Physics Kenya
Turning Effect of a Force
Turning Effect of a Force
Turning Effect of a Force
Moment of a force, unit of moment of a force
Moment of force about a point is the turning effect of the force about that point. It is obtained as the product of applied force and the perpendicular distance from the point of the application of the force.
Therefore, moment of a force depends on two factors namely;
- The applied force.
- The perpendicular distance from the point of action of the force to the turning point(fulcrum).
The change of state of a body can appear in several forms and the most common form is the turning effect which is referred to as moment of a force.
The unit for the moment of a force is Newton-meter (Nm).
The turning effect can bring about two kinds of moments;
- Clockwise moment -caused by the forces which tend to turn the body in a clockwise direction.
- Anticlockwise moment -caused by the forces which tend to turn the body in an anticlockwise direction
Consider the diagram below,
The above uniform rod AB is balanced about the turning point (fulcrum) f. The weight W1 acting at point A tends to turn the rod in anticlockwise direction. The weight W2 acting at point B and the weight of the rigid body W0 tends to move the rod in clockwise direction. Hence, we have two anticlockwise moments due to weight W2 and weight of the rod W0.
Principle of moments
The Principle of moments states that:
” If a body is in equilibrium under action of forces which lie on one plane, then, the sum of the clockwise moments is equal to the sum of the anticlockwise moments about any point in that plane.”
The equilibrium of state of a body is brought about by two conditions;
- The sum of anticlockwise moments is equal to sum of clockwise moments about the turning point.
- The sum of upward forces is equal to sum of downward forces acting on a body.
If the data for the weight (force) W, and distance d of the weight from the fulcrum can be obtained then the moment created about the fulcrum is obtained by multiplying the weight with the respective distance i.e Wd. According to the principle of moments, at equilibrium the sum of clockwise moments (Wd) and that of anticlockwise moments (Wd) must be equal. to make it simple the below table can be used to record and compare the moments;
| W1(g) | W2(g) | d1(cm) | d2(cm) | W1d1(gcm) | W2d2(gcm) |
Procedures
- Balance the meter rule horizontally on a knife edge.
- Mark a balance point as C. Use the marker pen to do that.
- Suspend a meter rule from a fixed axis through C. Suspend unequal weights W₁ and W₂ from the meter rule by using thin cotton threads.
- Adjust the distance d₁ and d₂ of the weights W₁ and W₂ from C until the meter rule balance.
- Repeat the experiment five times using different values of W₁ and W₂.Record the results on the table as shown below.
| W₁(g) | W₂(g) | d₁(cm) | d₂(cm) | W₁ d₁ (gcm) | W₂ d₂ (gcm) |
Observation:In each case it will be found that W₁ d₁ is equal to W₂ d₂.
Moment of a force is used in the following:
- Is applied by a hand to unscrew a stopper on the bottle.
- Is applied by a spanner to unscrew a nut on a bottle.
- Turning a steering wheel of a car.
Problems on principle of moments (consider single pivot only)
States of equilibrium
There are three types of equilibrium, namely:
- Stable equilibrium
- Unstable equilibrium
- Neutral equilibrium
Stable equilibrium:A body is said to be in stable equilibrium if is given with small displacement the center of gravity will be raised and the body returns to its original position after displacement
Unstable equilibrium:A body is said to be in unstable equilibrium if when given a small displacement the center of gravity will be lowered and the body doesn’t returns to its original position after displacement.
Neutral equilibrium:A body is said to be in neutral equilibrium when a small displacement doesn’t alter the position of the center of gravity; the body is at rest in whichever position it is placed, eg, rolling a sphere or a barrel.
Factors affecting stability
Applications of stability
Problems on centre of gravity and moments of a force (consider single pivot only)
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