Homework Help: Mechanics lift problem

1. Oct 22, 2012

bubakazouba

1. The problem statement, all variables and given/known data
a light scale pan is attatched to a vertical inextensible sring. The scale-pan carries two masses A and B. The mass of A is 400g and the mass of B is 600g. A rests on top of B, as shown in the diagram.
The scale-pan is raised verticaly, using the string with acceleration 0.5 m/s².
a)find the tension in the string.
b)find the force exerted on mass B by mass A
c)find the force exerted on mass B by the scale-pan.

visual representation of the problem http://i1269.photobucket.com/albums/jj597/bubakazouba/test_zps62ac1992.png [Broken]

2. Relevant equations
F=ma
g=9.8 m/s²

3. The attempt at a solution
I got a) correct, I just said
Tension-(0.4x9.8 +0.6x9.8)=0.4x0.5+0.6x0.5
Tension-9.8=0.5
tension =9.8+0.5=10.3N
--------------------
b)find the force exerted on mass B by mass A
I think the answer is mg its the weight, because its the only downward force on A so it should be 0.4x9.8=3.92, but its not right.
What am i missing here?

Last edited by a moderator: May 6, 2017
2. Oct 22, 2012

tiny-tim

hi bubakazouba!

3. Oct 22, 2012

bubakazouba

4. Oct 22, 2012

tiny-tim

you haven't used it

5. Oct 22, 2012

bubakazouba

why should I use it?

6. Oct 22, 2012

tiny-tim

i] it's in the question

ii] F = ma

7. Oct 22, 2012

bubakazouba

I dont get it why should I use any equation, isn't it obvious the only force that acts on B is the weight, mg

8. Oct 22, 2012

tiny-tim

that's the LHS of the equation

9. Oct 22, 2012

bubakazouba

what equation?

10. Oct 22, 2012

bubakazouba

I feel so stupid right now :'(

11. Oct 22, 2012

Staff: Mentor

Draw a free body diagram on A, showing the forces acting on it. Write a force balance on A, recognizing that A is accelerating upward at a rate of 0.5 m/s2.

12. Oct 23, 2012

bubakazouba

ok I got a force 4.12N upwards what does that mean?

13. Oct 23, 2012

Staff: Mentor

That's the contact force exerted by mass B on mass A. Now, from Newton's third law of action-reaction, what is the contact force that mass A exerts on mass B?

14. Oct 23, 2012

bubakazouba

4.12?
but why, why isn't it the weight?

15. Oct 23, 2012

bubakazouba

4.12?
but why, why isn't it the weight?

16. Oct 23, 2012

Staff: Mentor

Because mass A is accelerating upward, and so mass B has to exert enough upward contact force not only to support the weight of mass A (which would be the case if A were in equilibrium), but also to accelerate it. Also, by Newton's third law, the contact force A exerts on B is equal in magnitude and opposite in direction to the contact force B exerts on A.