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Moments: turning forces. All: calculate the moment of a force about a point (torque) (C) Most: apply principle of moments to simple balanced situations (B) Some: employ principle of centre of mass in calculations (A). Keywords: Force Moment Couple Torque COM Weight Turning.

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moments turning forces

Moments: turning forces

All: calculate the moment of a force about a point (torque) (C)

Most: apply principle of moments to simple balanced situations (B)

Some: employ principle of centre of mass in calculations (A)

slide2

Keywords:

Force

Moment

Couple

Torque

COM

Weight

Turning

All: calculate the moment of a force about a point (torque) (C)

Most: apply principle of moments to simple balanced situations (B)

Some: employ principle of centre of mass in calculations (A)

  • Key Equation:
  • Moment of a force = force x perpendicular distance
  • moment = fd
slide3

Keywords:

Force

Moment

Couple

Torque

COM

Weight

Turning

All: calculate the moment of a force about a point (torque) (C)

Most: apply principle of moments to simple balanced situations (B)

Some: employ principle of centre of mass in calculations (A)

  • What is the moment?
  • Remember the unit!
slide9

Keywords:

Force

Moment

Couple

Torque

COM

Weight

Turning

All: calculate the moment of a force about a point (torque) (C)

Most: apply principle of moments to simple balanced situations (B)

Some: employ principle of centre of mass in calculations (A)

Note definition

The Principle of moments:

When an object is in equilibrium:

Sum of anti-clockwise moments about any point

Sum of clockwise moments about any point

=

clockwise moments – anticlockwise moments = zero (resultant force)

stability

Stability

I’m unstable.

slide17

The lower the centre of mass the better the stability. The tightrope walker’s pole helps.

August 19th 1859. French tightrope walker “Blondin” crosses the Niagara Falls with his manager on his back.

the hare and the tortoise
The hare and the tortoise

Slow, steady and stable

slide19

My head is big for my body compared to an adult’s head. That’s why I find it harder to balance.

poor balance produces an overall turning force
Poor balance produces an overall turning force.

This requires a turning force in the opposite direction to counteract it.

slide24

Centre of gravity is well behind the heel of the leading leg. This allows progressive loading of the leading leg.

Centre of gravity much closer to heel. This means that the loads on the heel, ankle, knee and pelvis rise sharply.

slide26

The centre of gravity of an aeroplane is affected by the way it is loaded. If the centre of gravity is in front of the centre of lift, the aircraft will want to nose down. The pilot will counterbalance this tendency by “trimming” the elevators to push the tail down. The opposite applies when the centre of gravity is behind the centre of lift.

slide29

Keywords:

Force

Moment

Couple

Torque

COM

Weight

Turning

All: calculate the moment of a force about a point (torque) (C)

Most: apply principle of moments to simple balanced situations (B)

Some: employ principle of centre of mass in calculations (A)

Demonstration: spoon and door

  • Find centre of mass of a wooden spoon by balancing of finger
  • Cut spoon through COM
  • Weigh each piece
slide30

Keywords:

Force

Moment

Couple

Torque

COM

Weight

Turning

All: calculate the moment of a force about a point (torque) (C)

Most: apply principle of moments to simple balanced situations (B)

Some: employ principle of centre of mass in calculations (A)

Experiment 1: attaining equilibrium

  • Place the metre rule so it has a pivot point at 50 cm
  • Hang a 200g mass at 20 cm.
  • Use a 100g mass to restore equilibrium
  • Draw a diagram showing the arrangement and calculate the moments of each force acting
slide31

Keywords:

Force

Moment

Couple

Torque

COM

Weight

Turning

All: calculate the moment of a force about a point (torque) (C)

Most: apply principle of moments to simple balanced situations (B)

Some: employ principle of centre of mass in calculations (A)

0278

0278

(resourcefulphysics.org)

(resourcefulphysics.org)

Experiment 2

  • Calculate the centre of mass (COM) for a volunteer
  • Plank is placed at same height as scales
  • Measure:
    • change in weight, as shown on scales
    • Weight of student
    • The length of the plank
  • Can you use this information to calculate where the COM is positioned? Theory here

HINT HERE

slide32

Keywords:

Force

Moment

Couple

Torque

COM

Weight

Turning

All: calculate the moment of a force about a point (torque) (C)

Most: apply principle of moments to simple balanced situations (B)

Some: employ principle of centre of mass in calculations (A)

0278

(resourcefulphysics.org)

Experiment 2 Theory

  • Normal force = N, plank length = d, distance to COM = x, weight = W

Nd = Wx

slide33

Keywords:

Force

Moment

Couple

Torque

COM

Weight

Turning

All: calculate the moment of a force about a point (torque) (C)

Most: apply principle of moments to simple balanced situations (B)

Some: employ principle of centre of mass in calculations (A)

Experiment 3: calculate the weight of a metre ruler using moments

  • Can assume the rule is uniform. How can we check?
  • Place metre rule on a pivot at 15 cm.
  • Locate an object of known mass on the shorter end until the ruler is balanced i.e. the moments are in equilibrium
  • Draw a labelled diagram of the arrangement, noting the forces acting and their correct positions
  • Apply the principle of moments to calculate the mass of the ruler
slide34

Keywords:

Force

Moment

Couple

Torque

COM

Weight

Turning

All: calculate the moment of a force about a point (torque) (C)

Most: apply principle of moments to simple balanced situations (B)

Some: employ principle of centre of mass in calculations (A)

Calculations

  • Complete questions 1-4, page 98 in AQA book
  • Complete questions 6-10 on page 108-109
  • Home Learning: advanced warning! Complete all chapter 7 examination questions
slide35

Keywords:

Force

Moment

Couple

Torque

COM

Weight

Turning

All: calculate the moment of a force about a point (torque) (C)

Most: apply principle of moments to simple balanced situations (B)

Some: employ principle of centre of mass in calculations (A)

Experiment 1

  • Place the metre rule so it has a pivot point at 50 cm
  • Hang a 200g mass at 20 cm.
  • Use a 100g mass to restore equilibrium
  • Draw a diagram showing the arrangement and calculate the moments of each force
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