Which of the following is NOT a typical effect of applying a force on an object?

Change in its direction of motion

In a tug-of-war, Team A pulls with 300 N and Team B pulls with 320 N. The rope accelerates toward Team B. Which statement explains this?

The net force is 20 N toward Team B.

The net force is 620 N toward Team B.

Team A applies a greater force.

Assertion (A): When two forces of magnitudes 10 N and 6 N act on an object, the magnitude of the net force is always 16 N. Reason (R): Forces are vector quantities, so the net force depends on both the magnitudes and directions of the individual forces.

Both A and R are true and R is the correct explanation of A.

Both A and R are true but R is not the correct explanation of A.

Two forces of equal magnitude act on a stationary box from opposite directions. Will the box move? Explain your answer.

No, the box will not move. Since the forces are equal in magnitude and opposite in direction, their net force is zero. Balanced forces do not change the state of motion of an object, so the box remains at rest.

What is the SI unit of force and how is it defined? Name the instrument commonly used to measure force.

The SI unit of force is the newton (N). One newton is defined as the force required to produce an acceleration of 1 m/s² in a body of mass 1 kg. A spring balance is commonly used to measure force.

In an experiment, a stack of coins is launched by a stretched rubber band on three different surfaces: a wooden table, a laminated table, and a polished marble floor. The coins travel the shortest distance on wood and the longest on marble. Explain why, referencing the concepts of friction and net force on the coins after launch.

When the rubber band launches the coins, they initially gain kinetic energy. After launch, the only horizontal force acting on them is friction, which opposes motion. The magnitude of friction depends on the nature of the surface; it is largest for the rough wooden surface, smaller for laminated, and smallest for polished marble. A larger frictional force produces a larger deceleration (negative acceleration), causing the coins to stop sooner, thus a shorter distance. Marble, being smoother, offers the least friction, so the coins decelerate slowly and travel the farthest.

A wooden block of mass 2 kg is pulled on a surface by two horizontal forces: 12 N to the right and 5 N to the left. Calculate the net force and its direction. Hence, find the acceleration of the block.

Net force = 12 N - 5 N = 7 N to the right. Acceleration = net force / mass = 7 N / 2 kg = 3.5 m/s² to the right.

A block of mass 2 kg is placed on a smooth horizontal surface (neglect friction). A horizontal force of 10 N is applied to it. After 5 seconds, the force is doubled to 20 N and maintained for the next 10 seconds. Calculate: (a) the acceleration during the first 5 seconds, (b) the velocity at the end of 5 seconds, (c) the acceleration during the next 10 seconds, and (d) the total distance covered from the start until the end of the 15th second.

(a) Acceleration for first 5 s: a₁ = F/m = 10 N / 2 kg = 5 m/s². (b) Velocity after 5 s: v = u + a₁t = 0 + 5 × 5 = 25 m/s. (c) Acceleration for next 10 s: a₂ = 20 N / 2 kg = 10 m/s². (d) Distance in first 5 s: s₁ = ut + ½ a₁t² = 0 + ½ × 5 × 25 = 62.5 m. Distance in next 10 s: initial velocity for this phase is 25 m/s, so s₂ = ut + ½ a₂t² = 25 × 10 + ½ × 10 × 100 = 250 + 500 = 750 m. Total distance = 62.5 + 750 = 812.5 m.

Define balanced and unbalanced forces. Give one real-life example of each and explain how the net force determines the state of motion of an object in both cases.

Balanced forces are two or more forces acting on an object that are equal in magnitude and opposite in direction, resulting in a net force of zero. Example: a book resting on a table—gravity pulls it downward while the table exerts an equal upward normal force. Unbalanced forces occur when the net force on an object is not zero, causing a change in its state of motion. Example: kicking a stationary football—the applied foot force is greater than opposing friction, so the ball accelerates. The net force is the vector sum of all forces: if zero, the object remains at rest or moves with constant velocity (inertia); if non-zero, it accelerates in the direction of the net force.

During an experiment, a student pulls a wooden block on a horizontal table using a spring balance. She records that the block just starts to move when the spring balance reads 3.8 N. Once moving, a force of 2.9 N keeps it going at a constant speed. She then places a heavy book on top of the block and repeats the experiment. (1) What is the magnitude of the maximum static friction between the block and the table? (2) Why is the force required to keep the block moving (2.9 N) less than the force needed to start it moving (3.8 N)? (3) What change do you expect in the spring balance reading when the book is added, for both starting and steady motion?

Max static friction = 3.8 N. Kinetic friction is lower due to the nature of surfaces. Adding weight increases normal force, so friction increases, requiring larger forces.

Class 9 Science Chapter 6: How Forces Affect Motion — Important Questions & Sample Paper

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Class 9 Science Chapter 6, ‘How Forces Affect Motion’, delves into the ways forces alter the state of rest or motion of objects. It begins by defining force as a push or pull and categorizes its effects: a force can start or stop motion, change speed or direction, and even alter the shape of an object. The chapter then introduces the crucial distinction between balanced and unbalanced forces. When forces on an object are balanced, it remains at rest or continues moving uniformly; unbalanced forces cause acceleration. Students learn to compute net force by simple addition or subtraction when forces act along a straight line, and by vector addition (like the parallelogram or Pythagoras method) when forces act at an angle. Friction, a contact force that opposes motion, is examined in detail: static friction prevents initial movement, while kinetic friction (sliding and rolling) acts on moving objects. Activities comparing surfaces—such as coins pushed on wood, laminated table, and polished marble—illustrate that smoother surfaces offer less friction, allowing longer travel. Exam questions often ask students to explain why a heavy box requires a minimum force to budge, calculate the resultant of two perpendicular forces, or identify the SI unit of force (newton) and the instrument used to measure it (spring balance). A solid grasp of these concepts lays the groundwork for understanding Newton’s laws in subsequent chapters.

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Science — How Forces Affect Motion

Class 9Time: 3 hrsMax Marks: 80

SECTION A

1.
A 10 kg block is at rest on a horizontal floor. The maximum static friction between the block and the floor is 40 N. A horizontal force of 25 N is applied to the block. What is the magnitude of the friction force acting on the block?
(a) 0 N(b) 25 N(c) 40 N(d) 15 N
1
2.
Two forces of 9 N and 12 N act on a body simultaneously. The angle between them can be any value. Which of the following is NOT a possible magnitude for the net force?
(a) 3 N(b) 15 N(c) 21 N(d) 22 N
1
3.
Which of the following is NOT a typical effect of applying a force on an object?
(a) Change in its shape(b) Change in its speed(c) Change in its colour(d) Change in its direction of motion
1

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In a CBSE exam, this chapter typically contributes questions across the following types. The last column shows how many original questions of each type we have ready in our bank for this chapter:

Question type	Marks each	In our bank
Multiple Choice (MCQ)	1 mark	13
Assertion–Reason	1 mark	6
Short Answer	2 marks	8
Short Answer	3 marks	6
Long Answer	5 marks	5
Case Study	4 marks	6

44 original, exam-style questions in our bank for this chapter — with answers.

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Q1. A 10 kg block is at rest on a horizontal floor. The maximum static friction between the block and the floor is 40 N. A horizontal force of 25 N is applied to the block. What is the magnitude of the friction force acting on the block?
1 mark
Multiple Choice (MCQ)
(A) 0 N(B) 25 N(C) 40 N(D) 15 N
▸ Answer▾ Answer
25 N
Q2. Two forces of 9 N and 12 N act on a body simultaneously. The angle between them can be any value. Which of the following is NOT a possible magnitude for the net force?
1 mark
Multiple Choice (MCQ)
(A) 3 N(B) 15 N(C) 21 N(D) 22 N
▸ Answer▾ Answer
22 N
Q3. Which of the following is NOT a typical effect of applying a force on an object?
1 mark
Multiple Choice (MCQ)
(A) Change in its shape(B) Change in its speed(C) Change in its colour(D) Change in its direction of motion
▸ Answer▾ Answer
Change in its colour
Q4. In a tug-of-war, Team A pulls with 300 N and Team B pulls with 320 N. The rope accelerates toward Team B. Which statement explains this?
1 mark
Multiple Choice (MCQ)
(A) The forces are balanced.(B) The net force is 20 N toward Team B.(C) The net force is 620 N toward Team B.(D) Team A applies a greater force.
▸ Answer▾ Answer
The net force is 20 N toward Team B.
Q5. Assertion (A): When two forces of magnitudes 10 N and 6 N act on an object, the magnitude of the net force is always 16 N. Reason (R): Forces are vector quantities, so the net force depends on both the magnitudes and directions of the individual forces.
1 mark
Assertion–Reason
(A) Both A and R are true and R is the correct explanation of A.(B) Both A and R are true but R is not the correct explanation of A.(C) A is true but R is false.(D) A is false but R is true.
▸ Answer▾ Answer
A is false but R is true.
Q6. Two forces of equal magnitude act on a stationary box from opposite directions. Will the box move? Explain your answer.
2 marks
Short Answer
▸ Answer▾ Answer
No, the box will not move. Since the forces are equal in magnitude and opposite in direction, their net force is zero. Balanced forces do not change the state of motion of an object, so the box remains at rest.
Q7. What is the SI unit of force and how is it defined? Name the instrument commonly used to measure force.
2 marks
Short Answer
▸ Answer▾ Answer
The SI unit of force is the newton (N). One newton is defined as the force required to produce an acceleration of 1 m/s² in a body of mass 1 kg. A spring balance is commonly used to measure force.
Q8. In an experiment, a stack of coins is launched by a stretched rubber band on three different surfaces: a wooden table, a laminated table, and a polished marble floor. The coins travel the shortest distance on wood and the longest on marble. Explain why, referencing the concepts of friction and net force on the coins after launch.
3 marks
Short Answer
▸ Answer▾ Answer
When the rubber band launches the coins, they initially gain kinetic energy. After launch, the only horizontal force acting on them is friction, which opposes motion. The magnitude of friction depends on the nature of the surface; it is largest for the rough wooden surface, smaller for laminated, and smallest for polished marble. A larger frictional force produces a larger deceleration (negative acceleration), causing the coins to stop sooner, thus a shorter distance. Marble, being smoother, offers the least friction, so the coins decelerate slowly and travel the farthest.
Q9. A wooden block of mass 2 kg is pulled on a surface by two horizontal forces: 12 N to the right and 5 N to the left. Calculate the net force and its direction. Hence, find the acceleration of the block.
3 marks
Short Answer
▸ Answer▾ Answer
Net force = 12 N - 5 N = 7 N to the right. Acceleration = net force / mass = 7 N / 2 kg = 3.5 m/s² to the right.
Q10. A block of mass 2 kg is placed on a smooth horizontal surface (neglect friction). A horizontal force of 10 N is applied to it. After 5 seconds, the force is doubled to 20 N and maintained for the next 10 seconds. Calculate: (a) the acceleration during the first 5 seconds, (b) the velocity at the end of 5 seconds, (c) the acceleration during the next 10 seconds, and (d) the total distance covered from the start until the end of the 15th second.
5 marks
Long Answer
▸ Answer▾ Answer
(a) Acceleration for first 5 s: a₁ = F/m = 10 N / 2 kg = 5 m/s². (b) Velocity after 5 s: v = u + a₁t = 0 + 5 × 5 = 25 m/s. (c) Acceleration for next 10 s: a₂ = 20 N / 2 kg = 10 m/s². (d) Distance in first 5 s: s₁ = ut + ½ a₁t² = 0 + ½ × 5 × 25 = 62.5 m. Distance in next 10 s: initial velocity for this phase is 25 m/s, so s₂ = ut + ½ a₂t² = 25 × 10 + ½ × 10 × 100 = 250 + 500 = 750 m. Total distance = 62.5 + 750 = 812.5 m.
Q11. Define balanced and unbalanced forces. Give one real-life example of each and explain how the net force determines the state of motion of an object in both cases.
5 marks
Long Answer
▸ Answer▾ Answer
Balanced forces are two or more forces acting on an object that are equal in magnitude and opposite in direction, resulting in a net force of zero. Example: a book resting on a table—gravity pulls it downward while the table exerts an equal upward normal force. Unbalanced forces occur when the net force on an object is not zero, causing a change in its state of motion. Example: kicking a stationary football—the applied foot force is greater than opposing friction, so the ball accelerates. The net force is the vector sum of all forces: if zero, the object remains at rest or moves with constant velocity (inertia); if non-zero, it accelerates in the direction of the net force.
Q12. During an experiment, a student pulls a wooden block on a horizontal table using a spring balance. She records that the block just starts to move when the spring balance reads 3.8 N. Once moving, a force of 2.9 N keeps it going at a constant speed. She then places a heavy book on top of the block and repeats the experiment.
4 marks
Case Study
1. (i) What is the magnitude of the maximum static friction between the block and the table?1 mark
2. (ii) Why is the force required to keep the block moving (2.9 N) less than the force needed to start it moving (3.8 N)?2 marks
3. (iii) What change do you expect in the spring balance reading when the book is added, for both starting and steady motion?1 mark
▸ Answer▾ Answer
Max static friction = 3.8 N. Kinetic friction is lower due to the nature of surfaces. Adding weight increases normal force, so friction increases, requiring larger forces.

Frequently asked questions

Why does a box not move when a small force is applied, but starts moving when the force increases?

This happens because of static friction. Initially, the applied force is less than the maximum static friction, so the box remains at rest. When the force reaches the limiting friction, the box just begins to move, and then kinetic friction acts.

How do I calculate the net force when two forces act at a right angle?

You cannot simply add or subtract. Use the Pythagoras theorem: R = √(F1² + F2²). For forces F1 and F2 at 90°, the resultant is the square root of the sum of squares.

What is the SI unit of force and what instrument measures it?

The SI unit is the newton (N). One newton is the force required to accelerate a 1 kg mass by 1 m/s². A spring balance is commonly used to measure force.

Which surface offers the least friction: wood, laminated table, or polished marble?

Polished marble offers the least friction because it is the smoothest surface. Rough surfaces like wood cause more friction, making objects stop sooner.

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