Practical Activity for 14-16 16-19
- Activity time 10 mins
In this demonstration students see a simple rocket in action. You can use it to illustrate Newton’s third law of motion.
- Drinking straw
- Clothes peg or other clip
- Length of string
- Sticky tape
Locate suitable fixed points in the room (eg cupboard handles) to tie the length of string to.
- Pass the string through the straw.
- Attach the two ends of the string to the fixed points in the room.
- Inflate the balloon and use the clothes peg to close the mouth.
- Attach the balloon to the straw using sticky tape.
- Undo the peg to release the air.
- What keeps a balloon inflated?
- Are the forces balanced or unbalanced?
- Which force causes the balloon to speed up?
Students may refer to ‘action and reaction’ force pairs when describing the motion of the rocket. Emphasise that these can be misleading terms. They imply that one of the forces in Newton’s third law appears in response to the other. Discuss what’s happening inside the balloon to illustrate how the forwards force on the balloon arises at the same instant as a backwards push on air.
When the peg is attached, the balloon remains inflated because the air particles inside it are colliding with the inside surface. They push equally to the left, right, up and down and so the forces on the balloon are balanced (as are those on the air inside it).
When the peg is removed, the air particles no longer push on the open end of the balloon. The forward force on the front end of the balloon is no longer balanced by a backward force and so the balloon accelerates forwards. Similarly, if we consider the forces acting on the air in the balloon, we can see that there is a resultant force acting on it to the left, and so the air accelerates backwards.
Emphasise that, as with all Newton’s third law force pairs, the two forces that arise act on different objects (balloon and air).
Students describe how an air-filled balloon propels itself and identify the Newtons third law force pairs involved.
For a version of this activity for families and younger pupils to try at home, see Do try this at home: episode 7
This experiment was safety-checked in March 2020.