The chapter titled “Force and Laws of Motion” corresponds to chapter 8 in the NCERT textbook for Class 9.
Below, we have provided supplementary learning materials for this chapter.
- Concise Notes or Summary PDF
- Extra questions on each topic of the chapter is included.
- contains multiple choice questions (mcq)
- fill in the blanks
- true and false
- very short question answers
- short answer questions
- long question answers
- assertion reason question answers
Download NCERT Textbook for Class 9th
Table of Contents
Force and Laws of Motion Notes
In the chapter we will learn about the following topics:
- Cause of Motion
- Balanced and Unbalanced Forces
- First Law of Motion
- Inertia and Mass
- Second Law of Motion
- Mathematical formulation of Second Law of Motion
- Third Law of Motion
Cause of Motion
- Two scientists (Galileo Galilei and Sir Isaac Newton) discovered and deduced the cause of motion.
- Pushing, hitting and pulling of objects are all ways of bringing objects in motion.
- Force is the push or pull applied on a body.
- No person can see, taste or feel force.
- Force can change
- the magnitude of velocity of an object. (slower or faster)
- the direction of motion of an object.
- change the shape and size of objects.
Balanced and Unbalanced Forces
Balanced Forces:
- Balanced forces are equal and opposite forces acting on an object from both sides. These forces cancel each other out, resulting in no change in the object’s state of rest or motion.
- For example, if a wooden block is pulled from both sides with equal forces, it remains stationary because the forces are balanced.
Unbalanced Forces:
- Unbalanced forces occur when two opposite forces acting on an object have different magnitudes. In this case, the object moves in the direction of the greater force. This unbalanced force causes a change in the object’s state of motion.
- For instance, if one side of the wooden block is pulled with a greater force than the other, the block moves towards the direction of the stronger force.
Topic-based Question-Answers (Force and Laws of Motion)
Q.1 Why does a wooden box not move when children push it with a small force on a rough floor?
Ans: When children push a wooden box with a small force on a rough floor, the box doesn’t move because the frictional force from the floor opposes the children’s force. Both forces are equal but opposite, so they cancel each other out and the box stays still.
To move, the force must be greater than the frictional force, creating an unbalanced force. This unbalanced force causes the box to accelerate in the direction of the larger force and move.
Q.2 Why does a cycle stop moving when we stop pedalling?
Ans: When we stop pedaling a bicycle, the forward force ceases. Friction and air resistance, which are unbalanced forces, then act in the opposite direction, slowing the bicycle down. Because there is no equal forward force to counteract them, the bicycle eventually stops.
Deep Concepts:
- An object moves with a uniform velocity when the pushing force and frictional force are balanced and there is no net external force on it.
- If an unbalanced force is applied on the object, there will be a change either in its speed or in the direction of its motion.
- An object will accelerate until an unbalanced force acts on it.
Q.2 If a boy throws an object at 20 km/h, will it stop if no external forces act upon it?
Ans: No, if no external forces act upon it, the object will continue moving at 20 km/h indefinitely due to Newton’s First Law of Motion (inertia).
First Law of Motion
- Galileo observed that objects move at constant speed when no force acts on them. By studying a marble on an inclined plane, he noted its velocity increases when rolling down due to gravity and decreases when climbing up. On an ideal frictionless plane inclined on both sides, the marble reaches the same height from which it was released. If the opposite incline is gradually flattened, the marble travels further, maintaining motion indefinitely on a horizontal plane since no unbalanced force acts on it.
- In reality, friction opposes motion, because of which the marble stops after some time.
- Newton’s First Law of Motion states that an object remains at rest or in uniform motion in a straight line unless acted upon by an external force.
- The tendency of an object to stay at rest or to keep moving with the same velocity is called Inertia.
- The first law of motion is also known as law of inertia.
Questions on First Law of Motion (Force and Laws of Motion)
1. Why does our body tend to move forward even after the car slows down?
Ans: When a car suddenly slows down, our body tends to continue moving forward due to inertia. Inertia is the tendency of an object to resist changes in its state of motion. Since our body was moving forward with the car, it continues to move forward even when the car stops, causing us to lean forward.
2. Why do we tend to fall backwards when a bus suddenly starts moving?
Ans: When a bus suddenly starts moving, our body tends to remain at rest due to inertia. Inertia is the tendency of an object to resist changes in its state of motion. Since our body was at rest before the bus started moving, it tends to stay in that state, causing us to fall backward.
3. Why do we tend to get thrown to one side when a motorcar makes a sharp turn at high speed?
Ans: When a car makes a sharp turn at high speed, our body tends to continue moving in a straight line due to inertia. Inertia is the tendency of an object to resist changes in its state of motion. Since our body was moving in a straight line within the car, it continues to move in that direction when the car turns, causing us to feel like we are being thrown to the side.
4. What happens to the pile of carom coins when you attempt a sharp horizontal hit at the bottom of the pile using another carom coin or the striker?
Ans: When a carom coin or striker hits the bottom of the pile of carom coins with a sharp horizontal blow, the coin at the bottom moves due to the force, while the rest of the coins in the pile tend to remain stationary due to inertia. This causes the bottom coin to move away, and the rest of the coins fall in place, maintaining the structure of the pile.
5. Why does the coin fall vertically into the glass tumbler when the card it was on is flicked away quickly?
Ans: When a card is quickly flicked away from under a coin placed on a glass, the coin falls straight down into the glass. This happens because of inertia. The coin tends to stay in its state of rest. When the card is removed quickly, the coin’s downward motion is not obstructed, and it falls directly into the glass tumbler due to gravity.
6. Why does the water spill when we place a water-filled tumbler on a tray, and turn around as fast as we can?
Ans: When a water-filled tumbler is placed on a tray and the tray is turned around quickly, the water spills due to inertia. The water inside the tumbler tends to resist changes in its state of motion. When the tray and the tumbler suddenly change direction, the water, due to its inertia, tends to remain in its original position. This causes the water to move forward, and since the tumbler is moving, it spills over the side.
Inertia and Mass
- If a body is at rest, it tends to remain at rest; if it is moving it tends to keep moving. This property of an object is called its inertia.
- Inertia is the natural tendency of an object to resist a change in its state of motion or of rest. The mass of an object is a measure of its inertia.
- A heavier object has more inertia than a lighter object.
Second Law of Motion
- Momentum is defined as the product of mass and velocity i.e. p = mv.
- The second law of motion states that the rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.
- Unit of mass must be taken in kg, unit of velocity in m/s and acceleration in m/s2.
Third Law of Motion
- When a body exerts force on another body, the force applied by one body is called action force and the equal and opposite force applied by the other is called reaction force.
- The Third Law of Motion states that when one object exerts a force on another object, the second object instantaneously exerts an equal and opposite force back on the first.
- These two forces are always equal in magnitude but opposite in direction.
- To every action there is an equal and opposite reaction.
- The action and reaction always act on two different objects simultaneously.
Questions on Third Law of Motion
1. What force causes you to move forward when you start walking, and how does it relate to the second law of motion?
Ans: The force that causes you to move forward when you start walking is the force exerted by the ground on your feet in the opposite direction of your push. This relates to the second law of motion because the force you exert on the ground causes an equal and opposite reaction force (from the ground) that accelerates your body forward, changing its momentum according to the law.
2. Why do action and reaction forces, despite being equal in magnitude, not produce accelerations of equal magnitudes?
Ans: Action and reaction forces act on different objects, and because these objects have different masses, they experience different accelerations. This is due to the relationship between force, mass, and acceleration, as defined by Newton’s Second Law of Motion (F=ma).
3. What causes the recoil of a gun when it is fired, and why is the gun’s acceleration much less than that of the bullet?
Ans: The recoil of a gun is caused by the conservation of momentum. When a bullet is fired forward, the gun moves backward to conserve momentum. The gun’s acceleration is much less than the bullet’s because the gun has a much larger mass.
4. Why does the boat move backwards when a sailor jumps forward out of it?
Ans: The boat moves backwards when a sailor jumps forward due to the conservation of momentum. As the sailor jumps forward, an equal and opposite reaction pushes the boat backward to maintain the system’s total momentum.
5. How does placing two children one on a cart and one on another cart demonstrate the second law of motion?
Ans: Placing two children, one on each of two carts, demonstrates the second law of motion because if the children exert equal forces on their respective carts, the cart with the smaller mass will experience a greater acceleration. This illustrates that acceleration is inversely proportional to mass for a given force, as stated by Newton’s Second Law (F = ma).
Download the Summary of Force and Laws of Motion NCERT Chapter 8 for Class 9 pdf
Numerical Problems on Force and Laws of Motion
- A force acts on a 10 kg object for 4 seconds, changing its velocity from 2 m/s to 10 m/s. Calculate the magnitude of the applied force. If the same force acts for 6 seconds, what will be the final velocity?
- An object of mass 8 kg experiences a constant force for 3 seconds, increasing its velocity from 5 m/s to 11 m/s. Determine the applied force. What would be the object’s final velocity if the force was applied for 7 seconds?
- A 6 kg body is subjected to a force over 2.5 seconds, changing its velocity from 4 m/s to 9 m/s. Find the magnitude of the force. If the same force acted for 4 seconds, what would be the final velocity?
- A constant force is applied to a 12 kg object for 3 seconds, increasing its velocity from 1 m/s to 6 m/s. What is the magnitude of the force? If the force continues for another 5 seconds, what will be the object’s final velocity?
- An object with a mass of 15 kg has its velocity increased from 0 m/s to 5 m/s in 2 seconds under a constant force. Calculate the force involved. If the force was applied for 8 seconds instead, what would be the final velocity of the object?
- What is the force needed to accelerate a 3 kg mass at 4 m/s²?
- How much force is required to accelerate a 5 kg mass at 3 m/s²?
- Determine the force necessary to accelerate a 1 kg mass at 6 m/s².
- Calculate the force needed to accelerate a 6 kg mass at 2 m/s².
- What force is required to accelerate a 4 kg mass at 5 m/s²?
- A truck is travelling at a speed of 90 km/h and comes to a halt in 5 seconds after the brakes are applied. Determine the force exerted by the brakes if the truck’s mass is 2000 kg.
- A bike is moving at a velocity of 72 km/h and stops completely in 3 seconds after braking. What is the force applied by the brakes on the bike if its total mass is 150 kg?
- A bus with a mass of 5000 kg is moving at 60 km/h and comes to rest in 6 seconds once the brakes are applied. Calculate the braking force exerted on the bus.
- A car weighing 1200 kg is travelling at 80 km/h and takes 5 seconds to stop after the brakes are engaged. Find the force exerted by the brakes.
- A van moving at 100 km/h comes to a complete stop in 7 seconds after applying the brakes. If the mass of the van including passengers is 1800 kg, compute the force exerted by the brakes.
- A force of 8 N gives a mass m1 an acceleration of 4 m/s² and a mass m2 an acceleration of 6 m/s². What acceleration would it give if both the masses were tied together?
- A force of 12 N causes a mass m1 to accelerate at 3 m/s² and a mass m2 at 9 m/s². Find the resultant acceleration when both masses are connected.
- A force of 15 N produces an acceleration of 5 m/s² in mass m1 and 7.5 m/s² in mass m2. What will be the acceleration if both masses are tied together?
- A force of 20 N provides an acceleration of 2 m/s² to mass m1 and 4 m/s² to mass m2. Calculate the acceleration when both masses are combined.
- A force of 10 N results in an acceleration of 8 m/s² for mass m1 and 12 m/s² for mass m2. Determine the acceleration when both masses are tied together.
21. The velocity-time graph of a ball of mass 40 g moving along a straight line on a long table is given in Fig. 1. How much force does the table exert on the ball to bring it to rest?
22. The velocity-time graph of a car of mass 250 kg moving along a straight line on a road is given in Fig. 2. How much force does the brake exert on the car to bring it to rest?
23. A car of 300 kg is moving on the road and changes speed as shown in the velocity-time graph in Fig. 3. How much force should be applied by the car to attain the maximum speed in 5 seconds.
Please find solutions to the numerical problems here.