Turning Forces

Description

This is a basic physics GCSE tutorial reviewing the turning effect of forces (also known as a force's moment). Video tutorial to come.

What is a turning force?

The contact forces explored up until now have been concerned with forces that either change the shape of an object or move it in a linear fashion. But forces can also cause objects that are anchored or pivoted at a point to rotate or turn. This point is called the pivot or fulcrum. The effect of a force applied to an anchored object is called the turning effect.

How do we measure the turning effect of a force?

The turning effect of a force is called the moment of the force. The magnitude of the moment is the product of the applied force and its location in relation to the fixed pivot; it is reported in the unit newton metres or Nm.

or

M = f x d

Where

• moment (M) is measured in newton metres (Nm),
• force (f) is measured in newtons (N), and
• distance (d) is measured in metres (m).

Clockwise and Anticlockwise turns

We've learned that linear forces can be either positive or negative depending on the direction in which they act. Turning forces can also be given positive and negative attributes depending on the direction in which they act.

Turning forces can act in two directions: clockwise and anticlockwise. Clockwise forces are considered positive, whereas anticlockwise forces are considered negative. Getting these right is important for helping us balance moments, which we'll do in the next tutorial. Hint: hover over the below image to see which is positive and which is negative.

Some basic examples calculating moment

Let's calculate the moment experienced on a bolt by a spanner that has a force of 10 N applied to it at a perpendicular distance of 20 cm from the fulcrum.

When there is only one force acting on an anchored object, there are three basic steps needed to calculate the moment exerted on it.

Step 1: Write down the formula.
M = f x d

Step 2: Fill in the known values, making sure to include the units.
Because the force is acting in the clockwise direction, it is positive. Also, remember to convert the distance into metres (20 cm = 0.2 m).
M = +10 N x 0.2 m

Step 3: Calculate the moment, making sure to include the units, and an indication of the direction.
M = +2 Nm or 2 Nm clockwise

Let's do another example

Let's calculate the moment acting on a seesaw in a playground. Imagine a child sits on the seesaw, exerting a force of 300 N downwards on the left hand side of the pivot (which is in the centre in this case). The perpendicular distance between the force acting on the seesaw and the pivot is 1.6 metres. What is the magnitude and direction of the moment?

Let's follow the same steps as above.

Step 1: Write down the formula.
M = f x d

Step 2: Fill in the known values, making sure to include the units.
Because the force is acting in the anticlockwise direction, it is negative.
M = -300 N x 1.6 m

Step 3: Calculate the moment, making sure to include the units and an indication of the direction.
M = -480 Nm or 480 Nm anticlockwise

Exercise Worksheet

Below is the follow-on worksheet for this tutorial. It has five exercises (including an advanced one) for you to work through (with answers). To download the document, visit Slideshare here: