NCERT Solutions Class 9 Science Chapter 6 In-text Questions — How Forces Affect Motion

NCERT Class 9 Science — How Forces Affect Motion, the four Pause and Ponder boxes on pages 97, 101, 106 and 110. All 10 questions solved.

Also here. Solutions to the end-of-chapter questions for this chapter and the activities are on their own pages.

The four boxes in this chapter follow its argument step by step: first what it means for forces to be balanced, then what happens when the net force is zero, then Newton’s second law, and finally the third. Three ideas carry all ten questions:

• Net force. Several forces may act at once; only their combined effect decides the motion. Balanced forces give a net force of zero.
• Newton’s first law. With zero net force, an object at rest stays at rest and one already moving keeps moving with the same constant velocity.
• Newton’s second and third laws. $F = ma$, so for the same acceleration a larger mass needs a larger force; and every force comes in a pair, equal and opposite, acting on two different bodies.

Key insight. “Moving” and “being pushed” are not the same thing, and half of these questions are built on the gap between them. A net force is needed to change a velocity, never to maintain one. So a toy car at constant velocity has no net force on it, an object with no net force can be moving perfectly happily, and the moment something starts to tilt, speed up or slow down, you know the forces stopped balancing.

Pause and Ponder, page 97. The box that follows Section 6.2, on balanced and unbalanced forces.

Question 1

A weightlifter lifts a barbell (Fig. 6.8). List two forces that are acting on the barbell. Are these forces balanced if the weightlifter keeps the barbell steady?

Solution. Look only at the barbell and ask what is touching or pulling it.

The Earth pulls it down: that is the gravitational force, its weight. The weightlifter’s hands push it up: that is the upward force applied by the weightlifter, a contact force.

Now the second part. “Steady” means the barbell is at rest and staying at rest — its velocity is not changing. By Newton’s first law that requires a net force of zero, so the upward push must be exactly equal in magnitude to the weight and opposite in direction. The two forces are balanced.

Note what this does not say: the weightlifter is certainly working hard, but a large force and an unbalanced force are different things. It is the equality of the two that keeps the barbell still.

Question 2

Two players R and S are participating in an arm-wrestling match (Fig. 6.9). At the instant, when the arms tilt to the front direction (out of the page towards you), are the forces exerted by the players balanced? If not, which player exerted the larger force?

Solution. The test is the motion, not the effort. While the joined hands are still, the two pushes balance and the net force is zero. The moment the arms begin to tilt, the hands have started to move — a velocity has changed — so there must be a net force acting, which means the two pushes are not balanced.

The direction of that net force is the direction the hands move in: out of the page, towards the front. So the larger force is the one pushing that way, and the player exerting it is winning at that instant.

The book’s key names S as that player. The figure itself shows the two forearms with hands clasped, R on the left and S on the right, seen from the side of the table, and gives no cue as to which player is sitting nearer the reader — so it is the key, not the drawing, that settles which of the two is pushing out of the page.

Pause and Ponder, page 101. The box that follows Section 6.4, on Newton’s first law of motion.

Question 3

An object is moving with a constant velocity. Is there a net force acting upon it?

Solution. Constant velocity means both the speed and the direction are unchanging — nothing about the motion is changing at all. By Newton’s first law, that is exactly the state an object keeps when no net force acts on it.

So the net force is zero. Individual forces may well be acting: the box in Example 6.2 has a push forward and friction backward, and keeps moving at constant velocity because the two are equal and cancel. What must vanish is their sum, not each of them.

Question 4

Suppose, no net force is acting on an object. Which of the following situations are possible?

(i) Object remains at rest if at rest.

(ii) Object keeps moving with a constant velocity if already moving.

(iii) Object is moving with a constant acceleration.

Solution. Newton’s first law names precisely two possibilities for an object with no net force on it: stay at rest, or carry on at constant velocity. Situations (i) and (ii) are those two, so both are possible.

Situation (iii) is not. Acceleration means the velocity is changing, and by $F = ma$ a non-zero acceleration requires a non-zero net force. With no net force, the acceleration is zero — so a constant acceleration is only possible if that constant is zero, which is the case already covered by (i) and (ii).

Question 5

In the real world, it is difficult to find a situation where no forces are acting on an object. But by applying additional forces, a condition can be achieved where the net force on the object is zero. Explain with the help of an example.

Solution. The point of the question is the difference between no forces and no net force. Gravity acts on everything near the Earth, and surfaces and air push back, so an object with literally no force on it is not something you will find on a table. But forces can be arranged so that they cancel.

Take the box of Example 6.2, being pushed along the floor. Friction acts backwards on it, opposing the motion. If the person pushes forwards with a force exactly equal in magnitude to the friction, the two cancel: the net force is zero, and by Newton’s first law the box carries on at constant velocity even though two substantial forces are acting on it.

A book lying on a table is the same idea at rest: the Earth pulls it down with its weight, and the table pushes it up with an equal normal force, so the net force is zero and the book stays put.

Pause and Ponder, page 106. The box that follows Section 6.5, on Newton’s second law of motion.

Question 6

A toy car of mass 100 g is moving with a constant velocity of 0.5 m s⁻¹. What is the net force acting on the toy car?

Solution. The mass and the speed are both given, and neither is needed — which is the point of the question. What matters is the word constant.

Constant velocity means the acceleration is zero, and Newton’s second law gives

$$F = ma = (0.1\ \text{kg}) \times 0 = 0\ \text{N}$$

Multiplying the mass by the velocity, $0.1 \times 0.5 = 0.05$, would give a number, but that number is the momentum, not a force. A force is mass times acceleration.

Question 7

Two children of different masses are sitting on identical swings. To impart identical initial acceleration, for which child would you require to apply a larger force? Explain why.

Solution. Newton’s second law relates the three quantities directly:

$$F = ma$$

The acceleration $a$ is to be the same for both children, so the force needed is proportional to the mass. The child with the greater mass needs the greater force.

The reason behind the formula is inertia: a larger mass resists a change in its motion more strongly, so more force is required to produce the same change. A child of twice the mass needs twice the push for the same starting acceleration.

Question 8

How are glass items packed for transportation using a bubble wrap or hay protected from damage?

Solution. When a packed crate is jolted or dropped, the glass inside has to be brought to rest. What breaks it is not the change in velocity itself but how quickly that change happens: a sudden stop means a large acceleration, and by $F = ma$ a large force on the glass.

Bubble wrap and hay are soft and compress. They let the glass keep moving a little further as it is brought to rest, stretching the stop out over a longer time and a longer distance. The same change in velocity spread over a longer time is a much smaller acceleration, and so a much smaller force on the glass.

Landing on a bare floor stops the glass in a fraction of the time, so the force is many times larger — and that is what breaks it.

Pause and Ponder, page 110. The box that follows Section 6.6, on Newton’s third law of motion.

Question 9

Why does a fireperson sometimes struggle when holding the pipe issuing water?

Solution. This is Newton’s third law with a hose in it. The pipe pushes a large mass of water forwards, very fast, every second — that is a large force exerted by the pipe on the water.

Forces come in pairs. The water therefore pushes back on the pipe with an equal force in the opposite direction, backwards. Nothing about the water leaving reduces that reaction; the faster and heavier the jet, the harder the push back.

So the pipe is being driven backwards all the time the jet is running, and the fireperson has to exert a large force simply to hold it still. That backward push is exactly the recoil of a gun, and the reason a rocket moves at all.

Question 10

Suppose a spacecraft is moving in a region of space where the gravitational force acting upon it is negligible. Suggest how can it change its velocity.

Solution. With gravity negligible and nothing to push against, there seems to be nothing available to change the motion — and by Newton’s first law the spacecraft will otherwise drift on at constant velocity for ever.

Newton’s third law provides the way out: the spacecraft can push on something it carries with it. Firing its engines throws exhaust gases out of the back at high speed. The spacecraft exerts a backward force on those gases, so the gases exert an equal forward force on the spacecraft. That is a genuine net force on the craft, and it changes its velocity.

Firing thrusters sideways turns the craft or changes its direction the same way. No external surface, air or ground is needed — only something to throw away.

Common mistakes

• Answering Question 6 with $0.1 \times 0.5 = 0.05\ \text{N}$. Mass times velocity is momentum, not force. Force is mass times acceleration, and here the acceleration is zero because the velocity is constant.
• Saying a moving object must have a net force on it. This is the commonest error in the whole chapter, and Questions 3, 4 and 6 all test it. A net force is needed to change motion, not to continue it.
• Answering Question 1 with “no, they are not balanced, because the weightlifter is working hard”. Effort is not the test. The barbell is not accelerating, so the forces on it balance exactly.
• Treating a force and its reaction as balancing each other in Questions 9 and 10. They are equal and opposite but act on different bodies — one on the water, one on the pipe — so they never cancel. Balanced forces act on the same body.
• Explaining bubble wrap as “it absorbs the force”. What it actually does is lengthen the time of the stop, which reduces the acceleration and so the force. Saying why is the difference between the answer and a guess.
• Choosing (iii) in Question 4 because a constant acceleration “sounds steady”. Constant acceleration still means the velocity is changing, which needs a net force.

Practise next

• The end-of-chapter questions, Revise, Reflect, Refine, where the same three laws are applied to numerical problems on force, mass and acceleration.
• Chapter 7, Work, Energy, and Simple Machines — its in-text questions ask what these same forces do over a distance.