"Egg"streme force demonstration

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Applying Newton's laws can be tricky sometimes. Knowing when to apply the 2nd law and when to apply the 3rd law takes some practice. In this demonstration, an egg is placed between a big box and a little box. The boxes are pushed to the right with the force directed on the big box as shown in the picture. Then the boxes are pushed to the left with the SAME force, but this time the force is directed on the little box. Will the egg break?

To find out if the egg breaks, we first apply Newton's 2nd law, then the 3rd law. When a force is applied to our system, neglecting friction and assuming the egg is light compared to the boxes, its acceleration is F/(M1+M2) from the 2nd law. According the 3rd law, there are equal and opposite contact forces between the objects. These forces are shown in the diagram. Their magnitude, however, it NOT the same as the applied force. The force on M2 by M1 is the F21=M1a. This contact force is the force needed to give M1 the acceleration a. By Newton's 3rd law, this is also the force that is exerted on the egg. If M2 is light enough, the force will not be great enough to break the egg.

When the force is applied in the opposite direction, the force on the egg is the force needed to give M1 its acceleration. Since M1 is more massive than M2, the resulting contact force will be great enough to break the egg as shown in the movie.

1. Can there be a force without a reaction force?
2. Try this demo with a light book and a heavy book.(If you use the egg in between, put the egg in a platic bag to avoid damaging the books.)