Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS
BUILDING BIG
Bridges
Domes
Skyscrapers
Dams
Tunnels
Buy the DVD
Spacer image with 5 links within the image
The Educators Guide
How to use this guide
Program Descriptions
Planning Ideas
To Activities
Additional Resources
Buy the Video
  Mini-Activity: Forceful Furniture
Activites Index | Glossary Educator Support  

Definition 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.) 

Illustration: chair.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) 

Illustration: Chair3. Repeat Step 2, this time with a second kid pushing back on the chair in the opposite direction so that it doesn't move. Ask: Are any forces acting on the chair? Why doesn't it move? (Although two forces are acting on the chair, they are balanced. Therefore, the chair doesn't move.) 

Illustration: Chair.4. Now repeat Step 1. Explain that even without someone pushing on it, forces are acting on the chair. The force of gravity is pulling down on the chair. But since the chair is not moving, there must be an equal force in the opposite direction–the floor is pushing back up.  

Try the Forces Lab.  

BUILDING BIG Home Page BUILDING BIG Site Map BUILDING BIG Labs BUILDING BIG Databank BUILDING BIG Glossary