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Need Help on this AS Physics Question

So this question is based on Motion (chapter 3 of ocr a AS physics), and I don't seem to understand it. Could someone please help me understand this? Thanks :frown:

Reply 1

Plagioclase

Original post by iamfaizanabbas

So this question is based on Motion (chapter 3 of ocr a AS physics), and I don't seem to understand it. Could someone please help me understand this? Thanks :frown:

The average speed of the comet over one complete orbit is definitely not zero, speed is the magnitude of velocity (so it's always positive) which means the only way the average speed could be zero is if the comet is stationary the entire time, which clearly is not the case! You should know that the force (and hence acceleration) due to gravity is proportional to

\frac{1}{r^2}

and since the comet is not in a circular orbit,

r

varies so the acceleration varies too. The comet's energy is constant - energy can't be created or destroyed. When the comet approaches the sun, GPE is converted into KE and when it leaves the sun, KE is converted into GPE. So the KE is not constant and therefore the speed is also not constant.

The only correct statement is 1 because if you add up all of the velocity vectors for something travelling in a loop, the resultant is 0.