# Physics question

Hi, please can i have help on these 2 questions? I don’t understand question 13 and for question 14 I don’t understand how the force an be 0 because then wouldn’t the two weights separate?

Here is the question: https://app.gemoo.com/share/image-annotation/587385153465950208?codeId=M0Geqye0l7Xjo&origin=imageurlgenerator

Thanks!
Original post by anonymous294
Hi, please can i have help on these 2 questions? I don’t understand question 13 and for question 14 I don’t understand how the force an be 0 because then wouldn’t the two weights separate?

Here is the question: https://app.gemoo.com/share/image-annotation/587385153465950208?codeId=M0Geqye0l7Xjo&origin=imageurlgenerator

Thanks!

For 14, if youre "standing" in a lift in freefall, so both are falling at g, then youre in contact with the floor but you have zero force acting on it / it has zero reaction acting on you as youre both free falling due to gravity. If it had a reaction force acting on you, youd not be falling at g (accelerationn).

For 13, think about circular motion at constant speed. What is necessary to maintain circular motion?
(edited 3 months ago)
Original post by mqb2766
For 14, if youre "standing" in a lift in freefall, so both are falling at g, then youre in contact with the floor but you have zero weight (relative to it as your acceleration relative to the lift is zero) and it therefore has zero reaction.

For 13, think about circular motion at constant speed. What is necessary to maintain circular motion?

Oh so for 14, since we are both accelerating at g, the overall acceleration will be 0?
Sorry for 13 I don’t really get it, i know to stay in equilibrium, there must be no resultant moment?
Original post by anonymous294
Oh so for 14, since we are both accelerating at g, the overall acceleration will be 0?
Sorry for 13 I don’t really get it, i know to stay in equilibrium, there must be no resultant moment?

For 14 (modified the post slightly), the gravitational force would cause you to fall at an acceleration "g". As youre free falling at g, there must be no other forces acting on you. It must make sense if you think about it (astronauts floating/falling at the same rate in the vomit comet). Your acceleration relative to the lift is zero though.

For 13, there must be a non-zero radial force acting on a body for it to maintain circular motion. It must correspond to the centripetal force as you always accelerate towards the centre. You can derive it from the basic equation of a circle (constant speed assumption), but if it wasnt there, the body would travel in a straight line by Newton 1 (when you get thrown off a merry go round in a tangential direction as friction (centripetal) holds you in place until, ...)
(edited 3 months ago)
Original post by mqb2766
For 14 (modified the post slightly), the gravitational force would cause you to fall at an acceleration "g". As youre free falling at g, there must be no other forces acting on you. It must make sense if you think about it (astronauts floating/falling at the same rate in the vomit comet). Your acceleration relative to the lift is zero though.

For 13, there must be a non-zero radial force acting on a body for it to maintain circular motion. It must correspond to the centripetal force as you always accelerate towards the centre. You can derive it from the basic equation of a circle (constant speed assumption), but if it wasnt there, the body would travel in a straight line by Newton 1 (when you get thrown off a merry go round in a tangential direction as friction (centripetal) holds you in place until, ...)

Thanks, i get 13 now but for 14 how is a radial force used for this question? The answer is c but i really don’t understand it. Sorry if you’ve already explained it, we haven’t learnt about radial forces so I don’t understand it much.
Original post by anonymous294
Thanks, i get 13 now but for 14 how is a radial force used for this question? The answer is c but i really don’t understand it. Sorry if you’ve already explained it, we haven’t learnt about radial forces so I don’t understand it much.

Newton's first law of motion is often stated as "an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted on by an unbalanced force"
https://www.physicsclassroom.com/class/newtlaws/Lesson-1/Newton-s-First-Law

Circular motion constantly changes direction so it must not be in equilibrium and hence a non-zero force is acting on it. If there was no force acting on it, it would be moving in a straight line.
(edited 3 months ago)