Turning Effect of Forces
What is a turning effect?
The turning effect of a force is called the moment of the force. Moments act about a pivot in a
clockwise or anticlockwise direction. It depends on:
1. Size of the force
2. How far it is applied from the pivot/fulcrum
Moment of a force (Nm) = force (N) x perpendicular distance of the force from fulcrum (m)
Moment = Force x Distance
In a) the moment of F about O is
5 x 3 = 15 Nm
In b) the moment of F about O is
5 x 1.5 = 7.5 Nm
As the moment is greater in a, the gate
will be opened more easily when pushed
at the edge furthest from the hinge.
What is the principle of moments?
It states that when a body is in equilibrium, the sum of the clockwise moments about any
point equals the sum of the anticlockwise moments about the same point. There is no net
moment on a body that is in equilibrium.
For example, as in the diagram on the right, the
seesaw is in a body of equilibrium (not moving in
either direction) because the clockwise moment
(caused by F2), will be equal to the anti-clockwise
moment (caused by F1)
F2 x d2 = F1 x d1
The conditions for equilibrium are:
The sum of the forces in one direction equals the sum of the forces in the opposite direction.
The law of moments must apply.
Verifying the principle of moments
, 1) Balance a half-metre ruler at its centre, adding Plasticine to one side or the other until it
is horizontal.
2) Hang unequal loads m1 and m2from either side of the fulcrum and alter their distances d1
and d2from the centre until the ruler is again balanced. Forces F1 and F2 are exerted by
gravity on m1 and m2 and so on the ruler.
3) Record the results in a table and repeat for other loads and distances.
4) F1is trying to turn the ruler anticlockwise and F1 × d1is its moment. F2is trying to cause
clockwise turning and its moment is F2 × d2. When the ruler is balanced or, as we say, in
equilibrium, the results should show that the anticlockwise moment F1 × d1 equals the
clockwise moment F2 × d2.
Examples of turning effect
● Turning pencil in sharpener
● A worker applies a force to a spanner to rotate a nut.
● A person pushing a swing will make swing rotate about pivot.
● Turning stopcock of a water tap
● Turning the doorknob
What is the center of gravity?
What is a turning effect?
The turning effect of a force is called the moment of the force. Moments act about a pivot in a
clockwise or anticlockwise direction. It depends on:
1. Size of the force
2. How far it is applied from the pivot/fulcrum
Moment of a force (Nm) = force (N) x perpendicular distance of the force from fulcrum (m)
Moment = Force x Distance
In a) the moment of F about O is
5 x 3 = 15 Nm
In b) the moment of F about O is
5 x 1.5 = 7.5 Nm
As the moment is greater in a, the gate
will be opened more easily when pushed
at the edge furthest from the hinge.
What is the principle of moments?
It states that when a body is in equilibrium, the sum of the clockwise moments about any
point equals the sum of the anticlockwise moments about the same point. There is no net
moment on a body that is in equilibrium.
For example, as in the diagram on the right, the
seesaw is in a body of equilibrium (not moving in
either direction) because the clockwise moment
(caused by F2), will be equal to the anti-clockwise
moment (caused by F1)
F2 x d2 = F1 x d1
The conditions for equilibrium are:
The sum of the forces in one direction equals the sum of the forces in the opposite direction.
The law of moments must apply.
Verifying the principle of moments
, 1) Balance a half-metre ruler at its centre, adding Plasticine to one side or the other until it
is horizontal.
2) Hang unequal loads m1 and m2from either side of the fulcrum and alter their distances d1
and d2from the centre until the ruler is again balanced. Forces F1 and F2 are exerted by
gravity on m1 and m2 and so on the ruler.
3) Record the results in a table and repeat for other loads and distances.
4) F1is trying to turn the ruler anticlockwise and F1 × d1is its moment. F2is trying to cause
clockwise turning and its moment is F2 × d2. When the ruler is balanced or, as we say, in
equilibrium, the results should show that the anticlockwise moment F1 × d1 equals the
clockwise moment F2 × d2.
Examples of turning effect
● Turning pencil in sharpener
● A worker applies a force to a spanner to rotate a nut.
● A person pushing a swing will make swing rotate about pivot.
● Turning stopcock of a water tap
● Turning the doorknob
What is the center of gravity?
