Pushes, pulls and muscles
We exert force by contraction, in length, of muscles.
Apparatus and materials
For each student group
Mass, 1 kg
Mass, 100 g
Clamp and stand
Plywood or strong card (about 25 cm long), 2 lengths
Split pins (stationery variety), 3
Plywood or card to act as hand, small piece
String or other cord, approx 20 cm long
Health & Safety and Technical notes
Prepare the materials before the lesson. Cut the plywood or heavy card into lengths and drill a hole in each of them. Students will fit a split pin through these holes. Drill an additional hole in each of them to fit the other two split pins. Students will fit the ends of the string to these, to act as the "biceps muscle". Cut a slot into one end of one of the lengths. Students can use the slot for fitting the "hand".
a Line up the holes that are close to the ends of the two lengths of board. Push a split pin through both holes, to make the elbow pivot.
b Push split pins, in the same direction, through the other two holes.
c Fix the string to these two pins, so that when the string is tight the two halves of the arm are roughly at 90° to each other.
d Push the smaller piece of board into the slot, to make a hand.
e Clamp the 'upper arm' so that it is vertical.
f Put the mass onto the hand.
g Find out what you have to do to the string to lift the mass.
1 Before you set up the model, ask each student to hold a kilogram mass with their arm extended horizontally. Tell them to feel their biceps muscle and tendons as the load is raised.
2 Students must effectively shorten the string in the model to raise a load. When we raise a load in a real arm the biceps muscle contracts in length and also bulges. When the load is lowered the triceps muscle contracts.
3 A more elaborate alternative: use a piece (about 20 cm) of bicycle inner tube as the biceps muscle. Tie it tightly with string or wire to close it near each end. A rubber tube connected to a bicycle pump enters through the lower tie, so that this 'muscle' can be inflated - which will make it contract. Pieces of cord, representing tendons, run from the upper and lower ties (the upper arm and forearm). When the muscle is inflated it contracts and pulls the forearm up. If the ties are slightly leaky, the model imitates a characteristic of real muscles - the pumping has to be repeated to maintain tension.
This experiment was safety-checked in September 2004