I just want to make sure I'm understanding negative work correctly. An example problem where there is a bullet with an initial velocity of x m/s and a final velocity of 0 m/s. It is stopped by a wood block and the question asks to determine the amount of work done on the bullet by the block. If I use the Î”KE = W formula (we are assuming here that the only work being done is by block so W is just the W of the block) I will get a negative number because KE final is 0 but KE initial is a positive # (using the bullets movement along the positive x axis). Therefore, the work done on the bullet by the block is negative. This makes sense to me in that I understand negative work means the work done is in the opposite direction of the displacement. So in this instance, the change in KE and the amount of work done are the same and are both negative (change in KE is negative not KE itself).(adsbygoogle = window.adsbygoogle || []).push({});

To determine the average force the block exerts on the bullet I would use the Work equation: F (cos theta)(s). Because this is horizontal but the work being done is opposite the displacment, theta is 180 degrees, correct? Therefore, the average force exerted by the block will not be a negative number, correct? Example: Work= -X joules= Fcos180 x s (positive displacement #). Divide by s, then divide by cos 180 (-1) to get F. Negative divided by negative is positive so average force exerted is positive.

Do I have this correct? Am I understanding negative work properly and how to correctly use 180 for theta in horizontal situations?

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# Negative Work but Average Force positive in this situation?

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