You can easily apply a certain law of physics to kids: a child at rest tends to stay at rest and a child in motion tends to remain in motion. That's why your child should have no difficulty grasping the concept of inertia. Inertia measures the difficulty in changing the momentum of an object when it's in motion, either speeding up or slowing down. Kids learn that mass matters as heavier objects have more inertia.
Push and Pull
For this activity, you'll need a tray, wooden ball and steel ball. Have your child place the wooden ball in the center of the tray. Have him quickly pull the tray toward him. Have him repeat the action, but have him push the tray away from him the second time. Try it again with the steel ball. For fun, repeat the experiment another time, but place both balls in the center of the tray together. Ask him why the balls moved at different speeds.
A simple demonstration of inertia involves five books and a chair. Have the child stack the books on the edge of the chair's seat, far enough back that they won't topple off the edge while the chair is still. Leave the chair for a minute so she can see that the books stay stacked. Next, the child should push the chair forward quickly and stop suddenly. Stack the books again and try pulling the chair backward. Compare how the books reacted to the motion. Ask the child if she can think of different ways to stop the books from falling when the chair moves or when it suddenly stops.
Riding a Bike
Bike riding is a good demonstration of how inertia works. Get your child to hop onto his bike. When he first starts pedaling, he has to push hard to get the bike going. Once the bike is in motion, he doesn't stop instantly when he hits the brakes because of inertia. Ask your child if he's ever hit the brakes hard enough to stop the bike instantly and what happened. Explain that even though the bike stopped, he was still in motion because of the bike's momentum. That's why he moved forward -- and also why he should glide to a stop and not hit the brakes hard. You can also demonstrate inertia with a skateboard if your child prefers.
A large toy doll convertible or jeep and a pair of dolls or small stuffed animals are all you need to make a point about inertia and safety. The dolls need to fit into the front of the car. Ask your child what she thinks will happen if she pushes the car at a normal speed and then stops. Ask her what will happen if she pushes the car as hard as she can into the wall or other barrier. Lastly, ask her what will happen if you push it as hard as you can and she suddenly reaches out to stop it. Perform the experiment to see whether her predictions come true. After each drive, ask her how she thinks her dolls are doing and if they got hurt. Point out that the experiment also shows why we always wear seat belts in the car.