Inertia and Net Forces

Meet Inertia

Every object in the universe has a property called inertia: the tendency of an object to keep doing whatever it's already doing — staying still if it's still, or continuing to move in a straight line at a steady speed if it's moving — unless something pushes or pulls on it to change that. Inertia isn't a force itself; it's more like an object's built-in stubbornness about changing its motion.

When the crash-test dummy is sitting in the open truck bed while the truck speeds along, the dummy is moving forward at the same speed as the truck. Its inertia means it "wants" to keep moving forward at that same speed, in that same straight line, unless some force acts on it to slow it down or change its path.

What Happens When the Truck Suddenly Stops

The instant the truck crashes into the pole, the truck itself experiences a huge force from the pole that brings it to a screeching halt. But the dummy is a separate object, only loosely resting on the truck bed — nothing is gripping it or strapping it down. Because of inertia, the dummy keeps moving forward at its original speed, right past the point where the truck stopped. From the dummy's point of view, it just keeps going in a straight line — it's the truck that suddenly disappeared out from under it. That's exactly why the dummy flies off the back: not because some mysterious force throws it forward, but because nothing stopped it from continuing to do what it was already doing.

Mass, Inertia, and Net Force

Not all objects resist changes to their motion equally. The greater an object's mass — the amount of matter it's made of — the greater its inertia, meaning it takes a bigger force to speed it up, slow it down, or change its direction. A loaded moving truck has far more inertia than a skateboard, which is why it's so much harder to stop.

To actually change an object's motion, you need an unbalanced, or net, force acting on it. Forces are pushes or pulls, and objects often have multiple forces acting on them at once. When those forces cancel each other out completely, they're called balanced forces, and the object's motion doesn't change at all. But when the forces don't cancel out, there's a net force, and that net force is what changes an object's speed or direction. The seatbelt in a real car provides exactly this kind of net force on a passenger during a crash — pulling backward on them so they slow down along with the car, instead of continuing forward like our unfortunate crash-test dummy.