 Fig.1 John using a lever. John is moving a heavy stone with a strong steel bar. When he pushes down, the load goes up. When you use a rod in this manner to change a force in direction or size (magnitude) , we call it a LEVER. There are three different ways of using levers. It all depends on the order of using Effort, Pivot or Fulcrum, and Load. Class 1 EFFORT == PIVOT == LOAD or LOAD ==PIVOT == EFFORT. Class 2. EFFORT ==LOAD == PIVOT. Class 3. PIVOT==EFFORT == LOAD. The three types are illustrated here.  Calculations Lever problems can easily be solved by finding the Moments / Torque of a force. One may think that a lever can be balanced by making the forces turning clock wise equal to forces turning anti clock-wise. It is not so. You may have already seen a small kid lifting a bigger child in a seesaw. What becomes equal are the moments. What is a moment? A Moment or a Torque is the turning effect due to a force. This is obtained by multiplying the force by the distance to the pivot. Law of Moments If a Lever is balanced the sum of the clock-wise moments is equal to the sum of the anti-clockwise moments.  Example;- Find the effort required to lift the load. Cw Moments = ACW moments. E x 10 + 760 x 0.2 = 200 x 1 10 E + 15.2 =200 Effort =(200 - 15.2) /10 When the Pivot is not given? A log is supported by two pillers A and B. A Bunny telysits on it closer to B. It is clear that B gets most of the weight of Bunny. We have to find the support force at A and B separately. Now where is the Pivot?  Fig.4 Bunny on a uniform log.When the Pivot is not given as in this case , you can assume it to be anywhere and take moments around it. Let us assume P to be at A. As the log is uniform, the centre of mass will be at the mid point of 5m. log. The the CW moments are = (100x 2.5) + (40 x 4) ACW moments = B x 5 Therefore 5 B = 250 + 160 B = 410/5 = 82 N. We can make use of the parallel forces to find the force at A. Up forces = A + 82 Down forces = 100 + 40 Therefore A + 82 = 140 A = 140 - 82 = 58 N. Torques and Couples. How do you turn? Clockwise turn   Fig.5 Turning a car. Anti clockwise turn.
Have you ever wondered what makes an object turn or twist? For any kind of turn, there should be two forces acting in opposite directions. In physics, a combination of two such forces is called a ‘couple’[1] . We see the man standing on the plank. So his weight is acting down. There does not seem to be another force, but there is another force.  Fig. 6. Where is the other force? It is really the fixed point that gives the other force. This is really the reaction force as shown here . There are two directions of turn in any particular plane. Normally we consider the two important planes, the vertical plane and the horizontal plane. The sea-saw goes up and down in a vertical plane, while a merry-go-round turns in a horizontal; plane. Turning normally takes place around a point, which does not go up and down. In physics, we use three names for this. They are: Fixed point -- Fulcrum - Pivot. All three have almost the same meaning. In ordinary language words such as ‘hinge’ or ‘swivel’ are also usedA Couple is usually defined as two Equal, Parallel and opposite forces, not in the same line. Torque of a couple = A Single Force x Diameter.  Q.1.0  The uniform board is pivoted at the mid point.Bunny standing 2.5m away from pivot balances Lily who is 2m from Pivot. 1. Find the CW moments. 2. Find the ACW moments. (Use lily's weight as 'L'.) 3 Find the weight of Lily. 4. Find the support/reaction force. 5x4 = 20 marks
 1. Give the CW moments. 2. Give the ACW moments. 3. What will be the result due to the forces? 4. If the 600 N force is made to act perpendicular to the lever, what would be the ACW moment? 4. According to 4 what would be the result? 5 4 = 20 marks.
Q.Q. 3.0 Load distance = 40 cmEffort distance = 5 cm. Load = 25 N Ignore the weight of the hand. Consider the effort to be acting vertically. Find the following:-
5 x 6= 30 Marks. Q. 4.0
Q 5.0
5 Marks. For answers click Answers |
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