Question

Conceptual Work and Kinetic Energy

For each of the following statements, determine whether it is true or false.
1.)True/False  The spring force is a non-conservative force.
2.)True/False  The kinetic energy of an object does not depend on the reference frame of the observer.
3.)True/False  Spring B is stiffer than A (kA<kB). A person must do more work on the block connected to spring A if both springs are ideal and the blocks are pulled by the same amount.
4.)True/False  If the Moon revolves around Earth in a perfectly circular orbit, then Earth does do work on the Moon.
5.)True/False  Work cannot be done in the absence of motion.
6.)True/False  The kinetic energy can be negative.
7.)True/False  The work a person must do to raise a box onto a platform depends on how fast it is raised.
8.)True/False  The kinetic energy of an object depends on the direction of the motion involved.

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Answer 1

1.)False  The spring force is a non-conservative force.

Becasue, spring force is a conservative force.

2.) False  The kinetic energy of an object does not depend on the reference frame of the observer.

Because, the speed of an object depends on the frame of reference. so, kinetic energy also depends on frame of reference.
3.)False Spring B is stiffer than A (kA<kB). A person must do more work on the block connected to spring A if both springs are ideal and the blocks are pulled by the same amount.

if x is the extension of both springs. clearly, (1/2)*kB*x^2 > (1/2)*kA*x^2

4.)True If the Moon revolves around Earth in a perfectly circular orbit, then Earth does do work on the Moon.

because, W = F*d*cos(theta)

if theta = 90 degrees, W = 0

Because, Force is always perendicular to instantaneous displacement.
5.)True : Work cannot be done in the absence of motion.

Becasue, W = F*d*cos(theta)

if displacement, d = 0 , W = 0

6.) False  The kinetic energy can be negative.

we know, KE = (1/2)*m*v^2

m can never be negative. so, KE can nevre be negative.
7.) False  The work a person must do to raise a box onto a platform depends on how fast it is raised.

Because, Workdone does not depend on time taken.
8.) False  The kinetic energy of an object depends on the direction of the motion involved.

we know, KE = (1/2)*m*v^2

so, whether v is positive or negative KE is always positive.