Friction is a force that opposes motion. It is present whenever two surfaces rub over each other, such as when you rub your hands together, or when you apply the brakes on a bike or in a car.

Two forces can be added together to find a resultant force. In the diagram of a toy trailer below, when two children pull on the handle, one with a force of 3 newtons (N) to the left and one with ...

Martha loves to relate her science examples in practical, everyday terms, that the kids can identify with. For example: “Divide the speed of light (300 megameters per second) by the FM frequency on ...