Explain that gravity is the attraction between two objects. One way to demonstrate this is to drop a solid object such as a pencil to the ground. The free fall of the pencil toward the ground shows the pull of the earth's gravity on it.
Demonstrate friction, which is the force on an object against motion. Roll a ball down a long, smooth surface that is long enough for friction to stop it (such as a hallway). Have students observe how the ball slows and then stops. Roll the same ball over a carpeted surface to show how the extra friction from the carpet slows the ball even faster.
Contrast gravity and friction by dropping a ball from the top of a playground ladder. Don't throw the ball--simply let it drop from your hand. Now lightly roll the same ball across the blacktop. In the first instance, gravity pulls at the ball and it does not lose speed as it slips through the air toward the earth. No matter what height the ball is dropped from, it will fall without losing speed unless it encounters a substance other than air. The ball rolls on the blacktop, slows due to friction and eventually stops. It is impossible to roll a ball on any surface and have it continue moving without stopping.
Explain to students how the engine in a car helps the car to overcome friction and continue to move, as long as it has fuel. Conversely, a parachute provides friction to a skydiver to stop gravity's pull toward the earth.
Drop two balls of the same size but different weights, such as a basketball and a beach ball, from a ladder. This shows students how the friction of the air acts on the lighter ball, slowing its progress.