Homework Help: Physics Question involving a person pulling himself up

1. Oct 8, 2012

CoolGod

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.

Last edited by a moderator: May 6, 2017
2. Oct 8, 2012

256bits

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.

That would be a completely seperate problem.

Best to draw one or 2 FBD ( free body diagram ), perhaps one of the man and one of the platfrom.

3. Oct 8, 2012

CoolGod

Hmm 256bits i don't understand where you are confused. fundamentally is he going up or down?

4. Oct 8, 2012

5. Oct 8, 2012

CoolGod

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.

Last edited: Oct 8, 2012
6. Oct 8, 2012

256bits

Looks OK to me .