Forces without motion
What the activity is for
The purpose of this activity is to revise with pupils the idea that if an object is not moving there can be a number of forces acting on it that balance each other out.
What to prepare
- a book on a table
- a ping pong ball balanced on a jet of air
- a mass hung from a spring
- a person standing on the floor
- a tug of war rope with two people on each side holding the rope stationary
What happens during this activity
Demonstrate each of the above to the class and ask the following questions:
- What forces are acting on the object?
- What can you say about the direction of the forces?
- What can you say about the size of the forces?
- What can you say about the overall effect of the forces?
Conclude by getting the class to draw up a statement about what we know about such forces which add to zero (e.g. for a stationary object, the forces acting on it add to zero). This is a review of the work covered in episode 01 of the SPT: Forces topic.
Pupils may agree that equal and opposite forces can keep an object stationary, but it is more difficult to accept that when an object moves with a constant speed the forces are also equal and opposite. Encourage pupils to talk about how the speed of a cyclist changes as they start a journey and why. (Initially the driving force is greater than the retarding forces and the bike speeds up). Ask any cyclists in the class to explain why they don't keep going faster, even though they keep pedalling. This should lead to the idea of retarding forces (slip and drag) and the fact that these oppose motion. You can feel the effects of both at quite low speeds by using rough surfaces or large surface areas. The aim here is to encourage pupils to move away from the stationary object situation towards talking about forces acting on a moving object.