chapter 9 - conceptual questions Flashcards
If a particle’s speed increases by a factor of 3, by what factor does its kinetic energy change?
it increases by a factor of 9
Particle A has half the mass and eight times the kinetic energy of particle B. What is the speed ratio vA/vB?
speed ratio is 4
check flowery book for process
An elevator held by a single cable is ascending but slowing down. Is the work done by tension positive, negative, or zero? What about the work done by gravity? Explain.
w tension = positive
work by gravity = negative
force of gravity is in the opp direction of displacement
force of tension is in the same direction as displacement
The rope in Figure Q9.4 pulls the box to the left across a rough surface. Is the work done by tension positive, negative, or zero? Explain.
work done by tension is positive
since force of tension is in the same direction as displacement
A 0.2 kg plastic cart and a 20 kg lead cart both roll without friction on a horizontal surface. Equal forces are used to push both carts forward a distance of 1 m, starting from rest. After traveling 1 m, is the kinetic energy of the plastic cart greater than, less than, or equal to the kinetic energy of the lead cart? Explain.
they have equal kinetic energy
A particle moving to the left is slowed by a force pushing to the right. Is the work done on the particle positive or negative? Or is there not enough information to tell? Explain.
negative
You need to raise a heavy block by pulling it with a massless rope. You can either (a) pull the block straight up height h, or (b) pull it up a long, frictionless plane inclined at a 15° angle until its height has increased by h. Assume you will move the block at constant speed either way. Will you do more work in case a or case b? Or is the work the same in both cases? Explain.
same work done. its equal
by raising the block vertically, you exert more force but less distance is needed
while raising it at an angle of 15 degrees, cause you to use less force over a bigger distance.
therefore, they end up having the same amount of work done
A sprinter accelerates from rest. Is the work done on the sprinter positive, negative, or zero? Explain.
not sure about this question…
The left end of a spring is attached to a wall. When Bob pulls on the right end with a 200 N force, he stretches the spring by 20 cm. The same spring is then used for a tug-of-war between Bob and Carlos. Each pulls on his end of the spring with a 200 N force. How far does the spring stretch? Explain.
use newton third law here
the force of the wall on bob and the force of bob on the spring is equal of magnitude but opposite direction
same thing is happening when both bob and carlos are pulling the spring
hence, they move the same distance 0.2 m
