# Understanding Torque Balance and Center of Gravity in a See-Saw

• Wasseem92
In summary, the center of gravity of a poorly constructed see-saw with a fulcrum 2/3 of the way along its length is assumed to be in the center. In order to balance the see-saw with a weight of 30 kg, a child weighing 20 kg would have to sit at a distance of 1/4 of the length of the see-saw from the fulcrum. The distance of (L/6) from the fulcrum is used in the torque balance equation to find the distance x from the child to the fulcrum. This is because L/6 is the distance from the fulcrum to the center of mass of the see-saw, which can be regarded as
Wasseem92
A poorly constructed see-saw has a fulcrum 2/3 of the way along its length. a) If the see-saw weighs 30 kg, where would a 20 kg child have to sit in order to balance the see-saw? b) What is the least mass that a child must have in order to balance the see-saw?
well i tried solving this question and my teacher gave this as a solution:

The center of gravity of the see-saw is assumed to be in the center, assuming that it is uniform. The force diagram is therefore as shown below. mss = 30 kg is the mass of the see-saw, mc = 20 kg is the mass of the child, and L is the length of the see-saw.

Figure 8.3: Problem 8.1

In order to find the distance x from the child to the fulcrum we can do a torque balance about the fulcrum:
sum of torque = (L/6)mssg - xmcg = 0

so that
x = (L*mss)/(6*mc) = = (1/4) L.

My question is: why did he start of with a distance of (L/6). Please i need help, this really baffles me.
Thank you.

Welcome to PF!

Hi Wasseem92! Welcome to PF!
Wasseem92 said:
My question is: why did he start of with a distance of (L/6).

Because L/6 is the distance from the fulcrum to the centre of mass of the seesaw …

you can regard the whole mass of the seesaw as concentrated at its centre of mass.

(in fact, that's exactly why the centre of mass is defined, and is called that! )

alright man thank you a lot! that really helped!

## What is torque?

Torque is a measure of the force that causes an object to rotate around an axis. It is calculated by multiplying the force applied to an object by the distance from the axis to the point where the force is applied.

## Why is torque important?

Torque is important because it is a fundamental concept in understanding the movement of objects. It helps explain how objects are able to rotate and how much force is needed to cause a specific amount of rotation.

## What are some examples of torque in everyday life?

Some examples of torque in everyday life include opening a door, using a wrench to tighten a bolt, and riding a bike. In each of these cases, a force is applied to cause an object to rotate around an axis.

## How is torque related to angular acceleration?

Angular acceleration is directly proportional to torque and inversely proportional to the moment of inertia, which is a measure of an object's resistance to rotation. This means that the greater the torque, the greater the angular acceleration, and the greater the moment of inertia, the smaller the angular acceleration.

## How can torque be increased?

Torque can be increased by either increasing the force applied or by increasing the distance from the axis to the point where the force is applied. This is why longer wrenches can provide more torque than shorter ones, and why it is easier to open a door by pushing farther from the hinge rather than close to it.

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