NCERT Class 9 Science How Forces Affect Motion – Newton’s Laws Explained

Short Intro:
This chapter explores how forces affect motion, focusing on Newton’s three laws, friction, and real-world applications. Learn how balanced and unbalanced forces influence objects, and practice step-by-step solutions for various problems.

Quick Information Box:

• Chapter: 6 – How Forces Affect Motion
• Class: 9
• Key Topics: Force, Friction, Newton’s Laws of Motion, Acceleration, Mass, Systems of Objects
• Unit of Force: Newton (N)
• Key Concept: Force produces acceleration and occurs in action-reaction pairs

Concepts Used (Topics Covered):

• Definition of Force
• Measuring Force with Spring Balance
• Balanced and Unbalanced Forces
• Friction: Static and Kinetic
• Newton’s First Law (Law of Inertia)
• Newton’s Second Law (F = ma)
• Newton’s Third Law (Action-Reaction)
• Systems of Objects and Tension Forces

Important Formulas:

• Net Force: $F_\text{net} = ma$
• Gravitational Force: $F = mg$
• Acceleration from Kinematics: $a = \frac{v-u}{t}$
• Force from Momentum: $F = \frac{\Delta p}{\Delta t}$

Think It Over

1. Why does a canoe move forward when the canoeist pushes water backwards with their paddle and why does it move
   faster when they push harder?

2. Suppose the same canoeist uses the same paddle force in two different canoes, one empty and one carrying another passenger. In which case will the canoe move faster?

Pause and Ponder

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?

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

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

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

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.

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?

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.

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

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

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.

Revise, Reflect, Refine

1. Using a horizontal force F, a table is moved across the floor at a constant velocity. How much is the frictional force exerted by the floor on the table?

2. For a ball moving on a smooth frictionless surface, choose the appropriate option that will make the following statements physically correct.
   (i) If no net force is applied on the ball, the velocity of the ball will remain the same/increase/decrease.
   (ii) If a net force is applied on the ball in the direction of its motion, the magnitude of the velocity of the ball will remain the same/ increase/decrease.
   (iii) If a net force is applied on the ball in a direction opposite to the direction of its motion, the magnitude of the velocity of the ball will remain the same/increase/decrease.

3. Two blocks P and Q on a smooth horizontal surface are shown in Fig. 6.36a and Fig. 6.36b. Two forces of magnitudes 4 N and 5 N are acting in opposite directions on block P, while block Q is moving with a constant velocity.

Which of the following statement is correct?
(i) P experiences a net force and Q does not experience a net force.
(ii) P does not experience a net force and Q experiences a net force.
(iii) Both P and Q experience a net force.
(iv) Neither P nor Q experiences a net force.

4. While practising for the snake boat race (Vallum kalli in Kerala), 100 oarsmen are rowing a boat together. Out of these, 95 row backwards to propel the boat forward. But by mistake, 5 oarsmen row in the opposite direction. If each oarsman applies a horizontal force of 200 N, what is the net force on the snake boat? (Ignore drag forces, air friction, etc.)

5. When a net force acts on an object, we observe that the object accelerates: 
   (i) opposite to the direction of force, with acceleration proportional to the force acting on the object.
   (ii) opposite to the direction of force, with acceleration proportional to the mass of the object.
   (iii) in the direction of force, with acceleration inversely proportional to the force acting on the object.
   (iv) in the direction of force, with acceleration proportional to the force acting on the object.

6. The position-time graph for four objects A, B, C and D moving along a straight line are given in Fig. 6.37. A net force acts on:
   (i) Object A
   (ii) Object B
   (iii) Object C
   (iv) Object D

7. A sailor jumps out from a small boat to the shore (Fig. 6.38). As the sailor jumps forward, will the boat move? If yes, in which direction and why.

8. During a high jump event, a landing mat or sand bed is placed for the athlete to fall upon (Fig. 6.39). Explain the reason behind it

9. A hand cart loaded with vegetables collides with an identical but empty hand cart. During the collision:
   (i) the loaded cart exerts a force of larger magnitude on the empty cart.
   (ii) the empty cart exerts a force of larger magnitude on the loaded cart.
   (iii) neither cart exerts a force on the other.
   (iv) the loaded cart and the empty cart, both exert an equal magnitude of force on each other.

10. The acceleration-mass graph for the acceleration produced by a force on objects of different masses is plotted in Fig. 6.40. Plot the force-mass graph for this case.

11. The velocity-time graph of an object of mass 10 kg moving along a straight line is shown in Fig. 6.41. Calculate the force acting on the object by using the graph.

12. A bullet of mass 50 g moving with a speed of 100 m s–1 enters a heavy stationary wooden block and stops after penetrating a distance of 50 cm. Estimate the stopping force acting on the bullet (assume that the bullet undergoes constant acceleration within the block).

13. An ace footballer converted a penalty shot by kicking the football with a speed of 108 km h–1. The estimated force they imparted was 800 N. The mass of the football was 0.4 kg. Calculate the time of contact between their foot and the ball.

14. An object of mass 2 kg moving with a constant velocity of 10 m s–1 encounters a rough patch where the force of friction on the object is 7 N. At the same time, an additional constant force of 3 N opposing the motion is applied on the object. After entering the rough patch, how much distance does the object travel before coming to rest?

15. A tractor pulls a harrow (a ploughing tool) of mass m1 with a net force F resulting in an acceleration of a1 . The same tractor pulls a trolley of mass m2 with a force F producing an acceleration of a2 . If the tractor now pulls the trolley with the harrow placed on it (with the same force F ), then obtain an expression for the resulting acceleration in terms of a1 and a2 . Ignore friction.

16. When the pole of a bar magnet is brought close to a magnetic compass, the bar magnet and the compass needle which is also a magnet) exert a magnetic force on each other. As per Newton’s third law of motion, both the forces are equal in magnitude and opposite in direction. However, the compass needle moves, whereas the bar magnet does
    not move (Fig. 6.42). Explain why.

Common Mistakes:

• Forgetting friction acts opposite to motion
• Confusing balanced vs unbalanced forces
• Neglecting mass when calculating acceleration
• Misinterpreting action-reaction pairs

Exam Tips:

• Always draw a free-body diagram
• Identify all forces before applying Newton’s laws
• Check the direction of net force
• Use SI units consistently

Practice MCQs:

1. Two forces 4 N and 5 N act on block P in opposite directions. Net force? (Answer: 1 N)
2. A 0.1 kg bullet fired from 5 kg gun → recoil acceleration of gun? (Answer: $a = F/m = 0.4\, m/s^2$
3. Frictionless surface, net force = 0 → velocity? (Answer: constant)

FAQ Section:

• Q: Does a stationary object require a force to remain stationary?
  A: No, only to overcome friction or initiate motion.
• Q: Can acceleration be zero if a force is applied?
  A: Only if net force = 0.