Turning forces
Forces can turn objects, if they are attached to a pivot. Push a door, and it doesn’t jut fly out, it turns. Think of see saw, when no ones on it, it stays levelled. However when someone applies a force if turns on a pivot.
Turning forces around a pivot are called Moments.
When on a see saw, one side might have more fore than the other, however if it is balanced, we say the moments are equal and opposite.
To work out a moment, we need to know 2 things.
- The distance from the pivot that the fore is applied.
- The size of the force applied.
The equation is
Moment (Nm)= Force (newton) x Distance (meter)
So, if your closing a door, and you apply 10 N of force, and you’re 1 m from the hinges (pivot point) the equation would be
Force x Distance = Moment
10N x 2m = 20 Nm
Imagine if you bring your hand closer to the pivot, now you’re pushing only 20 cm from the hinge (pivot point). If you still pushed with 10 N the equation would be
Force x Distance = Moment
10N x .20m = 2 Nm
In order to push the door at the same moment (with out increasing distance from the pivot) you would need to increase Force.
Force x Distance = Moment
100N x .20m = 20 Nm
Try the question bellow
Observe the image to the left.
What is the moment of the 1000 gram weight?
What is the moment of the 2000 gram weight?
What will happen to the see saw if both weights where dropped on it, at the distance stated.
With the correct distance from an object, even something as small as a notepad could lift the empire state building!
If a nut is difficult to undo with a short spanner, a longer spanner will help.
This is because there will be a bigger moment on the nut, when the same force is applied further away.
Using the same principle you can increase the moment applied by a lever or a crowbar, and this can help you move heavy objects more easily.
Leaver are referred to as Force multipliers, since they reduce the effort needed to move the load.
Things on a pivot can only work in two directions.
Clockwise or anti-
If anti-
Questions
Short Answer Questions
1. Define a moment
2. What is meant by the term ‘force multiplier’
3. Explain what is happening when a see saw is balanced (3 marks)
4. Explain how a 50 kg person, can balance a see saw with a 100 kg person. Include the distance, Newton's, and Nm in your explanation
5. Copy the diagram into your book. Fill in the blanks
6. Copy the diagram into your book. Fill in the blanks and explain which way the see saw will go.
