Turning forces
Why is the door handle put as far from the hinge as it can go?
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Explain
Push far from the hinge.
Doors swing easily that way.
A force can make something turn. The turning effect is called a moment.
A bigger force turns things more, and so does pushing further from the turning point.
That's why door handles are far from the hinge, and why a long spanner makes a nut easier to turn.
M = F × d, where d is the perpendicular distance from the pivot to the line of the force. The unit is the newton metre (N m).
An object is balanced when clockwise moments equal anticlockwise moments.
Worked example: 20 N pushes a door 0.8 m from its hinge. So M = 20 × 0.8 = 16 N m.
- Force at a distance
- Makes a turning effect
- Bigger distance turns more
Want the full story?
A force through the pivot gives no moment, whatever its size, because d is zero. The door in the sim swings further for a bigger moment to show the idea, but a real door keeps swinging. Moments are sometimes called torques.
The Formula
moment = force × distance from the pivot
- the moment, or turning effect, measured in newton metres (N m)
- the force, measured in newtons (N)
- the shortest distance from the pivot to the line of the force, measured in metres (m)
Try the numbers
Moment: 16 N m
Try it at home
Ask a grown-up to help
You need a door, a clear doorway.
You need: a door, a clear doorway.
Push a door by the handle. Then near the hinge.
- Push a door open, at the handle, with one finger.
- Close it, and push close to the hinge with one finger.
- Feel the difference.
What's happening: the same force gives a much bigger moment further from the hinge.
Keep fingers away from the hinge side of the door as it closes.
Physics is life
Your body
Your muscles turn your joints.
Daily life
Spanners have long handles.
Nature
Trees stand against the wind.
Space
Spacecraft use tiny jets to turn.
Quick check
Question 1 of 3
Words to know
Moment: The turning effect of a force.