The Turning Effect of Forces
Section 1: Forces and motion
c) Forces, movement, shape and momentum
know and use the relationship:
moment = force × perpendicular distance from the pivot
know that the weight of a body acts through its centre of gravity
The moment of a force
Also known as the turning effect of a force.
The moment of a force about any point is defined as:
moment = force x perpendicular distance from the pivot
moment = F x d
Unit: newton-metre (Nm)
Moments can be either CLOCKWISE or ANTICLOCKWISE
Force F exerting an ANTICLOCKWISE moment through the spanner on the nut
Question
Calculate the moment exerted with the claw hammer if the person exerts a force of 80N and distance d equals 25cm.
moment = F x d
= 80N x 25cm
= 80N x 0.25m
= 20 Nm CLOCKWISE
Complete:
Answers
Force (N) | Distance | Moment (Nm) |
40 | 3 m | 120 |
200 | 5 m | 1000 |
50 | 4 m | 200 |
3000 | 20 cm | 600 |
120
5 m
50
600
Question
Choose appropriate words to fill in the gaps below:
The ‘moment of a force’ is another name for the ‘________ effect of force’.
The moment of a force is equal to the ________ multiplied by the perpendicular _________ between the line of ________ of the force and the turning point.
Turning effect is measured in _________ metres.
__________ can be either clockwise or anticlockwise.
moments
turning
action
newton
distance
WORD SELECTION:
force
moments
turning
action
newton
distance
force
Centre of gravity
The centre of gravity of a body is that point at which the weight of the body acts.
centres of gravity of regular shapes
The centre of gravity of a symmetrical body is along the axis of symmetry.
Centre of gravity is also sometimes called centre of mass.
If suspended, a body will come to rest with its centre of gravity directly below the point of suspension.
Centre of gravity
Finding the centre of gravity of a card
Centre of gravity
Pierce the card in at least two places.
Suspend the card from one of these holes.
Hang a plumbline from the point of suspension.
Using the plumbline as a reference draw a vertical line on the card.
Repeat for the other hole(s).
The centre of gravity is where the lines cross on the card.
Stability
A body is stable as long as its centre of gravity remains vertically above its base.
If this is not the case, the body will topple.
stable
Centre of gravity
unstable - toppling
Centre of gravity
balanced
Centre of gravity
Question
What factors make a modern racing car as stable as possible?
1. A wide wheel base.
2. A low centre of gravity.
Online Simulations
Balanced beam with precise placement - Freezeway.com
Arm movement distance multiplication - Freezeway.com
Balancing challenge- Freezeway.com
Levers ringing a bell - Freezeway.com
See-saw forces - uses g - NTNU
Lever - Fendt
Centre of mass - Explore Science
Stability of a block - NTNU
Blocks and centre of gravity - NTNU
BBC KS3 Bitesize Revision:
Moments - includes formula triangle applet