Everything Science Grade 11

12.6 CHAPTER 12. FORCE,MOMENTUM AND IMPULSE

Step 5 : Write the final answer

The torque from the perpendicular force is greater than the torque fromthe force

applied at 60 ◦. Therefore, the best angle is 90 ◦.

Mechanical Advantage and Levers ESBFO

We can use our knowledge about the momentsof forces (torque) to determine whether situations are
balanced. For exampletwo mass pieces are placed on a seesaw as shown in Figure 12.16. The one
mass is 3 kg and the other is 6 kg. The masses are placed at distances of 2 m and 1 m (respectively)
from the pivot. By looking at the clockwise andanti-clockwise moments, we can determine whether
the seesaw will pivot (move) or not. If the sum of the clockwise and anti-clockwise moments is zero,
the seesaw is in equilibrium (i.e. balanced).

2 m 1 m

3 kg

6 kg

F 1 F 2

Figure 12.16: The moments of force are balanced.

The clockwise momentcan be calculated as follows:

τ 1 = F⊥· r

τ 1 = (6kg)(9,8m· s−^2 )(1m)

τ 1 = 58, 8 N· m clockwise

The anti-clockwise moment can be calculated asfollows:

τ 2 = F⊥· r

τ 2 = (3kg)(9,8m· s−^2 )(2m)

τ 2 = 58, 8 N· m anti-clockwise

The sum of the moments of force will be zero:

The resultant moment is zero as the clockwiseand anti-clockwise moments of force are in opposite
directions and thereforecancel each other.

As we see in Figure 12.16, we can use different distances away from apivot to balance two different
forces. This principle isapplied to a lever to make lifting a heavy objectmuch easier.