1) The force you apply is not constant when you stretch the rubber band. At what
ID: 1427163 • Letter: 1
Question
1) The force you apply is not constant when you stretch the rubber band. At what point in the stretch is the force the least? At what point is the force the greatest?
a. The force is greatest when you first begin to stretch the rubber band and least when it is fully stretched.
b. The force is least when you first begin to stretch the rubber band and greatest when it is fully stretched.
c. The force is greatest when you first begin to stretch the rubber band, least at mid-stretch, and greatest again when it is fully stretched.
d.The force is least when you first begin to stretch the rubber band, greatest at mid-stretch, and least again when it is fully stretched.
2) As you lift the book, assume you lift it at a constant speed. Was the average force that you applied to the book as you lifted it
a. Less than the weight of the book
b. Greater than the weight of the book
c. Roughly equal to the weight of the book
3) Lift a book from the floor to the table. Did you do work? To answer this question, consider whether you applied a force parallel to the displacement of the book.
a. No work is done because I did not apply any force
b. No work is done because I applied a force opposite to its displacement
c. I did work because I applied a force parallel to its displacement
d. I did work because gravity helped me lift the book
4) While holding one end fixed in place, stretch a rubber band. Did you do work on the rubber band? To answer this question, consider whether you applied a force parallel to the displacement of the moving end of the rubber band.
a. No work is done because I did not apply any force
b. No work is done because I applied a force opposite to its displacement
c. I did work because I applied a force parallel to its displacement
d. I did work because the rubber band is very stiff and difficult to stretch
Explanation / Answer
1.answer b. The force is NOT constant. A rubber band can be thought of as a spring (for our purposes) and the force required to stretch a spring is proportional to the distance the spring is already stretched. The greater the stretch, the more force you need to stretch it. So...the force is the least when you're just starting, and greatest when you've stretched it the most.
3.This depends on how you define your system, actually.
If the system is just the book, and it begins and ends with zero speed, then no work was done on it. It starts and ends with zero kinetic energy. All the work you did was countered by work done by gravity.
If the system is the book and the Earth's gravity, then you're adding gravitational potential energy to the book. It still gains no kinetic energy, but now that you include gravitation potential as a place for energy to be stored, you had to add energy to the system, doing work.
If the system includes the book, Earth's gravity and YOU, then you don't do any work, but you do convert chemical potential energy (in your muscles) to gravitational potential energy.
If the system is you and the book but not gravity, then you do work on the environment, energy leaves the system (you burn chemical energy, which is then sucked out by the work done by gravity).
In any system where you and the book are on different sides of the dividing line, energy leaves you, so you do work, whether or not any work is done on the book.
Yes, energy can be kinda complicated, but if you make sure to define things carefully at the outset, it becomes a lot simpler. Work is any energy that enters (positive) or leaves (negative) a system. So it's very important to define what the system is.
4. answer cYes, you did work. By the physical definition, you do work whenever you apply a force parallel to the object's displacement.
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