Force A push or pull on an object. For example, gravity is the force that pulls objects towards Earth's center. When an object is at rest, any force acting on it is balanced by an equal force in the opposite direction. If a new, unbalanced force acts on the object, the object will move in the same direction as the new force.
Imagine a house being pulled toward Earth's center by gravity, while the ground is pushing back up on the house. If the forces are equal (the house is on solid ground), the house doesn't move. If the forces are unbalanced (the ground is soft and muddy and pushes back with less force), then the house sinks down into the ground. If a new force, such as a bulldozer, acts on the house without a balancing force in the opposite direction, the house will move in the direction the bulldozer is pushing.
It's hard to visualize that many different forces may be acting on a building that seems to be standing still. Try this activity to help kids imagine these unseen forces.
1. Place a chair in the middle of the floor. Ask: Are any forces acting on this chair? (Kids will probably say no.)
2. Invite a kid to gently push the chair a short distance across the floor. Ask: What force just acted on this chair? (a push that made it move; an unbalanced force)