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6/29/2015 Balancing forces | Nuffield Foundation
http://www.nuffieldfoundation.org/practicalphysics/balancingforces 1/2
Nuffield Foundation » Teachers » Practical Physics » > Forces and motion » Inertia and Newton's first law » Balancing forces
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Practical activities designed for use in the classroom with 11 to 19yearolds.
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Practical Work for Learning
Balancing forces
DemonstrationThe 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 catchthe retort stand should it topple over.
Read our standard health & safety guidance
Procedure
a Place the plank on the rollers and the two stands on the plank. Each stand carries one pulley fixed towardsthe base and a spring balance. For each one, a cord passes under the pulley and over another pulley, whichyou should clamp firmly to the end of the bench.
b Hang a 1kg mass from each cord. Adjust the positions of these masses so that when one mass is on thefloor 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 itsrollers, and quickly reaches a constant velocity. The additional mass should be such that the plank does notcontinue to accelerate (50 g is usually about the right value).
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6/29/2015 Balancing forces | Nuffield Foundation
http://www.nuffieldfoundation.org/practicalphysics/balancingforces 2/2
Page last updated on 24 February 2012
Teaching notes
1 The forces applied to the system are initially equal and opposite, so that the system does not accelerate. Itsvelocity remains zero. The fairly small added mass is sufficient to compensate for frictional force, so that totalhorizontal force on the system is again zero. When this condition is achieved, the system has zero accelerationand 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 orhas unchanging velocity) except when an unbalanced force acts on it. The law defines what force is: it is whatcauses acceleration.
This experiment was safetychecked in April 2005
Related guidance
Newton's laws of motion
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