Module 7 Class Activities
I. Weightlifter
A weightlifter grabs a 100 kg dumbbell that’s sitting on the floor and lifts it 2.0 meters to a point over her head. Then she holds the dumbbell over her head, motionless, for several seconds.
A. How much work did she do on the dumbbell while lifting it? Explain your reasoning.
B. How much work did she do on the dumbbell while holding it over her head? Explain your reasoning.
C. The weightlifter says,
“Don’t even try to tell me I’m not doing work while holding the dumbbell over my head. I’m working hard—look at me sweat, and look how tired I get!”
Reconcile your part B answer with the weightlifter’s feeling that she’s doing work on the dumbbell while holding it motionless. In other words, explain how it’s the case that the weightlifter gets tired even though she’s not doing work on the dumbbell.
II. Rocket
A toy rocket of mass 6.0 kg takes off from the ground and goes straight up. During the first 100 meters of its ascent, the engine exerts an 80 N upward force on the rocket.
A. How much work does the engine do on the rocket during those first 100 meters?
B. Assuming kinetic energy is the only kind of energy the rocket gains, how much kinetic energy does the rocket have at the moment it’s 100 meters above the ground? (Hint: You’ll need to include the work done by gravity in your calculation. To keep the math less messy, approximate g as 10 m/s2.)
C. At height 100 meters, the rocket has 1850 joules of kinetic energy, which is less than your part B answer. What’s going on? Where do you think the “missing” joules are?
III. Ramp vs. Straight Up
A lazy professor wants to lift a cart from the floor onto a table. She can either lift it straight up along path 1, or she can push it up the ramp along path 2 (see the dashed lines in the diagram). If she chooses path 2, the cart rolls with negligible friction. Whichever path she chooses, the cart starts at rest on the floor and ends at rest on the table. Here’s the issue: Along which path will the professor do less work on the cart?
A. Why might a smart student say the professor does less work lifting the cart along path 1?
B. Why might a smart student say the professor does less work pushing the cart along path 2?
C. Along which path, if either, does the professor do less work? Hint: Think about the work done by gravity in this case where the object gains no kinetic energy.
D. Reconcile your part C answer with the arguments given in parts A and/or B. In other words, show how elements of the “incorrect” ideas from parts A and/or B can be used to correctly explain why your part C answer makes intuitive sense.
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