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Unit 3: Forces, motion and pressure

Turning forces — HIA International - Grade 7 Revision Notes & Topical Questions

5 min read 8 parts Lesson 5 of 8 3 quizzes

📚 HIA International - Grade 7 Hub / Topical Revision
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Lesson summary

3.4 Turning forces — HIA INTERNATIONAL notes: Define the moment of a force and use moment = force x distance, including for a balanced seesaw.Examples — pushing a door handle, pushing a bicycle pedal and pulling a door all make things turn.Lever = an object that turns about a poi…

  • Body levers
  • Door handles
  • Clockwise and anticlockwise
  • Seesaw position
  • Finding distance
  • Small effort

Part 1 of 6Turning effects

Examples
  • Examples pushing a door handle, pushing a bicycle pedal and pulling a door all make things turn.
Lever
Lever an object that turns about a point.
Pivot
Pivot the point around which the lever turns.
Body levers
  • Body levers the arm is a lever with the elbow as the pivot.

Part 2 of 6Moment of a force

Moment
Moment the turning effect of a force.
It depends on two things.
  • The size of the force; a bigger force gives a bigger moment.
  • The distance from the pivot to where the force acts; a greater distance gives a greater moment.
Door handles
  • Door handles are placed far from the hinges to give a large moment.

Part 3 of 6Equation for moment

1Equation
  • Equation moment = force × distance.
2Distance
  • Distance measured from the pivot to the position where the force acts, perpendicular to the force.
3Unit
  • Unit newton metre, written Nm, with a capital N and a lower case m.
4Clockwise and anticlockwise
  • Clockwise and anticlockwise moments can turn either way.

Part 4 of 6Balancing

Seesaw
  • Seesaw a type of lever with the pivot in the middle.
Balanced
  • Balanced when the moments on both sides of the pivot are equal and opposite.
Rule
  • Rule clockwise moment = anticlockwise moment.
Seesaw position
  • Seesaw position a heavier person sits closer to the pivot than a lighter person to balance.
Finding distance
  • Finding distance distance = moment ÷ force.

Part 5 of 6Using levers

Small effort
  • Small effort a small force far from the pivot can balance or lift a large load close to the pivot.
Examples
  • Examples spanner, crowbar, scissors, wheelbarrow and brake lever.
Longer spanner
  • Longer spanner gives a larger moment for the same force.

Part 6 of 6Investigating moments

1
Predict how the force needed changes as the distance from the pivot decreases.
2
Set up a metre rule pivoting on a clamp stand with an elastic band and a newton meter on a loop of string.
3
Move the loop as far from the pivot as possible and record the distance.
4
Pull the rule down the same distance each time and record the force.
5
Move the loop closer to the pivot and repeat.
6
Plot distance on the horizontal axis and force on the vertical axis.
Result
  • Result the force needed rises as the distance from the pivot falls.
Safety
  • Safety clamp the stand firmly to the table.

Key terms

TermMeaning
MomentThe turning effect of a force.
LeverAn object that turns about a pivot.
PivotThe point a lever turns about.
Newton metreThe unit of moment, Nm.
BalancedHaving equal clockwise and anticlockwise moments.
SeesawA lever with a central pivot.
ClockwiseTurning the same way as clock hands.
AnticlockwiseTurning opposite to clock hands.

Avoid these mistakes

1
Measuring the distance from the end of the object — measure it from the pivot.
2
Giving moment in N or Nm^{-1} — the unit is Nm.
3
Saying moment depends only on force — it also depends on distance.
4
Saying balanced means no forces act — the clockwise and anticlockwise moments are equal.

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