Torque:

Question: A 5 Kg seesaw supports two people who mass is 45.5 Kg and  52.5 Kg, respectively. The fulcrum is under the center of gravity of the board. The heavier person is 1.50 m from the center. Where will the lighter person sit so the seesaw is balanced? Find the upward force exerted by the fulcrum on the board. Draw appropriate diagram to show the problem visually.(g = 10 m/s2)

Answer:

Let us first find the upward force:

Weight of first person is 45.5 Kg * 10 m/s= 455 N

Weight of second person is 52.5 Kg * 10 m/s= 525 N

Weight of board is 5 Kg * 10 m/s= 50 N

All these three forces act vertically downwards

So the normal force will be vertically upward and will balance these forces, hence = 455 + 525 + 50 = 1030 N

Now we will find the distance of the lighter person from the fulcrum:

525 N * 1.5 m = 455 N * x

x = 525 N *  1.5 m / 455 N

x = 1.73 m

 

Question: A 650 N person stands on a 15 Kg board. held up by two supports, as shown below. Calculate the normal force exerted by two supports? (g = 10 m/s2)

 

Answer: 

T NET = 0

We need to determine a pivot point for this. We can take the pivot point anywhere. Let us take it on the green point as shown in the diagram below. That will cancel the effect of Fbut which is undesirable as we know the value of FB and we will be left with two unknowns. So we will move our pivot point where the support on the right meets the board. This pivot point is shown by a RED coloured dot and will cancel the effect of FN2

T board  + T person T N1

150 * 4   + 650 * 2  = FN1 * 8

FN1  = 237.5 N

 

 

FNET = 0

Weight of Painter  +  Weight of Board =  Upwards force from first support + Upwards force from second support

       650 N          +        150 N           =             FN1                            +                    FN2

650 N + 150 N = 237.5 N + FN2

FN2 = 562.5 N