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How does Skipping a Stone on Water work?

The classic outdoors pastime, with a scientific twist

You must have done this at some point in your life, or at least have been interested in doing it. Skipping a stone across water is a classic activity to do outdoors, but have you ever wondered how it works? There’s actually a lot of physics behind it, but I’ll try and keep it as simple as possible.

It’s all about the way you throw it

When you throw the stone, its is important to keep in mind that it must be thrown at the right angle and also assuming you have the perfect stone size and shape (preferably flat) that is thrown with the right amount of force. Assuming all those conditions, we imagine that the stone is thrown with a certain velocity in a projectile like path. Just before it strikes the surface of water, the only force acting on the stone is gravity. Once the stone hits the surface, the stone gets partially submerged for a split second and multiple forces act on it.

The first one of interest is the drag force from the water, which acts in the opposite direction of the stone’s motion. It is a retarding force that reduces the velocity of the stone. At the same time, there is a reactionary force that is in the opposite direction of the force of gravity. This reaction force is what keeps the stone afloat for that split second and pushes the motion of the stone from a downwards direction to an upwards direction, which we see as “the bounce.”

Energy is not conserved

Unfortunately, in the real world, the energy is not conserved in this system because of the inelastic collision that occurred between the stone and the surface of the water. This is why the velocity decreases and continues to decrease each time the stone strikes the surface of the water, until finally coming to a point where the stone finally sinks.