Force used to kick a football
Practical Activity for 14-16
Using impact time and change of momentum of a football to measure the force needed to kick the ball.
Apparatus and Materials
- Scaler or electronic timer accurate to 0.001 s
- Round football (rugby type not suitable)
- 30 cm flexible leads with crocodile clips, 2
- Stopwatch or stopclock
- Balance (to measure mass of ball)
- Aluminium foil square, 15 cm by 15 cm
- Aluminium foil square, 7.5 cm by 7.5 cm
Health & Safety and Technical Notes
Take care that the football is aimed so that it does not cause damage, and there is no danger of the timer being knocked off the bench.
The large foil is Sellotaped to the football: the small foil is taped to the toe of the kicker’s foot.
Connections to the foil are made with crocodile clips. The other ends of the leads should be a loose fit in the ‘timer input’ sockets so that they will come out easily in the event of an accident. It is sensible to have a student holding the timer on the bench.
Providing that the flexible leads are arranged so that the period of contact takes place before the ball pulls the foil away from the crocodile-clip contact, no difficulties should arise. You should obtain consistent results.
It is possible to get a value for the time of flight of a ball kicked with medium force down a 10 m corridor (or even a 5 m laboratory).
- Place the ball on a laboratory table, using three small lumps of Plasticene to stabilize it.
- Kick the ball in a horizontal direction from a standing position, with only medium force. More vigorous kicks can be used out of doors to show the longer time of contact.
- Find the time of contact of the ball with the foot from the scaler or timer, t , seconds.
- Find the mass of the ball, m kg, using a balance.
- Measure how far the ball travels horizontally before it hits the floor, s , then s = vT.
- The time of flight, T , can be found from the height of the table, h = 1/2 gT 2 . The acceleration due to gravity = 10 m s-2 T 2 = 2h /g T = √2 h /g
- Substituting in the equation v = s /T gives a value for the initial velocity of the ball.
- Therefore using Ft = Δ( mv) the force, F, on the ball can be calculated.
- Alternatively, film the flight of the ball using a camcorder, and use frame-by-frame playback mode to calculate its speed.
- Once students have learnt about the conservation of momentum in a collision then a different method can be used to calculate the force. The football is kicked into a cardboard box which is fixed to roller-skates or a skateboard.
- The box should be made massive so that it moves slowly enough for the time of motion to be measured with a stopwatch. The flaps on the box should trap the ball. All the momentum of the ball is shared with the box.
- The momentum of the box is calculated from its mass and velocity (= distance travelled in the measured time). This is equal to the initial momentum of the ball after it is kicked. Using Ft = Δmv then the force can be calculated if the time of contact is measured on the scaler.