Question Opening prompt 1
Why is it harder to pedal a bicycle uphill than on flat ground?
Steps only
Uphill, the rider works against gravity.
Answer
More effort is needed to cycle uphill.
CLASS 8 · SCIENCE · CHAPTER 5
Exploring Forces
Simple answers to the opening prompts, all ten end questions, the question-making activity, and all five chapter projects.
33 worked answersComplete chapter answers
33 worked answers · short steps first, explanations when you need them
Probe and Ponder
Textbook pages 62
Question Opening prompt 2
Why is it easier to slip on a wet surface?
Steps only
Water can reduce the friction that helps our feet grip the ground.
Answer
Less grip makes slipping easier.
Question Opening prompt 3
Why might we feel light or as if floating when a swing reaches its highest point and starts coming down?
Steps only
The swing briefly slows and changes direction at its highest point.
Answer
The change in motion can make us feel briefly lighter.
Question Opening prompt 4
Share one question you have about forces.
Steps only
Ask a clear question that can be investigated fairly.
Answer
Example: How does the surface affect how far a toy car travels?
Keep the Curiosity Alive
Textbook pages 77–79
Question 1
Match each force with its example: muscular, magnetic, frictional, gravitational, and electrostatic force; examples include lifting a school bag, a compass pointing north, a cricket ball stopping, a fruit falling, and a rubbed balloon attracting hair.
Steps only
Match each example to the force that causes it.
Answer
Muscular—b; magnetic—e; frictional—a; gravitational—c; electrostatic—d.
Question 2
State whether these are true or false: (i) A force is always required to change an object's speed. (ii) Friction makes a ball rolling on flat ground speed up. (iii) There is no force between two charged objects close together.
Steps only
Check what forces do to speed and how charges interact.
Answer
- (i) True
- (ii) False
- (iii) False
Question 3
Two balloons rubbed with a woollen cloth are brought near each other. What happens and why?
Steps only
Both balloons get similar charges, and similar charges repel.
Answer
The balloons repel each other.
Question 4
Why does a coin sink in water while a bigger wooden block floats?
Steps only
Compare the object's weight with the water's upward buoyant force.
Answer
The coin's weight wins; the block displaces enough water to float.
Question 5
A ball is thrown upward and then falls. Name the forces on it and give their directions (i) while rising (ii) while falling (iii) at the highest point.
Steps only
Gravity always acts down; air resistance acts against the motion.
Answer
Gravity acts downward during the rise, fall, and at the top.
Question 6
A ball released from point P rolls down a slope and stops at A. How can you make it stop (i) before A (ii) after A?
Steps only
Increase friction to stop sooner; reduce friction to travel farther.
Answer
Use a rougher surface for before A and a smoother surface for after A.
Question 7
Why do we sometimes slip on smooth surfaces such as ice or polished floors?
Steps only
Smooth surfaces may provide too little friction for our feet to grip.
Answer
Low friction makes slipping more likely.
Question 8
Is a force being applied to an object moving non-uniformly?
Steps only
A change in speed or direction means a net force is acting.
Answer
Yes, a non-uniformly moving object has a net force on it.
Question 9
An object's weight on the Moon is one-sixth of its weight on Earth. Why? Does its mass also become one-sixth?
Steps only
Weight depends on gravity; mass does not depend on location.
Answer
Moon weight is about one-sixth; mass stays the same.
Question 10
Three same-sized and same-shaped objects made of different materials float at different depths in water (Fig. 5.17). Which relation is correct: (i) w₁ = w₂ = w₃ (ii) w₁ > w₂ > w₃ (iii) w₂ > w₃ > w₁ (iv) w₃ > w₁ > w₂?
Steps only
A floating object that dips deeper displaces more water and weighs more.
Answer
(ii) w₁ > w₂ > w₃.
Question Question-making activity
Prepare questions using prompts such as why, when, how long, where, and why not, based on the chapter.
Steps only
Make clear questions about forces, motion, and floating.
Answer
Answers vary; use the chapter's ideas to frame questions.
Discover, Design, and Debate
Textbook pages 79
Question Project 1
Rub different pairs of materials together and test whether they attract small pieces of paper. Record your observations systematically and write a short report.
Steps only
Rub material pairs, test near paper, repeat, and record observations.
Answer
Report which pairs attracted paper; results may depend on conditions.
Question Project 2
Imagine gravity disappears. Develop a story and present it as a cartoon strip.
Steps only
Show in a few panels what might happen to people and objects without gravity.
Answer
Create a short, imaginative cartoon about floating people and objects.
Question Project 3
Discuss whether friction is a necessity or a problem. Note when it is useful and when it causes problems.
Steps only
List ways friction helps us and ways it makes movement difficult.
Answer
Friction is useful for grip and stopping, but can cause wear and slow motion.
Question Project 4
With your teacher, make a spring balance, calibrate it using standard weights, measure different objects, and compare each object's weight-to-mass ratio.
Steps only
Mark a spring using known weights, then measure objects within its range.
Answer
On Earth, weight divided by mass is nearly the same for all tested objects.
Question Project 5
With your teacher, make an electroscope, test it, and explore other ways it can be used.
Steps only
Foil leaves spread apart when they receive similar static charges.
Answer
An electroscope can detect static charge; use it only with teacher guidance.
Activities and Observations
Textbook pages 63–76
Question Activity 5.1
What do you notice when you try moving a large cardboard box in different ways?
Steps only
What do you notice when you try moving a large cardboard box in different ways?
Answer
To move the box, we have to push or pull it.
Question Activity 5.2
What effects can a force have on an object? Use examples from the activity.
Steps only
What effects can a force have on an object? Use examples from the activity.
Answer
A force can make an object move or stop, change its speed or direction, or change its shape.
Question Activity 5.3
What happens when you gently push a flat object on a table, then push it the other way?
Steps only
What happens when you gently push a flat object on a table, then push it the other way?
Answer
The object slides for a short distance and then stops in either direction.
Question Activity 5.4
Does the same object travel the same distance on glass, cloth, wood, tile, and sand?
Steps only
Does the same object travel the same distance on glass, cloth, wood, tile, and sand?
Answer
No.
Question Activity 5.5
What happens when like poles of two ring magnets face each other on a stick? What happens when one magnet is reversed?
Steps only
What happens when like poles of two ring magnets face each other on a stick? What happens when one magnet is reversed?
Answer
Like poles repel, so the upper magnet can stay above the lower one and feel a push if pressed down.
Question Activity 5.6
What happens when a rubbed plastic scale or straw is brought near small paper pieces?
Steps only
What happens when a rubbed plastic scale or straw is brought near small paper pieces?
Answer
The paper pieces move toward and may stick to the plastic.
Question Activity 5.7
What happens when two rubbed balloons are released near each other, and when the woollen cloth is brought near a balloon?
Steps only
What happens when two rubbed balloons are released near each other, and when the woollen cloth is brought near a balloon?
Answer
The two balloons move apart because rubbing gives them similar charges, and similar charges repel.
Question Activity 5.8
What happens when a ball is thrown straight up, even if it is thrown harder?
Steps only
What happens when a ball is thrown straight up, even if it is thrown harder?
Answer
The ball comes back down in both cases.
Question Activity 5.9
Do objects with different masses stretch a hanging spring by the same amount?
Steps only
Do objects with different masses stretch a hanging spring by the same amount?
Answer
No.
Question Activity 5.10
What is the maximum weight shown by the spring balance in Figure 5.13?
Steps only
What is the maximum weight shown by the spring balance in Figure 5.13?
Answer
The shown balance measures up to 10 N.
Question Activity 5.11
The difference between two large marks is 1 N, with five small divisions between them. What weight does one small division represent?
Steps only
The difference between two large marks is 1 N, with five small divisions between them. What weight does one small division represent?
Answer
Divide the difference by the number of small divisions: 1 N ÷ 5 = 0.2 N.
Question Activity 5.12
How should you use a spring balance to measure objects safely and record the results?
Steps only
How should you use a spring balance to measure objects safely and record the results?
Answer
First check that each object is lighter than the balance’s maximum range.
Question Activity 5.13
What do you feel when you push a tightly closed empty bottle under water and then release it?
Steps only
What do you feel when you push a tightly closed empty bottle under water and then release it?
Answer
The bottle feels an upward push and rises when released.