What is inertia?
Inertia is the tendency of an object to resist changes in its motion. An object at rest stays at rest, and a moving object keeps moving in the same direction unless a force acts on it.
Inertia is described by Isaac Newton's first law of motion. The more mass an object has, the more inertia it has, and the harder it is to start, stop or change its direction. Pushing an empty shopping cart is easy, but a full one takes more effort to start moving and to stop.
You feel inertia every day. When a bus brakes suddenly, your body keeps moving forward because of inertia. When a car turns sharply, you feel pushed to the side as your body tries to keep going straight. Seat belts protect people by stopping their forward motion in a crash.
A classic trick
The tablecloth trick shows inertia at work. If a tablecloth is pulled away quickly enough, the dishes on top stay almost in place, because their inertia resists the sudden movement and friction acts only briefly. Italian scientist Galileo Galilei studied inertia before Newton described it in his laws.
Did You Know?
In space, where there is almost no friction or air resistance, a moving spacecraft can coast for years without its engines, thanks to inertia.