kinetic energy problem-need help conceptually

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cdistel

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there is a question in kaplan that says a box is initially at rest is pushed by a person at an angle with a force of F until it reaches a speed of v and has kinetic friction mu. which of the following relationships is true.

the answer is the work done by the person is greater in magnitude than the work done by the friction force and this makes sense since it is moving

however i dont understand why it cant be this answer
the KE gained by the box is greater than the energy dissipated by friction.

can someone please explain this

the kaplan instructor tried to explain it to me not sure if it is late or i dont understand but she said

Work net= work person- work friction = KE the work of friction can be higher than the work of person so that can disprove that answer-if this were true can the book still move??

thanks for the help in advance
-chris
 
KE = net work. The wrong answer choice is saying that the net work, or work person - work friction is necessarily greater than work friction, which is not true. What is true is that work person > work friction (i.e. that the net work is positive).

For example, imagine that the change in kinetic energy is 10J. This value has to be positive for the box to move, so work person - work friction = 10J. However, this could be true if work person was 10010J and work friction was 10000J, or any other numbers. The only requirement is that work person > work friction. Work friction can be a million times bigger than the kinetic energy gained.
 
KE = net work. The wrong answer choice is saying that the net work, or work person - work friction is necessarily greater than work friction, which is not true. What is true is that work person > work friction (i.e. that the net work is positive).

For example, imagine that the change in kinetic energy is 10J. This value has to be positive for the box to move, so work person - work friction = 10J. However, this could be true if work person was 10010J and work friction was 10000J, or any other numbers. The only requirement is that work person > work friction. Work friction can be a million times bigger than the kinetic energy gained.

YEAH Rabolisk!!

And cdistel...you are in good company...this question throws a LOT of students for a loop!
 
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