Question

3. When something is at rest, what do we know?


4. Scott Sterling drops a ball from rest over a pit 50 m deep. What is the acceleration of the ball 3 seconds after releasing it?
A. 9.8m/s2
B. 0m/s2
C. ­9.8m/s2
D. 30.31m/s2

Answer 1

for part 3) What is Newton's first Law?

for part 4) Use equations of motion (s=ut + 0.5 at^2) to work out how far it will fall in 3s
IF it has hit the bottom of the pit before 3s tehn its acceleration is 0 - otherwise it is falling under the acceleration due to gravity. You should know what that is....

Answer 2

A body continues in a state of rest or uniform motion in a straight line unless acted upon by a resultant external force.
If it's at rest, there's NO RESULTANT external force. So, ANY forces which ARE acting on it, cancel, effectively. Rain drops. The gravity force down is balanced by upthrust and friction. The drop moves with TERMINAL VELOCITY, which is why although you may get sopping wet, you don't get concussion/killed.
Mind you, I wouldn't like to hazard it with "super sized hail stones" coming at me. Ouch, that COULD hurt.
bon chance et bon voyage
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Answer 3

there's a wonderful section of a SHERLOCK HOLMES story by Sir Aurthur Conan Doyle, MD. He alludes to a "dog that didn't bark". So what, asks his hapless sidekick and fictional biographer, Dr Watson. SH suggests that the dog in question was a GUARD DOG, and the fact that it didn't bark makes SH think that the crime might have been an inside job in which the criminal spiked the dog.
I sometimes think that physics is a bit like that, and that this question "something's at rest ..." is sort of like this.
Pensez vous, tous les personnes, s'il vous plait (think about it, all you people, please)