Come for the cat video, stay for the science…

This showed up in my Facebook feed the other day.

As I watched this video about six or seven times in a row, you know I couldn’t help but consider the physics behind the poor kitty’s plight. We’ve discussed how an object only changes its motion (starts moving, stops moving, changes speed, changes direction) if an overall force acts on it. When a cat jumps off the floor, what’s applying the force that allows her to start moving upward?

Oddly enough, the floor is.

Explaining this situation starts with a discussion of Newton’s Third Law of Motion. You may have heard it phrased this way: “For every action, there is an equal and opposite reaction.” While technically accurate, this wording can lead to misconceptions. I usually say “for every force, there is an equal and opposite force.”

Newton’s Third Law means that if Object A exerts a force on Object B, Object B exerts an equally strong force on Object A in the opposite direction. For example, if a basketball hits the ground with a force of 25 Newtons, then the ground also exerts a 25 N force on the basketball. Note that a 25 N force causes the ball to bounce upwards but has no effect on the motion of the ground; the two objects have different reactions to equal forces because of Newton’s Second Law. Using the phrase “equal and opposite reaction” can lead people to people to believe that the floor should exhibit the same change in motion as the basketball.

Let’s apply Newton’s Third Law to our unfortunate cat. When he uses his paws to push down and backward on the floor, the floor exerts equal forces on him in the up and forward directions. On a non-waxed floor, these forces are strong enough to accelerate the cat into his cage, but a fresh coat of wax reduces the friction between the floor and the cat. Less friction means that less force needs to be applied to an object to make it start sliding. This reduces the maximum amount of force the floor can exert on the cat, and this reduced force is no longer strong enough to give the cat the necessary acceleration.

Between this and the whole Schrödinger thing, cats just can’t seem to catch a break in physics.

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