Inertia is the idea that things like to keep doing what they're already doing. If something is not moving, it will stay still unless you push or pull it. If it's moving, it will keep moving at the same speed and in the same direction unless something makes it stop or change direction.

Inertial mass tells us how hard it is to change the way something is moving. If something has a big inertial mass, it's tougher to make it speed up, slow down, or change direction.

The formula to figure this out is:

But don't worry about the formula too much—just remember that bigger things need more force to change their movement.

Key Idea

• If something is really heavy (has a large inertial mass), you need to push it harder to get it moving as fast as something lighter.

Real-Life Example Think about trying to push a car versus pushing a shopping trolley. The car is much harder to push because it has a bigger inertial mass. The trolley is easier to move because it's lighter and has a smaller inertial mass.