String with mass pulling block up frictionless ramp

  • #1
784
11

Homework Statement


A block of mass M has a string of mass m attached to it. A force F is applied to the string and it pulls the block up a frictionless plane that is inclined at an angle theta. Find the force that the string exerts on the block


Homework Equations


F=ma


The Attempt at a Solution


Force of the string on the block = Force of the block on the string = N
Force applied to string = F
acceleration of block = acceleration of string = a[/B]

My first equation was the net-force equation for the string:
ma = -mg*sin(theta) - N + F


My second equation was the net-force equation for the block:
Ma = -Mg*sin(theta) + N


solving for the acceleration in the top equation and plugging it into the second equation gives the following (after a small amount of algebra)

N = (F/m)*(1+M/m)^-1

Which is not the answer I am supposed to be getting. I am guessing the problem is with my net-force equations but I can't located a specific problem. Anyone want to explain what I'm doing wrong here?
 

Answers and Replies

  • #2
25
3
first try and write the net-force equation for the whole system (meaning substitute a block M and a string m, with a new object with mass M+m) Now we can forget about internal forces between mass and string. so we just have two force, the force f we apply and the one due to gravity. This way you can find a (acceleration) of the objects.
 
  • Like
Likes PsychonautQQ
  • #3
haruspex
Science Advisor
Homework Helper
Insights Author
Gold Member
2020 Award
37,150
7,273
Your initial equations are ok but you must have gone wrong in the subsequent algebra. Your final equation is not dimensionally consistent.
 
  • Like
Likes PsychonautQQ

Related Threads on String with mass pulling block up frictionless ramp

Replies
2
Views
24K
  • Last Post
Replies
13
Views
3K
Replies
1
Views
4K
  • Last Post
Replies
1
Views
5K
Replies
6
Views
2K
  • Last Post
Replies
4
Views
4K
Replies
0
Views
3K
Replies
5
Views
1K
Top