Rocket sets a toy car at the top of a book ramp. He lets go. It rolls down by itself!
"Nobody pushed it," he says. "How did it move?"
Raven watches it roll across the floor and stop. "What do you notice at the end?"
Nova hovers by the ramp. "Two hidden forces are at work," she says. "One pulls. One slows."
Gravity is a force that pulls things toward the center of Earth.
Gravity keeps you on the ground. It makes things fall when you drop them.
On a slope, gravity pulls a toy car down the ramp.
You cannot see gravity, but you can see what it does.
A rolling ball on the ground stops after a while.
A force that works against motion slows it down. That force is called friction.
Friction happens where two surfaces rub together.
Next week we will study friction closely.
Super science! Tomorrow we learn how to plan a fair push test, just like real scientists.