How Forces Affect Motion Class 9 New Science Book
Have you ever wondered why a football starts moving when kicked or why a bicycle slows down when brakes are applied? The answer lies in the concept of force. In our daily lives, we observe many situations where objects start moving, stop moving, speed up, slow down, or change direction. These changes occur because a force acts on the objects.
In this chapter, How Forces Affect Motion Class 9 New Science Book, you will learn what force is and how it influences the motion of objects. You will explore different types of forces, their effects, and how they can change the speed, direction, or shape of an object. Understanding these concepts will help you explain many everyday phenomena and build a strong foundation for further studies in physics.
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?
Answer
When a table moves with a constant velocity, its acceleration is zero. According to Newton’s First Law of Motion, the net force acting on the table must be zero. Therefore, the frictional force exerted by the floor is equal in magnitude and opposite in direction to the applied force F.
Frictional force = F
2. For a ball moving on a smooth frictionless surface, choose the appropriate option that will make the following statements physically correct.
Answer
(i) If no net force is applied on the ball, the velocity of the ball will remain the same.
(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 increase.
(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 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?
Answer
For block P, two opposite forces of 4 N and 5 N act on it. Therefore, the net force on P is:
Net force = 5 N − 4 N = 1 N
Hence, block P experiences a net force of 1 N.
Block Q is moving with a constant velocity. Therefore, the net force acting on Q is zero.
Correct option: (i) P experiences a net force and Q does not experience a net force.
4. While practising for the snake boat race (Vallum Kali 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.)
Answer
Force applied by 95 oarsmen = 95 × 200 = 19000 N
Force applied by 5 oarsmen in the opposite direction = 5 × 200 = 1000 N
Net force = 19000 N − 1000 N = 18000 N
Net force on the snake boat = 18000 N (forward).
5. When a net force acts on an object, we observe that the object accelerates:
Answer
According to Newton’s Second Law of Motion, acceleration is produced in the direction of the net force and is directly proportional to the force acting on the object.
Correct option: (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:
Answer
A net force acts on the object whose velocity changes with time, i.e., whose position-time graph is curved.
Correct option: (iv) Object D.
7. A sailor jumps out from a small boat to the shore. As the sailor jumps forward, will the boat move? If yes, in which direction and why?
Answer
Yes, the boat will move.
When the sailor jumps forward, he pushes the boat backward. According to Newton’s Third Law of Motion, the boat exerts an equal and opposite force on the sailor. Therefore, the boat moves backward while the sailor moves forward.
8. During a high jump event, a landing mat or sand bed is placed for the athlete to fall upon. Explain the reason behind it.
Answer
A landing mat or sand bed increases the time taken by the athlete to come to rest after landing. Since the change in momentum occurs over a longer time, the impact force is reduced. This helps prevent injuries.
9. A hand cart loaded with vegetables collides with an identical but empty hand cart. During the collision:
Answer
According to Newton’s Third Law of Motion, when two objects interact, they exert equal and opposite forces on each other.
Correct option: (iv) the loaded cart and the empty cart both exert equal magnitude forces 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.
Answer
From Newton’s Second Law:
F = ma
Using the values from the graph:
- At m = 1 kg, a ≈ 10 m/s² → F = 10 N
- At m = 2 kg, a ≈ 5 m/s² → F = 10 N
- At m = 4 kg, a ≈ 2.5 m/s² → F = 10 N
- At m = 5 kg, a ≈ 2 m/s² → F = 10 N
Thus, the force remains constant at 10 N for all masses.
Therefore, the force-mass graph is a horizontal straight line parallel to the mass axis at F = 10 N.
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.
Answer
Given:
- Mass of the object, m = 10 kg
- Initial velocity, u = 10 m/s
- Final velocity, v = 30 m/s
- Time taken, t = 8 s
First, calculate the acceleration:
Now, using Newton’s Second Law:
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.
Answer
Given:
- Mass of bullet,
- Initial speed,
- Final speed,
- Distance penetrated,
Using the equation of motion:
(The negative sign indicates retardation.)
Now, using Newton’s Second Law:
The negative sign shows that the force acts opposite to the motion.
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.
Answer
Mass, m = 0.4 kg
Initial velocity, u = 0
Final velocity, v = 108 km/h = 30 m/s
Force, F = 800 N
Acceleration:
a = F/m
a = 800/0.4
a = 2000 m/s²
Using:
v = u + at
30 = 0 + 2000t
t = 30/2000
t = 0.015 s
Time of contact = 0.015 s.
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?
Answer
Total opposing force = 7 N + 3 N = 10 N
Mass = 2 kg
Acceleration:
a = F/m
a = −10/2
a = −5 m/s²
Using:
v² = u² + 2as
0 = 10² + 2(−5)s
0 = 100 − 10s
s = 10 m
Distance travelled before stopping = 10 m.
15. A tractor pulls a harrow 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, obtain an expression for the resulting acceleration.
Answer
For the harrow:
F = m₁a₁
m₁ = F/a₁
For the trolley:
F = m₂a₂
m₂ = F/a₂
Combined mass:
M = m₁ + m₂
M = F/a₁ + F/a₂
Resulting acceleration:
a = F/M
a = F / [F(1/a₁ + 1/a₂)]
a = 1 / (1/a₁ + 1/a₂)
Therefore,
a = (a₁a₂)/(a₁ + a₂)
16. When the pole of a bar magnet is brought close to a magnetic compass, the bar magnet and the compass needle exert equal and opposite forces on each other. However, the compass needle moves, whereas the bar magnet does not move. Explain why.
Answer
According to Newton’s Third Law, the bar magnet and the compass needle exert equal and opposite forces on each other.
However, the compass needle has a very small mass and can rotate freely, so the magnetic force produces noticeable motion. The bar magnet has a much larger mass and is usually held firmly, so the same force produces negligible acceleration.
Therefore, the compass needle moves while the bar magnet appears stationary.
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