Editing issues with the homework problems:
#53 on page 5: The written problem uses a given of 49 J, but the solution on p. 16-17 uses 46 J.
#59 on page 5: The written problem uses a given of 2400 kJ, but the solution on p. 17 uses 2400 J.

Yuriy Zavorotniy • 4 years, 7 months ago
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Thank you for the wonderful questions on the chapter problems. However, please look into this queston. This one is a bit tricky. Q4. How much work is required to lift a 500 kg block 12 m? Wouldln't the force depend on the acceleration of the lifting of the block? F = m(g+a).

Joan, excellent question. And, yes, the force depends on the acceleration of the block being lifted. The presentation talked about lifting objects with a constant velocity. The question should also say that. So, if it is being lifted with a constant acceleration, then your use of Newton's Second Law has a = 0, and the force required is in fact F = mg. Then the work is Fd.
Of course, the block has to be accelerated when it is first lifted and when it comes to a stop, but the problem is looking for what happens in between the initial and final states - and that's where we say a = 0. A lot is assume in this problem, and not a lot written! Thanks, John

Matthew Reischer • 7 months, 4 weeks ago
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Problem 99 should asks for 2 free body diagrams. It should ask for forces AND acceleration vectors to be drawn. It only asks for forces. The solutions and worked solutions should be updated to add acceleration as well.

Matthew - free body diagrams just include the forces. I always tell my students to add the expected acceleration vector to help them assign signs to the forces. I think we're ok in not asking for the acceleration vectors for this problem - they should do that to solve the problem. John

## 10 Comments

Erik Yates •4 years, 8 months ago • login to replyYuriy Zavorotniy •4 years, 7 months ago • login to replyJoan Yoon •1 year, 1 month ago • login to replyJohn Ennis •1 year, 1 month ago • login to replyMatthew Reischer •1 year ago • login to replyJohn Ennis •1 year ago • login to replyMatthew Reischer •8 months ago • login to replyJohn Ennis •7 months, 4 weeks ago • login to replyMatthew Reischer •7 months, 4 weeks ago • login to replyJohn Ennis •7 months, 3 weeks ago • login to reply