component of W down the slope=Wcos (90º- Ɵ)= W sin Ɵ

F=mgsinƟ

a=mgsinƟ/mg = gsinƟ

**Moment of force**

The moment of force is its turning affect about a fixed point.

The magnitude of the moment is given by:

Moment = force × perpendicular distance from force to the fixed point

moment=Fs

In the given figure, the force is not acting perpendicularly to the fixed point.

The perpendicular or the closest distance is scos Ɵ

The moment is given by moment=FscosƟ

We can also use value of s that would give:

moment=FcosƟs

**Couples**

A couple is a pair of equal forces acting in opposite directions. If a couple acts on an object it rotates in position.

The moment of couple is called torque. The torque is calculated as:

Torque = force × perpendicular distance between forces

torque=Fs

According to diagram,

torque=FscosƟ

**Balance**

When an object is balanced or at equilibrium:

the total moments acting clockwise=total moment acting anticlockwise

If a seesaw is left to balance then clockwise moment must be equal to anticlockwise moments.

Clockwise moment due to 3 and 4

moment=F3s3 + F4s4

Anticlockwise moment due to 1 and 2

moment= F1s1 + F2s2

So,

F3s3 + F4s4 = F1s1 + F2s2