9.1 Work, Power, and the Work–Energy Theorem
True or false—While riding a bicycle up a gentle hill, it is fairly easy to increase your potential energy, but to increase your kinetic energy would make you feel exhausted.
Which statement best explains why running on a track with constant speed at 3 m/s is not work, but climbing a mountain at 1 m/s is work?
- At constant speed, change in the kinetic energy is zero but climbing a mountain produces change in the potential energy.
- At constant speed, change in the potential energy is zero, but climbing a mountain produces change in the kinetic energy.
- At constant speed, change in the kinetic energy is finite, but climbing a mountain produces no change in the potential energy.
- At constant speed, change in the potential energy is finite, but climbing a mountain produces no change in the kinetic energy.
9.2 Mechanical Energy and Conservation of Energy
True or false—The formula for gravitational potential energy can be used to explain why joules, J, are equivalent to kg × mg2 / s2 . Show your work.
True or false—A marble rolls down a slope from height h1 and up another slope to height h2, where (h2 < h1). The difference mg(h1 – h2) is equal to the heat lost due to the friction.
9.3 Simple Machines
Why would you expect the lever shown in the top image to have a greater efficiency than the inclined plane shown in the bottom image?
- The resistance arm is shorter in case of the inclined plane.
- The effort arm is shorter in case of the inclined plane.
- The area of contact is greater in case of the inclined plane.
Why is the wheel on a wheelbarrow not a simple machine in the same sense as the simple machine in the image?
- The wheel on the wheelbarrow has no fulcrum.
- The center of the axle is not the fulcrum for the wheels of a wheelbarrow.
- The wheelbarrow differs in the way in which load is attached to the axle.
- The wheelbarrow has less resistance force than a wheel and axle design.
A worker pulls down on one end of the rope of a pulley system with a force of 75 N to raise a hay bale tied to the other end of the rope. If she pulls the rope down 2.0 m and the bale raises 1.0 m, what else would you have to know to calculate the efficiency of the pulley system?
- the weight of the worker
- the weight of the hay bale
- the radius of the pulley
- the height of the pulley from ground
True or false—A boy pushed a box with a weight of 300 N up a ramp. He said that, because the ramp was 1.0 m high and 3.0 m long, he must have been pushing with force of exactly 100 N.