NCERT Class 8 Science Curiosity Chapter 05 Exploring Forces PDF Download

Chapter 5: Exploring Forces

Probe and ponder

• Why does it feel harder to pedal a bicycle when going uphill than on flat ground?
• Why is it easier to slip on a wet surface?
• Why do we feel 'light' or like we are 'floating' just after our swing reaches its highest point and begins to come down?
• Share your questions

[Figure: Two students cycling through a village landscape with hills, thunder and lightning visible in the distance]

It was a windy day. Sonali and Ragini were excited to go cycling. Their summer vacation had just begun, and they wanted to explore the beautiful landscapes around their village. After pumping air into their bicycle tyres, they set off. As they rode through the village, the wind rushed past them. "Oh no! The wind is pushing me hard!" said Ragini. Smiling, Sonali replied, "We are riding against the wind. We must push our pedals harder to move faster."

Their ride took them up a long path to a hilltop. Some parts of the road were rough where they found it hard to pedal, while other parts were smoother. When they reached the top and were enjoying the view, they heard thunder and saw flashes of lightning at a distance. Even though it looked beautiful, they decided to head back immediately. On the way back, while passing a herd of sheep, they pressed their bicycle bells and turned the handles to change direction.

As they were coming down the slope of the hill, they realised that their bicycles were moving down at a great speed even though they were not pedalling! Sonali yelled, "It's thrilling! It seems something is pulling us downhill, what could it be?"

5.1 What Is a Force?

Let us try to experience the push and the pull.

Activity 5.1: Let us explore

• Take a large cardboard box.
• Try moving the box in as many different ways as you can think of.

Did you move the box in any other way than shown in Fig. 5.1? In all the ways that you might have used to move the box, you had to apply a push or pull to the box. Generally, the push or pull applied on an object is called force in science.

[Figure 5.1: Moving a box in different ways (a) Pushing; (b) Pulling; (c) Lifting (pulling up), and carrying]

Teacher's Note

A force is simply any push or pull. When you lift something, you are pulling it upward against gravity. When you press something flat, you are pushing it. In exams, always identify what object is pushing or pulling on what other object - that is how you spot where the force is being applied.

5.2 What Can a Force Do to the Objects on Which It Is Applied?

We experience push or pull in our daily lives all the time, often without even realising it. Let us recall some of these experiences and analyse them.

Activity 5.2: Let us analyse

• Think of situations where a force (push or pull) is applied and list them in Table 5.1.
• Analyse each situation and write the effect of the force in Table 5.1. Some situations and their effects are already listed for you.

Table 5.1: Different actions and their effects

What do you conclude from these examples? Does a force cause a moving object to stop? Can it change speed, or direction of motion, or change the shape of an object?

[Figure 5.2: Applying force on objects]

Teacher's Note

Force has three main effects: it can start motion (push a stationary ball), stop motion (catch a moving ball), or change direction (hit a ball to the side). It can also change the shape of something, like squashing a balloon. When answering questions about forces, always say which of these effects you observe.

Key Points

• A force is a push or pull applied to an object by something else.
• Forces can start motion, stop motion, change the direction of a moving object, or change the shape of an object.
• In daily life, we experience forces all the time - when we ride a bicycle, catch a ball, or press something.

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