Exploring Force Pulleys: The Surprising Truth Behind Non-Net Forces on Objects

[aspodkfpo]

Homework Statement

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Relevant Equations

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If a free body diagram is constructed for 1kg, one might be inclined to draw an arrow downwards with only the weight of 10N. However, the downward, non-net, force on 1 kg is (1+5) x 9.8. Why is this the case? It seems very counterintuitive.

 

[ehild]

Homework Statement:: n/a
Relevant Equations:: n/a

View attachment 267189

If a free body diagram is constructed for 1kg, one might be inclined to draw an arrow downwards with only the weight of 10N. However, the downward, non-net, force on 1 kg is (1+5) x 9.8. Why is this the case? It seems very counterintuitive.

It is not true, you misunderstood something. There are to forces acting on the 1 kg mass, its weight, 1*g downward, and the tension T1 in the string, upward.

 

[aspodkfpo]

It is not true, you misunderstood something. There are to forces acting on the 1 kg mass, its weight, 1*g downward, and the tension T1 in the string, upward.

Consider the entire system.
(1+5) x 10 is the force acting on the entire system. This means that the system travels with an acceleration of 5ms^-2. The tension in the string to the right of 6kg is 30N. 5kg has to have a net force of 25N, so between 5kg and 1kg, string is at 55N. 1kg has to travel at 5ms^-2, so downwards on 1kg is 60N.

I don't quite see how it is intuitive to realize it is 60N pointing in the downward direction on the 1kg.

 

[Lnewqban]

Cut the string between the two hanging masses and remove the smallest mass.
How much force your hand should pull down the section of remaining string hanging from mass 5kg to keep the original tension in longer string?

 

[kuruman]

Homework Statement:: n/a
Relevant Equations:: n/a

View attachment 267189

If a free body diagram is constructed for 1kg, one might be inclined to draw an arrow downwards with only the weight of 10N. However, the downward, non-net, force on 1 kg is (1+5) x 9.8. Why is this the case? It seems very counterintuitive.

I can think of two forces acting on the 1-kg mass: The tension in the string (up) which is exerted by the string and gravity (down) which is exerted by the Earth. Their sum is the net force and that is directed down. What force is the "downward non-net force" and what entity exerts it?

 

[ehild]

Consider the entire system.
(1+5) x 10 is the force acting on the entire system. This means that the system travels with an acceleration of 5ms^-2. The tension in the string to the right of 6kg is 30N. 5kg has to have a net force of 25N,

correct so far, but wrong after.

so between 5kg and 1kg, string is at 55N. 1kg has to travel at 5ms^-2, so downwards on 1kg is 60N.

I don't quite see how it is intuitive to realize it is 60N pointing in the downward direction on the 1kg.

There are three forces acting on the 5 kg mass. The tension in the upper string, 30 N upwards
the weight, 50 N, downwards
and the tension T1, connecting 5kg an 1 kg. It acts downward. The 5 kg mass accelerates downward with 5m/s₂, so the net force acting on it is T1+50-30=25 N. what is the value of T1 then?