The Student Room Group

Multiple Choice Q



Hi guys, I can't for the life of me figure out why the answer is A? I thought it would be D. If someone could explain to me why it's A that'd be great! Thanks


Edit: Also need help on understanding this one, I thought it was A since 100x diameter = 100x greater radius, then F/100^2 = 0.0008

(edited 6 years ago)
Reply 1
Avatar for have
have
17
Original post by iMacJack


Hi guys, I can't for the life of me figure out why the answer is A? I thought it would be D. If someone could explain to me why it's A that'd be great! Thanks


Edit: Also need help on understanding this one, I thought it was A since 100x diameter = 100x greater radius, then F/100^2 = 0.0008



For the first question, it wants you to find when the resultant force is at a minimum. The way I would go about this, is looking for the point when there is no resultant force acting on it. Since 0 is the smallest possible magnitude.
By N1L, no resultant force means no acceleration. So we're looking for the point where there is no acceleration.

At the very bottom or at the very top, it's instantaneous velocity is 0. BUT, It's acceleration is definitely not 0.
At the very bottom, it has an acceleration upwards, which causes it to change from going downwards, to instantaneously at rest, to upwards.
At the very top, it has an acceleration downwards, which causes it to change from going upwards, to instantaenously at rest, to going downwards.

So at the very bottom, it has an upwards acceleration, and at the very top it has a downwards acceleration. Therefore my intuition tells me that at the centre, it should have 0 acceleration, because somewhere in the middle there, it has to change between having a positive (upwards) acceleration to a negative (downwards) acceleration.

Now this actually makes sense, because at the centre, it is at its equilibrium position. Key word: Equilibrium, Definition: No resultant force. Therefore at it's centre, there is no resultant force.

For your second question, your right in that you should divide by 100^2 since the radius is 100x as large. But you're forgetting one detail. The fact that the density is the same but diameter is larger means that its mass will also be much larger.
100x diameter actually means 1000000x larger Volume (you can prove that for yourself however you want to, scale factors etc.).
Therefore mass will be 1000000x larger
And therefore you should times by 1000000 as well
So 8.0 * 1000000 / 10000 = 800 N kg^-1

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you