Turning Effect of Forces Chapter 3 What we

Turning Effect of Forces Chapter 3

What we have learnt: • Newton’s Laws of Motion deal with ‘bulk’ motion • Here in this topic, we want to look at how forces can cause a turning effect.

Describing the turning effect: • Direction: – CLOCKWISE, or – ANTI-CLOCKWISE (COUNTER CLOCKWISE) • Magnitude: – Magnitude of force – Perpendicular distance of force from pivot WS 3. 1 Points 1 -3

Physical Quantity • The physical quantity used to describe the ‘turning effect’ of forces is called the Moment of a Force = Force x perpendicular distance

Calculating moment of a force WS 3. 1 Q 4(b)

Questions Now that we have learnt to quantify the turning effect, can we compare the moments of different forces? What if there are many forces acting on one object? Is there such a thing as net moment? (similar to net force? ) If so, how do I calculate the net moment?

Resultant moment of forces WS 3. 1 Q 5(b)

WS 3. 1 Q 5 (v) Moment due to F 1 = 15 N x 0. 0 m = 0. 0 Nm CCW Moment due to F 2 = 15 N x 2. 0 m = 30 Nm 1 2 CW Moment due to 10 N force = 10 N x 1. 0 m = 10 Nm Magnitude of CCW moments > Magnitude of CW moments Resultant Moment (CCW moments – CW moments) = 30 Nm – 10 Nm = 20 Nm CCW CW: Clockwise CCW: Counter-Clockwise

WS 3. 1 Q 5 (vi) CW Moment due to F = 15 N x (4. 0 – 2. 7) m = 19. 5 Nm CW: Clockwise CCW: Counter-Clockwise CCW Moment due to 10 N = 10 N x 2. 0 m = 20 Nm Magnitude of CCW moments > Magnitude of CW moments Resultant Moment (CCW moments – CW moments) = 20 Nm – 19. 5 Nm = 0. 5 Nm CCW

Key things to note when calculating moments=force x perpendicular distance • Identify point of pivot – in some cases it is not so obvious where the rotation is about • Identify the line of action of the force creating the moment • Mark the perpendicular distance from the line of action of the force to the pivot.

Identifying key points to consider in a rotation WS 3. 1 Q 6