Turning Effect of Forces

Moment

The turning effect of a force is known as the moment. It is the product of the force multiplied by the perpendicular distance from the line of action of the force to the pivot or point where the object will turn.

When undoing a nut fastened to a screw by hand one realises that the amount of force required is a lot greater than when undoing the same nut using a spanner. The spanner increases the distance between the fulcrum and the line of action of the force, thus for the same force a greater moment is obtained.

SMALL MOMENT The distance from the fulcrum to the line of action of force is very small	LARGE MOMENT The distance from the fulcrum to the line of action of force is large

Principle of Moments

The principle of moments states that when in equilibrium the total sum of the anti clockwise moment is equal to the total sum of the clockwise moment.

When a system is stable or balance it is said to be in equilibrium as all the forces acting on the system cancel each other out.

 In equilibrium

Total Anticlockwise Moment = Total Clockwise Moment

This principle can be explained by considering two people on a seesaw.

 

Moments Acting On A Seesaw Both people exert a downward force on the seesaw due to their weights. Person A’s weight is trying to turn the seesaw anticlockwise whilst person B’s weight is trying to turn the seesaw clockwise. Person A’s Moment = Force x perpendicular distance from fulcrum 1000  x 1  = 1000 Nm Person B’s Moment = Force x perpendicular distance from fulcrum 500   x 2   = 1000 Nm Persons A’s moment  = Persons B’s Moment Anticlockwise moment = Clockwise moment Therefore seesaw is in equilibrium. Circular Motion & Centripetal Force From Newton’s first law of motion it is known that an object will remain   stationary, or keep moving at constant velocity in a straight line unless acted upon by an unbalanced force. When an object moves in a circular path its direction is changing all the time therefore according to Newton’s first law there must be an unbalanced force acting upon it all the time. When an object moves in a circle although its speed is constant the direction is continuously changing. Therefore its velocity is continuously changing as velocity is speed in a particular direction. The changing velocity in time means the object is accelerating all the time. The resultant force which causes this acceleration is the centripetal force. The centripetal force always acts toward the centre of the circle. The centripetal force is determined from the following equation: If a ball is tied to the end of a strong string and swung in a circle, the ball accelerates towards the centre of the circle. The centripetal force which causes the inwards acceleration is from the tension in the string caused by the person’s hand pulling the string. If the string breaks there is no longer a resultant force acting on the ball, so it will continue its motion in a straight line at constant speed. The centripetal force required to make an object perform circular motion increases in the following cases: If the mass of the object increases. If the velocity of the object increases. If the radius of the circle decreases. The above points are evident when considering the equation for centripetal force.