Teacher resources and professional development across the curriculum

Teacher professional development and classroom resources across the curriculum

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Science in Focus: Force and Motion
About the Workshops
1. Making an Impact
2. Drag Races
3. When Rubber Meets the Road
5. Keep on Rolling
6. Force Against Force
7. The Lure of Magnetism
Supplemental Resource List

Workshop 2 Web Highlights

"Balanced Forces and Net Force"

If an object is not moving, we can be sure that the forces acting on it are balanced. There is no net force. As we will see in a later workshop, it is also true that there are no net forces acting on an object that moves at a constant speed in a straight line. No net force means no acceleration, and in both of these cases, the acceleration is zero. So, while we have seen that an object "at rest" has no force acting on it, it is also possible that a moving object has no net force acting on it either.


"Unbalanced and Balanced Forces"

If an object is speeding up, slowing down, or changing direction, a net force is needed to cause that change in motion. We use the term "net force" since there could be many forces acting on an object and we will only see the change if the forces are unbalanced or do not add up to zero. The leftover force is the net force that causes the object to accelerate.

In the simplest cases where only two forces act on an object, like a ball resting on your hand or a weight hanging from a spring scale, the two forces pushing upward and downward are equal and opposite. They add up to zero.


"Rolling Friction"

Friction is a force that opposes motion. It occurs when the surfaces of objects roll on or slide past each other. In the cars used in Barbara's class, the wheels fit in axles that in turn fit in holes on blocks mounted on either side of the car. The students adjusted the axles, wheel, and frame of their cars to minimize the amount of friction. If the wheels rub and do not turn freely, the car will be harder to move and will not roll as far. If the frame is bent rather than square, the front and rear axles will not be parallel, causing the wheels to slide rather than roll. We will be looking more closely at friction in later workshops.

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