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# Physics circular motion question

1. A student swings a bucket of water, radius 1.3m it has total mass of 2.5kg. T=1.4ms-1
From this, at the bottom of the swing, what force does the rope exert to the students hand:
A(90N B(63N C(52 D(36N

I thought, well since it's at the bottom of the swing it's easier just to resolve the force by drawing a free body diagram. Weight acts down (2.5g) and since the bucket isnt levitating or falling for no reason there is no acceleration so the normal force (tension) to counter the weight is towards the hand, 2.5g which comes out as about 25N. apparently it's 90. Can someone explain why?
If it was anywhere else i could explain it as centripetal force acting towards the centre but this suggests it's at the bottom
2. (Original post by Toasticide)
A student swings a bucket of water, radius 1.3m it has total mass of 2.5kg. T=1.4ms-1
From this, at the bottom of the swing, what force does the rope exert to the students hand:
A(90N B(63N C(52 D(36N

I thought, well since it's at the bottom of the swing it's easier just to resolve the force by drawing a free body diagram. Weight acts down (2.5g) and since the bucket isnt levitating or falling for no reason there is no acceleration so the normal force (tension) to counter the weight is towards the hand, 2.5g which comes out as about 25N. apparently it's 90. Can someone explain why?
If it was anywhere else i could explain it as centripetal force acting towards the centre but this suggests it's at the bottom
Hello Toasti !

In a nutshell, you've missed out the fact that the swinging mass has a kinetic energy component.

The first thing to recognise is that the system forms a circular motion with a net centripetal force so the problem is a bit more involved than your answer.

No matter where the bucket is on the swing, the tension will always point along the radius to the pivot.

There is also a part of the gravitational force tugging away from the centre at a tangent to the radius.

Hence the gravitational force must be resolved into two components.

Take a look at this video which explains the concept well:

3. ah thanks!
Didn't expect you to reply, would've repped your post but i already repped one of yours :s

That video helped quite a bit, took a couple of watches as compared to what i normally have in the questions that was a lot, but i got it in the end. Rather devious of my textbook though, the questions were supposed to increase slowly in difficulty and it was stuck right at the beginning with simple substitution questions!

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