Sorry, Sandra Bullock: A Fire Extinguisher Is a Lousy Thruster

If you stop pushing on it, it stops moving.That force model seems to work just about all the time. So if you push on an object with a force in the same direction that the object is moving, it will speed up.Here is that same cart, but now the fan is pushing in the opposite direction. I have to give the cart a push, and then this happens.In this case, the backwards force makes the cart slow down. Once it stops, the cart starts moving back to the right and speeds up—since it's now a forwards force.I couldn't do this with the cart, so I used a yo-yo instead. Since friction is just about everywhere, we think that pushing with a constant force makes something move at a constant speed. There is a constant force from the fire extinguisher that pushes in the direction of the arrow.