Two blocks sliding on top of each other

[pharoh123]

A 4 kg block is placed on top of a 8kg block, placed on a table with no friction,which is subject to force of 30N. What is the coefficient of friction needed between the two blocks so that the the top doesn't slide on the other one?

What i did was find the acceleration of the bottom block and then i found the force needed to push the top block at that acceleration. I considered that force the friction and found the coefficient. However, I don't think i got it right since i thought friction couldn't actually move an object and is only a resistance. So did i get right or not?
Thanks

 

[Redbelly98]

Welcome to PF

did i get right or not?

If you show your work, we can tell you. At the very least post your answer, but showing the entire work is better.

 

[tomcue]

for your question. Your approach to finding the coefficient of friction is a valid one. However, you are correct in thinking that friction cannot actually move an object. Friction is a force that opposes motion, and in this case, it would act in the opposite direction of the applied force of 30N.

To find the coefficient of friction between the two blocks, you can use the formula μ = Ff/N, where μ is the coefficient of friction, Ff is the force of friction, and N is the normal force (in this case, the weight of the top block).

In this scenario, the force of friction would be equal to the applied force of 30N, as the blocks are not sliding and therefore the friction force is equal to the applied force. The normal force, or weight, of the top block would be 4kg multiplied by the acceleration due to gravity (9.8 m/s^2), which is equal to 39.2N.

Therefore, the coefficient of friction would be μ = 30N/39.2N = 0.76. This means that for the top block to not slide on the bottom block, the coefficient of friction between them must be at least 0.76.

I hope this explanation helps. Keep up the good work in your scientific thinking and problem-solving!