# Stuck on a mechanics Q

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#1
Here is the Q below

I have made an attempt at it, but I'm getting nowhere as I end up getting 2 unknowns at the end therefore I can't solve for what the mass is which means I can't find out the weight

Any help would be nice. Thanks
Last edited by Yatayyat; 1 year ago
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1 year ago
#2
(Original post by Yatayyat)
Here is the Q below

I have made an attempt at it, but I'm getting nowhere as I end up getting 2 unknowns at the end therefore I can't solve for what the mass which means I can't find out the weight

Any help would be nice. Thanks
What would be the acceleration when the mass is just about heavy enough to cause movement?
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#3
(Original post by mqb2766)
What would be the acceleration when the mass is just about heavy enough to cause movement?
Well the block can only move to the right once the tension on the rope becomes more than or equal to the frictional force pushing it back.

If tension balances with the frictional force then it could supposedly be moving at a steady speed since the resultant force becomes zero?

So in order for the block to accelerate forward, it must exceed the frictional force there. I know that the frictional force is '0.26 * 0.10g = 0.2548N' equating that to 'ma' where F (force) is 0.2548N, then a = 0.2548/0.10 = 2.548 ms^-2.

So the acceleration must be bigger than 2.548 ms^-2 for the block to cause movement?
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1 year ago
#4
(Original post by Yatayyat)
Well the block can only move to the right once the tension on the rope becomes more than or equal to the frictional force pushing it back.

If tension balances with the frictional force then it could supposedly be moving at a steady speed since the resultant force becomes zero?

So in order for the block to accelerate forward, it must exceed the frictional force there. I know that the frictional force is '0.26 * 0.10g = 0.2548N' equating that to 'ma' where F (force) is 0.2548N, then a = 0.2548/0.10 = 2.548 ms^-2.

So the acceleration must be bigger than 2.548 ms^-2 for the block to cause movement?
The acceleration is zero? Increeasing the mass just enough will break the equilibrium.
So solve for T and m with a=0.
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#5
(Original post by mqb2766)
The acceleration is zero? Increeasing the mass just enough will break the equilibrium.
So solve for T and m with a=0.
I don't quite get why a= 0 for the block to move?
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1 year ago
#6
(Original post by Yatayyat)
Here is the Q below

I have made an attempt at it, but I'm getting nowhere as I end up getting 2 unknowns at the end therefore I can't solve for what the mass is which means I can't find out the weight

Any help would be nice. Thanks
It's a static problem - you need W to just match the frictional force. There's no need to have equations with acceleration or tension.
Last edited by RogerOxon; 1 year ago
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1 year ago
#7
If the block is light, there is no movement so a=0. Its in equilibrium.
Increasing the mass slowly there will become a critical point where movement is just about to start. Here a=0, but your equations hold.
When the mass in larger than the critical value, the acceleration will be higher.
Last edited by mqb2766; 1 year ago
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1 year ago
#8
(Original post by Yatayyat)
I don't quite get why a= 0 for the block to move?
You're looking for the point where the frictional force is just overcome, i.e. by an infinitesimal amount.
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#9
Yes, thank you I can see now why a = 0 here

I have tried subbing in a = 0 and have got around to finding the weight to be 0.2548N

So in actual fact I could have just worked out what the frictional force was to be and know that would be equal to the weight hanging down. As we're only looking for the point where frictional force is overcome like you have said for movement just about to occur.
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1 year ago
#10
(Original post by Yatayyat)
Yes, thank you I can see now why a = 0 here

I have tried subbing in a = 0 and have got around to finding the weight to be 0.2548N

So in actual fact I could have just worked out what the frictional force was to be and know that would be equal to the weight hanging down. As we're only looking for the point where frictional force is overcome like you have said for movement just about to occur.
Yes, in equilibrium the tension makes the forces equal.
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1 year ago
#11
(Original post by Yatayyat)
Here is the Q below

I have made an attempt at it, but I'm getting nowhere as I end up getting 2 unknowns at the end therefore I can't solve for what the mass is which means I can't find out the weight

Any help would be nice. Thanks
Well, I don’t know for certain that what I’m suggesting is right but it might give you some inspiration. The block isn’t actually moving you just need it to move. So perhaps there’s no acceleration yet.
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