that the top mass does not slip during the oscillation? b) In a system that undergoes this limiting motion: What is the total energy of the system?
3. Stand by the wall and push on it as hard as you can for one minute. Do you feel tired? Did you do work? Did you expend energy? Where did it 4. go? Split into two groups. Simultaneously, one group should graph on the board the force of a spring as a function of the distance the spring is compressed or extended beyond its equilibrium position, while the other group should plot the potential energy of the spring as a function of the distance the spring is compressed or extended beyond its equilibrium position. Each group should explain their graph to the other group. Everyone should agree that the graphs are correct. Now, compare the two graphs and see that they qualitatively fulfill the mathematical relationship between force and potential energy.
5.
Two atoms in a molecule are separated by a distance r. The potential energy between these two particular atoms depends on the separation of the atoms and is shown below as a function of the distance between the atoms (on some arbitrary scale). Note: a realistic potential function ...