Horizontal Blocks in Contact
Horizontal blocks in contact is a classic problem that considers both Newton’s 2nd and 3rd laws. Consider two objects A and B, with masses of 5kg and 3kg respectively, that are being pushed with a force of 40N to the right as shown below:
Newtons 3rd Law predicts that A exerts a force on B and B exerts an equal but opposite force on B:
We can consider a few aspects of this problem:
If the total force acting on both objects is 40N and they have a combined mass of 8kg, according to Newton’s 2nd law the acceleration of both objects:
We can now determine the net force acting on each object:
Based on Newtons 3rd we can see that the reaction force of B on A;
We can now observe the net force acting on A, which agrees with our working above.
Solving these problems requires an understanding of the forces analysed above.
In the diagram below, box A has a mass of 8kg and box B has a mass of 5kg. A force of 78N acts on the boxes to the right. Calculate the following:
a) the acceleration of the boxes
b) the force acting on box A and B
c) the reaction force of B on A
a) Using Newtons 2nd law:
b) Again, using and the answer to part a:
c) The reaction force of B on A is equal and opposite to the force of A on B: