Teacher resources and professional development across the curriculum

Teacher professional development and classroom resources across the curriculum

Workshop 6: Force Against Force

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Workshop 6's Featured Classroom

Harwich Elementary School, Harwich, Massachusetts, Grade 4,
Teacher: Janet Smithers

Answer to Workshop 6's Going Further
Can You Find All the Forces?

1. We see the pan balance in equilibrium right before the force is broken. The forces are balanced. The right pan has the weight of the washers and cup and the left pan has both the weight of the magnet and cap plus the force due to the attraction of the two magnets. Did you remember that the fulcrum has to push up with a force equal to the sum of the left and right hand pans? You may even have thought of other forces like the base of the balance pushing on the table and the table pushing on the base of the balance!

2. Regardless of the mass of the two people, regardless of how hard they pull, the two forces will be equal. The effects of the forces may be different but the magnitudes will be the same.

3. Were you fooled by the table? Since the forces are equal and opposite the two weights will be in equilibrium. The pulley only changes the direction of the forces. In fact the two weights will be stable at any position until one of them touches the pulley or the floor. This principle of using a counterweight to balance a force is used in elevators and some draw bridges. Moving the system only requires a small force, to overcome friction.

4. This system is in equilibrium, too. Some people think that the scale reading should be either 0 or 20 Newtons, but remember that forces come in pairs. For the spring to stretch and move the pointer to a 10 Newton reading the other end of the spring attached to the tube must also be pulled with a force of 10 Newtons in the opposite direction. Think about simply hanging the 10 Newton weight on a scale attached to the ceiling.