Missed A Mechanics Lesson: Hinges?

Original post by Retsek

How would I go about doing questions like this? (Finding F in this case)
I missed the lesson in class and want to catch up over the weekend.

Assuming the hinge is at A, it is irrelevant since you only care about the forces acting on the particle which are F, the weight of the particle and the tension in the string.

This is assuming there's no other information in the question. If there is, please post everything.

Reply 3

OP

Original post by Notnek

Assuming the hinge is at A, it is irrelevant since you only care about the forces acting on the particle which are F, the weight of the particle and the tension in the string.

This is assuming there's no other information in the question. If there is, please post everything.

Whoops this is just a question I made up as I thought it would be a simple example to start with and forgot to add tension. I see now that the fact it is hinged at point A makes no difference, in what cases would this effect the question? (Also is this in M1 or M2? Trying to find examples of hinge questions in a textbook).

Reply 4

Original post by Retsek

Whoops this is just a question I made up as I thought it would be a simple example to start with and forgot to add tension. I see now that the fact it is hinged at point A makes no difference, in what cases would this effect the question? (Also is this in M1 or M2? Trying to find examples of hinge questions in a textbook).

It would affect the question when the main object in the question is hinged to something. E.g. a rod might be hinged to a wall and you need to consider the forces on the rod. You'll find these questions in Edexcel M2 in the moments section.

Was this a further maths lesson you missed?

Reply 5

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Original post by Notnek

It would affect the question when the main object in the question is hinged to something. E.g. a rod might be hinged to a wall and you need to consider the forces on the rod. You'll find these questions in Edexcel M2 in the moments section.

Was this a further maths lesson you missed?

Yeah we have 4 teachers, two for pure, one for stats and one for mechanics and it was a mechanics lesson I missed. The idea is for us to have been taught the entirety of the old A-level in a couple months time (with our applied teachers switching to teach more pure soon as we have gotten through the applied quicker).

(edited 6 years ago)

Reply 6

Original post by Retsek

Yeah we have 4 teachers, two for pure, one for stats and one for mechanics and it was a mechanics lesson I missed. The idea is for us to have been taught the entirety of the old A-level in a couple months time (with our applied teachers switching to teach more pure soon as we have gotten through the applied quicker).

Yes this is a topic in the new A Level maths and not further maths - I forgot. It's referred to as "equilibrium of rigid bodies" in the spec and comes under moments.

Reply 7

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Original post by Notnek

Yes this is a topic in the new A Level maths and not further maths - I forgot. It's referred to as "equilibrium of rigid bodies" in the spec and comes under moments.

Thanks Notnek :smile:

Reply 8

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Original post by Notnek

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Hey Notnek I'm doing some questions and it asks for the reaction at the hinge, I can work it out easy enough but I struggle somewhat to actually understand what the reaction means.

I get it in the horizontal, the rod pushes against the wall and the wall pushes back so the push back is the horizontal reaction.

But the vertical reaction has me confused, is it the weight of the rod pulling down combined with the vertical component of the tension? Now I'm sure that one of these is bound to win over every time but idk which or why.

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(edited 6 years ago)

Reply 9

Original post by Retsek

Hey Notnek I'm doing some questions and it asks for the reaction at the hinge, I can work it out easy enough but I struggle somewhat to actually understand what the reaction means.

I get it in the horizontal, the rod pushes against the wall and the wall pushes back so the push back is the horizontal reaction.

But the vertical reaction has me confused, is it the weight of the rod pulling down combined with the vertical component of the tension? Now I'm sure that one of these is bound to win over every time but idk which or why.

Look at the diagram in your OP, ignore the mass and imagine you are moving the 5m rod up and down. It is hinged at A so will rotate about A but will remain in equilibrium if you keep it in equilibrium with the force from your hand.

Depending on how high/low you move the rod up, the hinge will have to act a force on the rod (the reaction force) with a different magnitude/direction. E.g. if you stopped holding the rod then it would hang vertically downwards so the hinge would need to act on the rod vertically upwards to keep it from falling. If you moved the rod up with your hand then the reaction would change direction to maintain equilbrium.

It is usually not obvious to know what the magnitude/direction of the reaction is so you what you normally do is show the vertical and horizontal component of this reaction force since every force can be split into two perpendicular components. And usually the best thing to do is to take moments about a hinge so you don't have to involve these two forces. But if you need the reaction at the hinge then you have to include these two forces in your diagram.

Reply 10

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