Beam Support Lab

A light beam rests on two supports. Drag the object along the beam and watch each support's scale.

Turning effects

Choose a support to use as the pivot. Work out the clockwise and anticlockwise moments about it for the object's current position, type them in, and check. When both are right they appear on the diagram with a balance verdict.

moment (N m) = force (N) × perpendicular distance from the pivot (m)

Pivot

Your results

Read both scales yourself and type every value, including the total. Nothing is filled in for you.

position x (m) reading at A (N) reading at B (N) A + B (N)
reading at A reading at B
Guided Investigation

Before you measure

  • Before you drag anything, predict: when the object sits directly over support A, what will each scale read? Write your prediction down, then move the object over A and test it.

Finding the pattern

  • Move the object to several positions between A and B, reading both scales each time and recording them. What happens to the reading at A as the object moves towards B? What happens to the reading at B?
  • Look at your two readings at each position together with the object's weight. Is there a relationship between them? Does it hold at every position you try?
  • Is there a position where the two readings are equal? Is there more than one?
  • Change the object's weight with the slider and repeat a couple of positions. Does the relationship you found still hold for a different weight?

Moving across

  • Start with the object over A and move it slowly all the way to B. In words, describe how the reading at A changes across the whole journey, and how the reading at B changes. Are the changes steady, or do they speed up and slow down?

Explaining why

  • Open the Turning effects view and choose support A as the pivot. With the object somewhere between the supports, which way does the object's weight try to turn the beam about A — clockwise or anticlockwise? Which force turns it the other way?
  • The beam stays still. What must be true about these two turning effects for that to happen?
  • Use this idea to explain why moving the object closer to B makes the reading at B larger. Refer to the turning effect of the object's weight about A, and name its direction.
  • Now switch the pivot to B and explain, in the same way, why the reading at A behaves as it does.
  • Put it together: an object starts at support A, then moves all the way to B. Using turning effects about a pivot, explain how and why the force at each support changes as it moves. Name the direction of each turning effect you mention.