Workshop 6: Force Against Force
Web Highlights
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.
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