It's all about resultant force
What the activity is for
The purpose of the activity is to provide a memorable event that offers the opportunity to talk about whether the resultant force is zero or not. You'll be focusing on horizontal forces only. Depending on the class, you may need to make this explicit.
What to prepare
- rope with marker (possibly a red ribbon) in the middle
Use a substantial rope, perhaps a climbing rope. Take care than any pupils who stumble do not fall back onto furniture. Arranging the activity in a playground or a corridor is a safer option.
What happens during this activity
Start with the rope and red ribbon and no pupils pulling. Emphasise the point that we are interested in the state of motion of the ribbon. Why does the marker remain initially unmoved? (no forces acting – equilibrium). Then encourage two pupils on each end of the rope to
take the strain so that the marker still does not move. Lead a discussion focusing on the idea that the resultant force acting on the rope must be zero. This is another equilibrium situation.
Now add an extra two people to one side of the rope, and get the pupils to
take the strain again. This time, the marker will change its motion: moving towards the side that has more people on it. Invite pupils to suggest why. Encourage the use of the terms
changing motion and
The marker moves because the forces acting on it do not add to zero. Four people provide a greater force than two people and so the marker, which is initially at rest, speeds up and moves in the direction of the greater force (towards the four people). The general point is that a change of motion always follows when a resultant force acts on an object.