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# I Need Help With an AP Physics Problem.

. Posts: 1,170 Senior Member
edited October 2005
88.) Two blocks are connected by a rope that passes over a set of pulleys. One block has a weight of 412 N, and the other has a weight of 908 N. The rope and pulleys are massless and there is no friction.

(a.) What is the acceleration of the ligher block?

(b.) Suppose that the heavier block is removed and a downward force of 908 N is provided by someone pulling on the rope.

(c.) Explain why the answers in (a) and (b) are different.

I only need help with (a) and (b), I understand that the acceleration is going to be much less in (a), because the two blocks share inertia while in (b), only the smaller block remains.

But how do you calculate the acceleration in (a) and (b)?

I've been spending a long time on this problem and I would be grateful for help.

Thanks!
Post edited by CoolaTroopa on

## Replies to: I Need Help With an AP Physics Problem.

• Registered User Posts: 40 Junior Member
Draw a free mass diagram for both of them. What is the net force on the blocks?
• Registered User Posts: 774 Member
(a) The mass of the lighter block is 412/9.8 kg, the heavier one is 908/9.8 kg. The total mass being accelerated is (412 + 908)/9.8 kg, the net force applied is 908-412 N.
Acceleration = F/m = (908-412)/ ((412+908/9.8)

(b) Total mass is now 412/9.8 kg, net force = (908-412) N. Acceleration = F/m = (908-412) / ((412/9.8))

(c) Total mass being accelerated is different in (a) and (b)
• . Posts: 1,170 Senior Member
Thanks a lot!!

I didn't realize that there was no need to calculate the Tension force in the problem. I spent so much time trying to calculate Tension that it stopped me from finding the answer.

Yet again, thanks!
This discussion has been closed.