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# How can i get 5 marks from this :-o watch

1. A tennis ball connected to a long piece of string is swung around in a horizontal circle above the head of a pupil.

The pupil feels that there is a tension in the string and argues that for equilibrium there must be an outward "centrifugal" force acting on the ball. Criticise his argument and explain why there is a tension in the string. [5]

The pupil lets go of the string. Draw a free-body force diagram for the ball at the instant release. [2]

i appreciate any help, as it is a very long question!

For
A) Im thinking of writing:

The pupil feels the tension because this is the centripetal force. There is an unbalanced force on the ball which is the tension, as newtons' first law argues. the velocity is constantly changing as the direction is perpindicular to the string.

How can i improve/add to that? Cheers

for the free-body diagram, i'm confused of what to put down
2. for a mathematician, the tension doesn't need an outward centrifugal force because the ball isn't in equlibrium - it is accelerating towards the centre of the circle in order to stay in circular motion. but i'm not sure how a physicist would argue it.

the free body diagram is simply the projectile at the moment of release, having lost the tension and so the only force on it is its weight
3. A physist would argue similarly. Recall Newton's 3rd Law. F = ma. If there is a force acting upon said mass, then said mass must be accelerating in that direction. Acceleration is any change in velocity, recall velocity is a vector quantity, hence you are right to state that the velocity is in fact changing. This also means there is acceleration, which means a force, which means, a lack of an equalibrium.
The centralfugal force that one learns in primary school is the result of a referance frame that is being accelerated, thus it feels like, in the frame, being accelerated the other way.

Me thinks the diagram is just a blob, which is the mass. Then arrows pointing off of it in the dirrection of any forces that act upon it (ie just weight = mg).
4. Thanks, But it's two marks for the last bit. Probably a trick question.
I think them comments with the newtons 1st law comment will get 5 marks. Cheers for the help
5. (Original post by nas7232)
A tennis ball connected to a long piece of string is swung around in a horizontal circle above the head of a pupil.

The pupil feels that there is a tension in the string and argues that for equilibrium there must be an outward "centrifugal" force acting on the ball. Criticise his argument and explain why there is a tension in the string. [5]

The pupil lets go of the string. Draw a free-body force diagram for the ball at the instant release. [2]
for the first question,the tention provides the centripetal force and also it provides and opposite and equal force to the weight,so Tcosx=mg
more over mv^2/r equals the centripetal force which is Tsinx ,now the tention provides the centripetal force and so there is no outword force acting on it,you can say this the way you want also indicate that the centripetal force acts towards the centre Aka Tsinx
for the second one the diagram would be stright line tangetial to the motion

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