Newton's Second Law -- Accelerations in a pulley system

  • #1

Homework Statement


A 20-kg block with a pulley attached slides along a frictionless ledge. It is connected by a massless string to a 5.0-kg block via the arrangement shown in Figure. (a) draw complete free body diagrams for the masses and Find (b) the acceleration of each block and (c) the tension in he connecting string.

Homework Equations


net force= ma
Fweight=ga
Fnormal=Fweight

The Attempt at a Solution


aTension=aTension
Fweight-Ftenison=5a
Ftension=20a
Fweight-20a=5a
ga=25a
 

Answers and Replies

  • #2
gneill
Mentor
20,804
2,784
Hi Jonathan, Welcome to Physics Forums.

The problem mentions a figure that is absent from your post. Without it it would be difficult to solve the problem or evaluate your solution efforts. Can you post the image? Did you have a particular question about the work you've shown?
 
  • #3

Homework Statement


A 20-kg block with a pulley attached slides along a frictionless ledge. It is connected by a massless string to a 5.0-kg block via the arrangement shown in Figure. (a) draw complete free body diagrams for the masses and Find (b) the acceleration of each block and (c) the tension in he connecting string.
upload_2017-2-28_10-35-32.png


Homework Equations


net force= ma
Fweight=ga
Fnormal=Fweight

The Attempt at a Solution


aTension=aTension
Fweight-Ftenison=5a
Ftension=20a
Fweight-20a=5a
ga=25a
 
  • #4
gneill
Mentor
20,804
2,784
How does the pulley arrangement affect the relative rates of motion of the two blocks (do they both experience the same acceleration)? How does the pulley arrangement affect the net force delivered through string tension to each block?

In other words, does mechanical advantage play a role in this problem? If so, does your current work reflect this?
 
  • #5
They both experience the same acceleration. The tension pulling both blocks will be the same because the string is massless
 
  • #6
I don't believe mechanical advantage would play a role in this problem.
 
  • #7
gneill
Mentor
20,804
2,784
They both experience the same acceleration. The tension pulling both blocks will be the same because the string is massless
I don't believe mechanical advantage would play a role in this problem.
Better check again! :wink:

Make yourself a sketch of just the block on the horizontal surface's pulley and its string connections. Suppose the pulley moves distance x to the right. How far does the top string have to move to take up the slack created?

upload_2017-2-28_16-4-58.png
 
  • #8
No it actually doesn't play a role in this problem. We havent learned that yet.
 
  • #9
gneill
Mentor
20,804
2,784
No it actually doesn't play a role in this problem. We havent learned that yet.
Nevertheless, solving this problem requires that you take into account the relative motions of the two blocks and the sum of tension forces at each block. If the problem requires concepts that you haven't learned yet, you should speak to your instructor.
 

Related Threads on Newton's Second Law -- Accelerations in a pulley system

Replies
7
Views
1K
  • Last Post
Replies
11
Views
15K
  • Last Post
Replies
4
Views
10K
  • Last Post
Replies
8
Views
39K
Replies
2
Views
4K
  • Last Post
Replies
2
Views
1K
Replies
15
Views
11K
Replies
3
Views
2K
Replies
14
Views
1K
Replies
4
Views
727
Top