# Understanding Forces Diagrams

A force interacts with another object.

But before you read on, you might want to download this entire revision notes in PDF format to print it out for your child, or to read it later.

This will be delivered to your email inbox. What is interaction? Like two persons interacting with each other, it involves two objects / surfaces.

If you encounter problems on the OEQ for forces, click on this article to find out more:

Common mistakes on forces

Basic science knowledge – Visualising the forces

Let’s recall the science knowledge: When one, two or three forces are acting on the object / interacting with the object, it becomes complicated for some students.

How to understand this? Or How to see where will the object go / what becomes of the object?

Does the object go up? Down? Move to the left? Or Right?

To infer what happens to the object, we have to visualise by drawing arrows to show a force and its direction.

A force, represented by an arrow, gives the direction. Direction tells you where will the object go.

• Look at the steel block resting on the horizontal ground: Gravitational force is acting on the steel block.

• If the block is sliding down along the slope, there are two forces acting on the object. Now, let’s look at the question and ask yourself what happens to the weight of the object.

## Question 1

In diagram 1, Magnet Y weighs 80 units (as shown from the machine). Magnet Y is resting on the machine. Magnet Y does not exert any magnetic force of attraction on the machine.

Next, Magnet Z was placed close to Magnet Y as shown in diagram 2. Is the weight more than, less than or equal to 80units?

Explanation  *Draw arrows to represent each type of force*

As you can see from Diagram 2, there are two forces acting on Magnet Y: The gravitational force pulling the magnet down and the magnetic force of attraction pulling the magnet up.

Take note: The reading on the machine measures the Total force acting on magnet Y.

When two forces are pulling Magnet Y in opposite directions (one is downward force and the other is an upward force), the Total force on the magnet is smaller.

Why is the total force less than 80units?

• Gravitational force pulling the object down is the same (80 units)
• There is magnetic force of attraction exerted by Magnet Z on Magnet Y and is trying to pull Magnet Y upwards. However, as the magnetic force of attraction is weak, it is not able to overcome all the gravitational force acting on Magnet Y.

After taking into consideration the magnetic force of attraction (1 downward force and 1 upward force), the total force acting on magnet Y is less than 80units.

Conclusion: Less than 80units

Still cannot understand?

You can use simple mathematics to work on this:

Gravitational force acting on Magnet Y is 80units.

Based on Diagram 2, Magnet Z is resting on the machine (Magnet Z does not move up). That means the magnetic force of attraction is weak and is not strong enough to overcome all the gravitational force.

Hence, we give a number that is smaller than 80. Let’s assume the magnetic force of attraction is 10units.

We can see the total force acting on Magnet Y is 80 – 10 = 70units.

Conclusion: Less than 80units

## Question 2

In diagram 3, the Magnet Y weights 80 units (as shown from the machine). Magnet Y is resting on the machine. Magnet Y does not exert any magnetic force of attraction on the machine.

Next, Magnet Z was placed close to magnet Y as shown in diagram 2. Is the weight more than, less than or equal to 80units?

Explanation  *Draw arrows to represent each type of force*

As you can see from Diagram 4, there are two forces acting on Magnet Y: The gravitational force pulling the magnet down and the magnetic force of repulsion pushing Magnet Y down.

Take note: The reading on the machine measures the Total force acting on magnet Y.

When two forces acting on Magnet Y are in the same downward direction, the Total force on the Magnet Y increases.

Conclusion: the reading on the machine is greater than 80units.

Learning pointers from Questions 1 and 2:

1) The reading on the machine is actually measuring the Total force acting on the magnet Y.

(Many students have the assumption that the machine measures the gravitational force acting on the Magnet Y only. We can assume the machine measures the gravitational force only when no other forces are acting on magnet Y.)

2) When there is no magnet nearby, only gravitational force acting on Magnet Y.

3) Draw arrows to visualise the force.

• If the direction of a force is upwards and the other is downwards, the Total force acting on that object is smaller.
• If the direction of both forces is downwards or upwards, the total force acting on the object is larger.

4) If the arrows are hard for you to visualise and compare, you can use mathematics to help you with this. The advantage of using numbers is we can easily compare them or draw conclusions from them.

5) Take note of the explanations I’ve provided. They are useful in the open-ended questions on forces.

Between pointers 3 and 4, I leave you to decide your preference.  🙂

Think you’ve mastered the concept? Let’s try out this question.

Apply the same learning pointers we have just covered in this question.

## Question 3

Object X and Magnet P are balanced on a rod as shown in the diagram below. Will the mass of the object X be the same, less than or more than 200g?

Explanation

Let’s look at Magnet P first.  🙂  *Draw arrows to represent each type of force*

There are two forces acting on magnet P:

• There is the gravitational force acting on Magnet P. The direction of force is downwards.
• As like poles repel, there is magnetic force of repulsion between the two magnets. The direction of force is upwards.

Based on the diagram, Magnet P does not move up. That means the magnetic force of repulsion is weak and is not strong enough to overcome all the gravitational force on Magnet P.

With a force in the upward direction and the other force in the downward direction, we can determine the Total force acting on magnet P is smaller than the gravitational force acting on Magnet P.

Still cannot visualise?

Let’s assume the gravitational force acting on Magnet P is 200units.

Let’s assume the magnetic force of repulsion is 40units (you can give any number smaller than 200 units). The reason is the magnetic force of repulsion is weak and is not strong enough to overcome all the gravitational force on Magnet P (the evidence is shown in the diagram where Magnet P did not move upwards).

Total force acting on Magnet P is 200 – 40 = 160units.

You can see the total force of 160units is smaller than the gravitational force of 200units

It is time to look at Object X

Looking at Object X, only one type of force is acting on Object X: Gravitational force.  *Draw arrows to represent each type of force*

Remember this:

The plastic rod is balanced, that means the gravitational force acting on object X must be the SAME as the Total force acting on Magnet P.

Since the total force acting on magnet P is smaller (as explained above when we are analysing Magnet P just now), the gravitational force acting on Object X has to be equally smaller!

Science knowledge: Since the gravitational force is affected by the mass of Object X. The smaller the mass of the object, the smaller the gravitational force acting on Object X.

Conclusion: The mass of object X has to be lesser than 200g (less than the mass of Magnet P).

Learning pointers:

1) Look at each object and determine the types and direction of force(s) acting on each object.

2) Draw arrows to visualise the direction and understand the total force acting on the object(s).

• If the direction of a force is upwards and the other is downwards, the total force acting on that object is smaller.
• If the direction of both forces is downwards, the total force acting on the object is larger.

3) Take note of the explanations I provided. They are useful in the open-ended questions on forces.

This will be delivered to your email inbox. How the online course from ScienceShifu is like?

At ScienceShifu, we are not interested in going through worksheets after worksheets. We have structured our lessons in such a way that your child will master each concept in a step-by-step approach. This is shown from the 3 questions provided.

That way, the learning becomes more engaging (it’s like a journey of discovery) and we feel the lesson is easy to follow.

In the online courses, all the concepts will be taught in pre-recorded lesson videos. There are no background noises from other students.

We call this the 4R approach:

1) Recall the science knowledge;

2) Realign and resolve any misconceptions;

3) Review the questions and hone the answering techniques and

4) Reapply the concepts and answering techniques to the experimental results or real-life questions.

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