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True or False

by PhysicsPhun
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PhysicsPhun
#1
Mar1-04, 03:50 PM
P: 55
The question asks for which of these statements is true.

a) The Kinetic Energy of an object depends on the direction of the motion involved.

b)The kinetic energy of any object is either positive or Zero.

c) If the scalar product (also called the dot product) of two nonzero vectors is zero, then the vectors must be perpendicular to each other.

D) Spring A and B are identical except that A is stiffer than B; that is Ka > Kb. More work must be expended on spring A if both springs are stretched by the same amount.

E) If the earth revolves around the sun in a perfectly circular orbit, then the sun does not do any work on the earth.

F) The work done to raise a box onto a platform depends on how fast it is raised.



What i thought was that BCD and F were all true.. but apparently that's incorrect.
Does anyone have any ideas?
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Njorl
#2
Mar1-04, 04:02 PM
Sci Advisor
P: 875
Originally posted by PhysicsPhun
The question asks for which of these statements is true.

a) The Kinetic Energy of an object depends on the direction of the motion involved.

b)The kinetic energy of any object is either positive or Zero.

c) If the scalar product (also called the dot product) of two nonzero vectors is zero, then the vectors must be perpendicular to each other.

D) Spring A and B are identical except that A is stiffer than B; that is Ka > Kb. More work must be expended on spring A if both springs are stretched by the same amount.

E) If the earth revolves around the sun in a perfectly circular orbit, then the sun does not do any work on the earth.

F) The work done to raise a box onto a platform depends on how fast it is raised.



What i thought was that BCD and F were all true.. but apparently that's incorrect.
Does anyone have any ideas?
F is false. It does not matter how fast you raise the box, provided it is at rest when you are done. The work done upon it is mgh, m=mass, g=gravitational acceleration, h=height. This is the change in the box's potential energy. While it is true that you also added kinetic energy while the box was being raised, while it is at rest on a shelf, it has 0 kinetic energy.

Njorl
PhysicsPhun
#3
Mar1-04, 04:10 PM
P: 55
BCD is not the correct answer either.. Did i mess up somewhere else?

Doc Al
#4
Mar1-04, 06:33 PM
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True or False

E is true as well. The force is perpendicular to the displacement, so no work is done.
PhysicsPhun
#5
Mar1-04, 07:25 PM
P: 55
Thanks to both of you. Much appreciated.
Dreamer
#6
Mar1-04, 07:40 PM
P: 3
I know answer B is also true.
Njorl
#7
Mar2-04, 09:34 AM
Sci Advisor
P: 875
Originally posted by PhysicsPhun
BCD is not the correct answer either.. Did i mess up somewhere else?
BCDE are true.
AF are false.

D has a special case. If "the same amount" is zero, then the same work is done on both springs. It is a false statement for that special case.

Njorl


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