Forces: Identifying Factors That Affect Frictional Force

Introduction

Have you ever tried pulling a heavy object like a box full of toys? Did you use any tool to pull it easier?

In this Forces blog post, we will be reviewing how frictional force acts on the surface of the floor and the material that you are pulling.

We will also discuss what factors might make an object easier or harder to pull.

Alternatively, you can also watch my explainer video for free by visiting our YouTube channel.

Let’s Take A Look At This Forces Question

Source: Nanyang Primary School – P5 SA2 Examination Paper [Q25]

The question wants us to choose the scenario where Kenneth will find it the most difficult to move the box along the floor.

Let us examine the photos in each option. There is one object that is common among all four options that Kenneth is using to pull the box. What is it?

It is the rope!

Source: Nanyang Primary School – P5 SA2 Examination Paper [Q25]

But looking at the images, you will see that there are some options wherein aside from the rope, Kenneth added something else to the set-up.

Which options are these?

In option (1), Kenneth used a cart with wheels.

In option (3), Kenneth applied oil at the bottom of the box.

In option (4), Kenneth rubbed the rope with oil.

Meanwhile, in option (2) Kenneth only used a rope without adding anything.

So which option should we start analysing from?

We can start analysing from option (2), where Kenneth only used the rope without adding anything else. We can use it as a basis for comparison to other options.

Let’s Analyse Option (2)

Let us identify the forces acting on the box.

Kenneth is pulling the box to the right side using a rope.

Are there any other forces acting on the box?

Remember that there is one force that acts on all objects at all times. What do you call that force?

It is gravitational force! Let us label our diagram with GF for gravitational force.

Is there any other force acting on the box?

There is! While Kenneth is pulling the box, the box is rubbing against the floor. What do you call the force between the two surfaces rubbing against each other?

It is frictional force!

In which direction should we draw frictional force?

Let’s recall. Does friction go along or oppose motion? Friction opposes motion!

Since Kenneth is pulling the box to the right, friction should be to the left. We will use FF to label frictional force.

After labelling our forces, we have to figure out what might cause Kenneth to find it difficult to pull the box.

Since Kenneth is pulling the box to the right, there must be a force holding him back to make it difficult for him to pull the box.

Is it frictional force or gravitational force? It is the frictional force that is causing him to find it difficult to pull the box to the right!

Let’s Compare Options (2) & (1)

Let’s compare option (2) with option (1).

In option (1), a cart with wheels was added to the setup.

What do wheels do to the frictional force between the box and the floor?

It reduces the friction between the box and the floor.

Since the wheels reduce friction, do you think Kenneth will find it easier or harder to pull the box as compared to option (2)?

It will be easier for Kenneth to pull the box in option (1) than in option (2).

Let’s Compare Options (2) & (3)

Now, let us compare option (2) with option (3).

In the third setup, oil was added at the bottom of the box.

🔎 What Will Oil Do To The Setup? 🔎

Oil is a lubricant. Lubricants make the surface they are applied to smoother.

Because the oil acts as a lubricant, the frictional force between the box and the floor will be reduced.

Therefore Kenneth will find it easier to pull option (3), the box with oil at the bottom compared to option (2).

Let’s Compare Options (2) & (4)

Finally, let us analyse option (4), wherein the rope was rubbed with oil.

Have you touched oil before?

It’s very slippery, right?

When you add oil to the rope, do you think Kenneth will find it easier or harder to grip onto the rope?

He will definitely find it harder to grip the rope!

This means that between options (2) and (4), Kenneth will find it more difficult to pull (4) because the rope with oil will be harder to grip.

After analysing all four options, in which scenario will Kenneth find it most difficult to move the box along the floor?

It will be option (4).

Answer For Q12

Option (4)

Conclusion

The different forces acting on the box are:

1️⃣ Pull Force

2️⃣ Gravitational Force

3️⃣ Frictional Force

Because friction opposes motion, it makes pulling the box harder. So when you use other tools like a cart with wheels or put oil at the bottom of the box, it becomes easier to pull the box because these factors reduce friction.

Stay tuned for more Forces blog posts!