Hello I have a test in 2 days and I know problems like this will be on it but I don’t understand it at all, could someone answer this question so I have something to work of off.

Three boxes are being pushed together on a surface with friction - A followed by B and then C. The first box has a mass of 25 kg while the second box has a mass of 45 kg and the third box he has a mass of 15 kg if an applied force of 275 N is applied on the box, the acceleration is measured at 1.4 m/s^2 [forward]

calculate the coefficient of kinetic friction and the reaction force of the box C on box B.

If someone could explain the process or order In how to get to the point of calculating. Like do I get the acceleration and from there the force then from there the weight. Which order do I calculate in?

I’m not asking for you to do the question I want to know how I get to the point where I can answer the question. What’s do I solve for first?

Thanks for anyone who helps in advance


1 Answer 1


For finding friction you can treat the three blocks as a single block and write equations accordingly from this you can get the coefficient of friction.

The reason why we do this is that we don't have to account for the forces between the blocks themselves which we can find later with the coefficient of friction.

Now all you need to do is draw free body diagrams for blocks A and C, It will be unnecessary for B. With A's free-body diagram you will get the reaction forces between A and C which will be useful in the free body diagram for C.

Hope this was easy to follow and good luck on the test!


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