Rotational Force – GCSE Physics

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Introduction

  • Rotational force is the force that causes an object to rotate around a point or axis (pivot point) instead of moving in a straight line.
  • This force is also called:

Moment

Torque

  • Rotational force plays a crucial role in daily life and machines because it helps us turn, rotate, or twist objects using force applied at a distance from the axis.

Daily-Life Example:

Examples of rotational force in daily life showing opening a door, turning a screwdriver, and seesaw movement – GCSE Physics

What is Moment?

  • A Moment (in physics) refers to the rotational effect produced by a force acting at a distance from a pivot point (axis of rotation).
  • It is essentially a turning force that causes an object to rotate.
  • Moment is another name for rotational force.

Moment Formula:

Where:

    • M = Moment in – Nm
    • F = Force applied in – N
    • d = Perpendicular distance from the pivot in – m

certified Physics and Maths tutorSolved Example

Problem: A student applies a force of 20 N at the end of a spanner to loosen a nut. The distance from the nut to the point where the force is applied is 0.3 m. Calculate the moment (rotational force) about the nut.

Solution: 

Step #1: Given

    • F = 20N
    • d = 0.3m

Step #2: Using the formula:

The moment about the nut is 6 Nm.

Final Answer: 6 Nm

  • Moment tells us how strong the turning effect of a force is where torque is a special type of moment that not only makes something turn but also causes it to spin faster or slower (rotational acceleration) around an axis.
  • Torque is another name for moment; both mean the turning effect of a force about a point.
  • Torque is a specific term for the turning effect around the axis of rotation, especially used in mechanics, engines, and rotational systems.
  • Example: When you push a door to open it, you are using moment and torque together: Moment explains how your push causes the door to rotate around its hinges and torque explains how strong that rotation will be.

Formula For Both:

When the force is perpendicular to the pivot point:

Formula for moment showing Moment equals Force times distance for rotational force GCSE Physics

Where:

    • F = Force applied in – N
    • d = Perpendicular distance from the pivot in – m

When the force is at any angle or not perpendicular:

Moment formula with angle showing Moment equals r times F times sin theta for rotational force GCSE Physics

Where:

    • F = Force
    • r = Distance from axis to point where force is applied.
    • θ = Angle between F and r.

How to Calculate Rotational Force?

  • Calculation for rotation in terms of moment involves finding how much a force causes an object to turn around a point or pivot.

To Calculate Moment in Physics, We Follow These Simple Steps:

  • Step#1: Identify the given values.
  • Step#2: Apply the formula and plug in the values.
  • Step#3: Calculate the moment.

certified Physics and Maths tutorSolved Example

Problem: A force of 12 N is applied perpendicularly at a distance of 0.4 m from the hinge of a gate. Calculate the moment.

Solution: 

Step#1: Identify the given values:

Given

    • F = 12N
    • d = 0.4m

Step#2: Apply the Formula and plug in the values:

The formula for moment is:

Now plug in the values:

Step#3: Calculate the Moment:

The moment is 4.8 Nm in the anticlockwise direction.

Final Answer: 4.8 Nm

certified Physics and Maths tutorSolved Example: 

Problem: A flagpole painter applies a force of 150 N perpendicularly on a brush attached to a rope that is tied 250 cm from the base of the flagpole to rotate and clean it. Calculate the moment about the base of the flagpole.

Man holding a paint roller next to a pulley system demonstrating rotational force GCSE example

(Rotational Force GCSE Questions)

Solution: 

Step#1: Identify the given values:

Given

    • F = 150N
    • d = 250cm

Step#2: Apply the Formula and plug in the values:

Convert cm to m:

Conversion of 250 cm to 2.5 m

The formula for moment is:

Now plug in the values:

Moment calculation using force and distance

Step#3: Calculate the Moment:

Final answer of moment calculation as 375 Nm

The moment about the base of the flagpole is 375 Nm in the anticlockwise direction.

Final Answer: 375 Nm

certified Physics and Maths tutorSolved Example

Problem: A shopkeeper pushes down on the handle of a heavy shop shutter with a force of 400 N perpendicular to it, producing a moment of 800 Nm about the hinge. Find the distance from the hinge where the force is applied.

Shop worker standing beside closed shutter showing rotational force GCSE example

Solution: 

Step#1: Identify the given values:

Given

    • F = 400N
    • M = 800Nm

Step#2: Apply the Formula and plug in the values:

The formula for moment is:

Formula for calculating moment showing M equals F times d for rotational force GCSE Physics

Rearranged it:

Formula showing distance equals moment divided by force for rotational force GCSE

Now plug in the values:

Example calculation showing distance equals 800 Nm divided by 400 N for rotational force GCSE

Step#3: Calculate the Moment:

Distance equals 2 metres final answer for rotational force GCSE calculation

The perpendicular distance is 2 meters.

Final Answer: 2 Meters

certified Physics and Maths tutorSolved Example

Problem: Child B weighs 350 N and sits 1.6 m from the pivot on a balanced seesaw. Calculate the moment of child B about the pivot. Give your answer in newton-metres (Nm).

Seesaw example showing rotational force GCSE problem with two children, pivot, force, and distance labelled

Solution: 

Step#1: Identify the given values:

Given

    • F = 350N
    • d = 1.6m

Step#2: Apply the Formula and plug in the values:

The formula for moment is:

Formula for calculating moment showing M equals F times d for rotational force GCSE Physics

Now plug in the values:

Rotational force GCSE calculation showing moment equals force times distance formula with example

Step#3: Calculate the Moment:

Rotational force GCSE final answer showing moment equals 560 Nm

The moment is 560 Nm.

Final Answer: 560 Nm.

Frequently Asked Questions

Solution:

Rotational force (torque or moment) is the tendency of a force to cause an object to rotate around a point or axis.

Solution:

Yes, both measure rotational effect of a force.

Solution:

Formula for moment:

M = F × d

Solution:

Newton-meter (Nm).

Solution:

  • If the force causes clockwise rotation – moment is negative.
  • If it causes anticlockwise rotation – moment is positive.

Solution:

The moment increases, making it easier to rotate heavy objects.

Solution:

Opening a door, using a spanner, turning a steering wheel, or pushing a swing are daily examples of rotational forces.

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