Physics Question involving a person pulling himself up

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In summary, a person is sitting on a swing/seat/platform and pulling himself up with a rope attached to a spring scale. The scale reads 250n, indicating that the person is exerting 250n of force. The person weighs 320n and the swing weighs 160n, resulting in a net downward force of 480n. However, due to the mechanical advantage of 2 created by the pulley system, the person is actually moving upwards with an acceleration of 0.40875 m/s^2. The force exerted by the person on the seat is 83.3333 N downwards.
  • #1
CoolGod
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A person is sitting on a swing/seat/platform and pulling himself up with a rope. Don't worry about the swing part. One end of the rope is attached to 2 other metal wires connected to the seat. The other end of the rope is attached to a spring scale and is being pulled by the person. The rope goes above and passes through a pulley. Like in this diagram. http://imageshack.us/a/img845/3343/phydia2.png [Broken]
the swing weighs 160n, the person weighs 320n and the scale reads 250n.Is the swing/seat/platform going up or down? What is the force exerted by the person on the seat. What is the acceleration of the system.
Since scale reads 250n that should be tension in the rope. he must be exerting 250n. Thus 320+160 >250 thus he is going down.
The hard part of this question is that when the person pulls the rope/scale does the rope or scale pull him back up? Because if it does then technically he'd be going up. In this question can i replace his pull with a 250n block of mass? If he is going up, how is it possible for someone to exert 250n and move 480n up without mechanical advantage.
 
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  • #2
Since scale reads 250n that should be tension in the rope. he must be exerting 250n. Thus 320+160 >250 thus he is going down.

Didn't I just read that the rope goes over a pulley. the rope is attached to the platfrom and the man is pulling on the other end.

Because if it does then technically he'd be going up. In this question can i replace his pull with a 250n block of mass

That would be a completely separate problem.

Best to draw one or 2 FBD ( free body diagram ), perhaps one of the man and one of the platfrom.
 
  • #3
Hmm 256bits i don't understand where you are confused. fundamentally is he going up or down?
 
  • #4
CoolGod said:
Hmm 256bits i don't understand where you are confused. fundamentally is he going up or down?

That's for you to figure out. Please show us your FBD(s).
 
  • #5
i solved for the answer and he is going up. Since he pulls 2 m for every m he goes up, and work is same then it is similar to mechanical advantage of 2. Thus ((2*250)-480)/(480/9.81)=0.40875 m/s^2. so chair is accelerating at 0.40875m/s^2 and the force acting on it is fg 160n down fpc (missing force)down and tension 250n up so we get (250-160)-(0.40875*(160/9.81)) for fpc or 83.3333 N down.
 
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  • #6
Looks OK to me .
 

1. How does the weight of the person affect the force required to pull themselves up?

The weight of the person affects the force required to pull themselves up because according to Newton's Second Law of Motion, the force required is directly proportional to the mass of the person. This means that the heavier the person, the more force is needed to overcome their weight and pull them up.

2. What role does gravity play in a person pulling themselves up?

Gravity plays a significant role in a person pulling themselves up because it is the force that pulls the person down towards the ground. In order to overcome this force, the person must exert an equal and opposite force to lift themselves up.

3. Is it easier to pull yourself up using a rope or a bar?

This depends on the specific situation and the individual's strength. Pulling up with a rope allows for a wider grip and the use of arm muscles, while pulling up with a bar allows for a more narrow grip and the use of back muscles. Ultimately, the easier method will vary from person to person.

4. Can a person pull themselves up indefinitely?

No, a person cannot pull themselves up indefinitely. This is due to the fact that muscles require energy to contract, and eventually they will become fatigued and unable to continue exerting force to pull the person up.

5. How does the angle at which the person pulls themselves up affect the required force?

The angle at which the person pulls themselves up affects the required force by changing the direction of the force needed to overcome their weight. For example, pulling straight up requires more force than pulling at an angle, as the force is split between pulling up and pulling towards the side.

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