Balancing forces

Demonstration

The effect of friction is reduced so that you can see that a pair of balanced forces produce zero acceleration - but not necessarily zero velocity.

Apparatus and materials

Plank, smooth

Rollers, 10

Retort stands, bosses and clamps, 2

Demonstration spring balances, 2

Single pulleys on clamps, 4

Masses, 1 kg, 2

Weight hanger with slotted weights, 10 g

Cord

Health & Safety and Technical notes

The person standing near the additional (upper) mass to read the spring balance should be prepared to catch the retort stand should it topple over.

Procedure

a Place the plank on the rollers and the two stands on the plank. Each stand carries one pulley fixed towards the base and a spring balance. For each one, a cord passes under the pulley and over another pulley, which you should clamp firmly to the end of the bench.

b Hang a 1-kg mass from each cord. Adjust the positions of these masses so that when one mass is on the floor the other is almost at bench height.

c Add an additional mass to the upper mass. If necessary, give the system a small push so that it moves on its rollers, and quickly reaches a constant velocity. The additional mass should be such that the plank does not continue to accelerate (50 g is usually about the right value).

Teaching notes

1 The forces applied to the system are initially equal and opposite, so that the system does not accelerate. Its velocity remains zero. The fairly small added mass is sufficient to compensate for frictional force, so that total horizontal force on the system is again zero. When this condition is achieved, the system has zero acceleration and constant velocity. Explain this.

2 Whenever forces are not balanced, the system accelerates.

3 This resulting motion is consistent with Newton's first law. Any object has zero acceleration (so stays still or has unchanging velocity) except when an unbalanced force acts on it. The law defines what force is: it is what causes acceleration.

This experiment was safety-checked in April 2005