In this topic you will learn about how pushes and pulls change the motion of objects.

Some things are harder to push or pull than others. You might use a big push to push your bike up a hill. A small pull might be needed to open a book. A spring scale can be used to measure pulls. Spring scales measure in newtons. A newton is the unit used to measure pushes and pulls.

A force is a push or a pull. A force can change the motion of an object. You see examples of forces in your everyday life. You use a pulling force when you put on your backpack. You use a pushing force when you open a door.

One force that is everywhere is the force of gravity. Gravity is the pulling force between two objects. It attracts or pulls objects together. It pulls everything on Earth. The force of gravity between two objects depends on two things. It depends on how much matter makes up the objects and how close they are to each other. The more matter there is in objects, and the closer the objects are to each other, the stronger the force of gravity between them will be. Things fall to Earth because they are pulled by Earth's gravity. The pull between Earth and other objects is very strong.

Some objects are heavy. Others are light. You can measure how heavy or light something is by measuring its weight. Weight is the pull of gravity on an object. Objects that weigh more have more matter in them. The more matter there is in an object, the greater the pull of gravity is on that object. Scientists measure weight in newtons. People in the United States measure weight in pounds. A pound is the unit used to measure forces (such as weight) in the English system of measurement. A newton is the unit of force in the metric system.