Class 9th- Force & Laws of Motion (Prashant Kirad) (1).pdf

Force and Laws of Motion  Topics to be Covered Force Galileo’s Analysis about Motion Newton’s Laws of motion  Rest and Motion: An object is at rest if its position relative to its surroundings doesn't change over time. An object is in motion if its position do...

Force and Laws of Motion  Topics to be Covered Force Galileo’s Analysis about Motion Newton’s Laws of motion  Rest and Motion: An object is at rest if its position relative to its surroundings doesn't change over time. An object is in motion if its position does change with time.  FORCE “Force is defined as a push or pull on the body.” Force is an external effort that can change an object's state of motion or rest, or change its shape. Force is a vector quantity. Its SI Unit is Newton (N). Its CGS unit is dyne. 1 N = 100000 dyne Effects of Forces: → Force can make a stationary body move. e.g.: a football can be set to move by kicking it i.e. by applying a force. → Force can stop a moving body. e.g., by applying brakes, a running cycle or a running vehicle can be stopped. → Force can change the direction of a moving object. e.g: by applying force i.e. by moving steering, the direction of a running vehicle is changed. → Force can change the speed of a moving body. When you push a swing, you apply force to make it go higher and faster. → Force can change the shape and size of an object. e.g.: by hammering, a block of metal can be turned into a thin sheet. By hammering, a stone can be broken into pieces.  Balanced Force Unbalanced Force 1. When two or more forces act on a body and 1. When two or more forces act on a body and produce a net force equal to zero, the forces produce a net force not equal to zero, the are called balanced forces. forces are called unbalanced forces. 2. A balanced force does not produce any 2. An unbalanced force can produce a change change in the state of uniform motion or rest in the state of uniform motion or rest of the of the body. body. 3. A balanced force does not cause a body to 3. An unbalanced force can accelerate a body. accelerate. 4. In balanced forces, the forces are equal in 4. In unbalanced forces, the forces are magnitude and opposite in direction, unequal in magnitude, causing the object to canceling each other out. move in the direction of the greater force.  RESULTANT FORCE The resultant force is the total force acting on an object when multiple forces are combined. It's also known as the net force. The resultant force has the same effect on an object as the original system of forces. 730 N 950 N 760 N 500 N 1050 N  Galileo’s Analysis on Motion: Aristotle's Belief: Natural state of bodies is rest. Galileo's Opposition: Challenged Aristotle's view. Galileo's Observation: Ball rolling down an inclined plane: Speed increases. Ball rolling up an inclined plane: Speed decreases. Experiment on Horizontal Plane: On smooth surface, ball continues to move. Speed remains constant with no external force or friction. Conclusion: Bodies naturally oppose changes in their state of rest or motion.  Inertia: Inertia is defined as a property of matter by which it tries to maintain its state of rest or of uniform motion along a straight line Inertia of an object is measured by its mass. Inertia is directly proportional to the mass. This means inertia increases with increase in mass and decreases with decrease in mass. Inertia of Rest: Inertia of Motion: An object stays at rest, and it remains at rest until an An object will continue to be in motion until a external force affects it. Example: When a car accelerates, force acts on it. Example: A hockey puck will passengers may feel as though their bodies are moving continue to slide across the ice until acted upon backwards. In reality, inertia is making their bodies stay in by an outside force. place as the car moves forward. Q. Why does a person standing in a bus fall backward when the bus suddenly starts moving? Initially, both the bus and the person are at rest. When the bus suddenly starts moving, the legs of the person move with the bus, but the upper part of their body tends to remain at rest due to inertia, causing them to fall backward if they are not alert. Q. Why does a person standing in a moving bus fall forward when the driver suddenly applies the brakes? When the bus is moving, the person is also in motion along with it. When the brakes are suddenly applied, the bus slows down or stops abruptly, but the upper part of the person’s body tends to remain in motion due to inertia, causing them to fall forward. Q. Why are seat belts used in cars and other vehicles? Seat belts are used to prevent passengers from being thrown forward in case of sudden braking or an accident. When the vehicle stops suddenly, passengers tend to continue moving forward due to inertia, and the seat belt restrains them and prevents injuries. Q. Why do we give many jerks to wet clothes before hanging them to dry? Jerks cause the water droplets in the clothes to separate from the fabric. When the clothes are suddenly moved, the water droplets tend to remain at rest due to inertia and fall out, reducing the water content in the clothes and helping them dry faster. Q. Why does the coin fall into the glass tumbler when the card is flicked away quickly? The coin remains in its state of rest due to inertia. When the card is flicked away quickly, the coin's inertia causes it to stay in place while the card moves, so the coin falls straight down into the tumbler. Q. Why does water spill when you turn around quickly holding a tray with a water-filled tumbler? The water tends to stay in its state of rest due to inertia. When the tray is turned quickly, the water does not follow the motion of the tray immediately, causing it to spill out. Q. Why is the head of a hammer tightened by banging the handle against a hard surface? When the handle is struck against a surface, the handle comes to rest, but the head of the hammer, due to inertia, continues moving downward and tightens onto the handle after repeated strikes. Q. Why does only the bottom coin move when a pile of coins on a carom board is hit with a striker? When the striker hits the pile of coins, only the bottom coin moves due to the force applied, while the rest of the coins in the pile tend to remain at rest due to inertia, causing them to drop vertically and stay in place. Q. Between a football and rock of same size which one will have more inertia? CBSE 2016,2018  Newton’s First Law of Motion Newton’s first law of motion states that: A body remains in the state of rest or uniform motion in a straight line unless and until an external force acts on it. There are two conditions on which the 1st law of motion is dependent: Objects at rest: When an object is at rest, velocity (v = 0) and acceleration (a = 0) are zero. Therefore, the object continues to be at rest. Objects in motion: When an object is in motion, velocity is not equal to zero (v ≠ 0), while acceleration (a = 0) is equal to zero. Therefore, the object will continue to be in motion with constant velocity and in the same direction. Newton’s First Law of Motion: Examples A person standing in a bus falls backward when bus starts moving suddenly. A person standing in a moving bus falls forward if driver applies brakes suddenly. Before hanging the wet clothes over laundry line, usually many jerks are given to the clothes to get them dried quickly.  MOMENTUM Momentum is the quantity of motion an object has The product of velocity and mass is called the momentum. Momentum is denoted by ‘p’. p=mxv where, p = momentum, m = mass of the object and v = velocity of the object. Impulse = (Force × time) Momentum: Examples Even a small bullet is able to kill a A person get injured in the case of person when it is fired from a gun hitting by a moving object, such as because of its momentum due to stone, pebbles or anything because great velocity. of momentum of the object. A person get injured severely when hit by a moving vehicle because of momentum of vehicle due to mass and velocity. 20 m/s 10 m/s v v  -1 Q. What is the momentum of a body of mass 5 kg moving with a velocity of 0.20 ms. CBSE 2016,2019 Q. The mass of a goods lorry is 4000 kg and the mass of goods loaded on it is 20000 kg. If the lorry is moving with a velocity of 2 m/s what will be its momentum ?  Newton’s Second Law of Motion The rate of change of momentum of an object is directly proportional to the applied unbalanced force in the direction of the force. Newton's second law of motion states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. If any object is in the state of rest, then it will remain in rest until a external force is applied to change its state. Similarly, an object will remain in motion until any external force is applied over it to change its state. This means all objects resist to in changing their state. The state of any object can be changed by applying external forces only. Q: A person is prone to more serious injuries when falling from a certain height on a hard concrete floor than on a sandy surface. Explain why. [CBSE 2011, 2012, 2015] Ans: When a person falls from a height on a hard concrete floor, he immediately comes in rest position. It means change in momentum is taking place in an extremely short time and consequently, force exerted by the floor on the person to destroy its momentum is extremely large. Hence, chances of more injuries. When a person falls on a sandy surface, the surface gets compressed downward and it increases the time of fall. As a result for same change in momentum force exerted by sandy surface on the person is less and chances of his being hurt are less. Q. Why does a continuous push over time help to accelerate a car with a dead battery more effectively than a sudden push? Continuous pushing applies a steady force over time, gradually changing the car's momentum, which is more effective than a sudden push that does not provide enough time for momentum change. Q. Why does a fast-moving cricket ball hurt a spectator more than a table tennis ball hitting a player? A fast-moving cricket ball has greater momentum due to its higher mass and velocity, which results in a more forceful impact compared to a table tennis ball.  Mathematical Expression: Newton’s second law of motion states that the force exerted by a body is directly proportional to the rate of change of its momentum. mass = ‘m’ velocity changes from u to v time = ‘t’, force = ‘F’  Newton’s Second Law of Motion F = ma S.I. Unit of Force is kg m/² or Newton. 1 Newton: When an acceleration of 1 m/s² is seen in a body of mass 1 kg, then the force applied on the body is said to be 1 Newton.  Force Momentum Force = mass * acceleration (F = ma) Momentum = mass * velocity (p = mv) Forces can exist even when the object is Momentum for stationary objects is always stationary. zero. The direction of the force depends on the The direction of momentum depends on the acceleration. direction of velocity. Force is inversely proportional to time (F ∝ Momentum is directly proportional to time 1/t). (p ∝ t). Momentum changes with constant Force remains constant for constant acceleration, but remains constant if acceleration. velocity is constant. Q. What will be the effect on force if: a) mass of the body is doubled b) acceleration of the body is halved Q. Why does a fielder pull his hands backwards while catching a fast-moving cricket ball? Answer: The fielder pulls his hands backward to increase the time over which the ball's momentum decreases to zero. This reduces the acceleration of the ball and thus the force of impact, which helps in reducing the chance of injury. Q. Why are athletes in high jump events made to fall on cushioned or sand beds? Answer: Athletes fall on cushioned or sand beds to increase the time over which their momentum decreases to zero. This reduces the rate of change of momentum and thus the force of impact, minimizing the risk of injury. Q. Why does a karate player use a quick, powerful strike to break a slab of ice? Answer: A karate player uses a quick, powerful strike to achieve a high acceleration of the fist, which generates a large force over a very short period. This force is sufficient to break the slab of ice despite its resistance. Q. The velocity-time graph of a car of 1000 kg mass is given alongside. From the graph answer the following: [CBSE 2010, 2016, 2022] (a) When is the maximum force acting on the car? Why? (b) What is the retarding force? (c) For how long is there no force acting? Q. If the time taken to bring a ball to rest from a certain velocity v is reduced to half, what will be the changes in values of: (a) initial and final momentum (b) change of momentum (c) rate of change of momentum. [CBSE 2012, 2013, 2018] Q: Define the SI unit of force. A force of 2N acting on a body changes its velocity uniformly from 2 m/s to 5 m/s in 10 s. Calculate the mass of the body. (CBSE 2018, 2022) Q: A 8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a horizontal track. If the engine exerts a force of 40000 N and the track offers a friction force of 5000 N, then calculate the net accelerating force.  Newton’s Third Law of Motion Newton’s 3rd law states that every action has an equal and opposite reaction. Action and reaction forces are equal, opposite and acting on different bodies. “When one body exerts a force on the other body, the first body experiences a force which is equal in magnitude in the opposite direction of the force which is exerted”. The mathematical representation of Newton’s third law of motion is let A be the body exerting force F₁ on the body B, then body B too exerts a force F₂ on body A, which is given as: F₁ = -F₂ Q. Why do both players get hurt when they collide while playing football? Answer: Both players get hurt because, according to Newton's third law, each player exerts an equal and opposite force on the other. These action and reaction forces are equal in magnitude but opposite in direction, resulting in both players feeling the impact. Q. Why do you push the ground backward when trying to walk forward? Answer: When you try to walk forward, you push the ground backward. According to Newton's third law, the ground exerts an equal and opposite reaction force on your feet, which propels you forward. Q. Why does a gun recoil when a bullet is fired? Answer: When a gun is fired, it exerts a forward force on the bullet, and the bullet exerts an equal and opposite reaction force on the gun. This reaction force causes the gun to recoil backward. The difference in mass between the gun and the bullet results in different accelerations. Q. Why does a rowing boat move backward when a sailor jumps out of it? Answer: When a sailor jumps forward from a rowing boat, the force exerted by the sailor on the boat has an equal and opposite reaction force on the boat, causing it to move backward. This demonstrates Newton's third law of motion. Q. Why does a swimmer push water backward to move forward in the water? Answer: A swimmer pushes water backward using their arms and legs. According to Newton's third law, the water exerts an equal and opposite force on the swimmer, propelling them forward. The reaction force from the water helps the swimmer move forward in the water. Q. Why does a rocket move upwards when it is launched? Answer: A rocket moves upwards because it expels gases downward. According to Newton's third law, the gases push back on the rocket with an equal and opposite force, causing the rocket to accelerate upward. Q. Why is it difficult to walk on ice compared to a rough surface? Answer: Walking on ice is difficult because there is less friction between the ice and your shoes. According to Newton's third law, the reaction force you exert on the ice is the same in magnitude but opposite in direction to the friction force, which is much lower on ice than on a rough surface. MORE QUESTIONS Q. Give a reason of the following: (a) A footballer kicks a ball, which rolls on the ground and after covering some distance comes to rest. (b) Only the carrom coin at the bottom of a pile is removed when a fast moving striker hits it. [CBSE 2015, 2018, 2022] Ans:(a) When a football is rolling on the ground, a force of friction acts on it due to ground in a direction opposite to its motion. As a result, the motion of football gets slowed down and after covering some distance it comes to rest. (b) The carrom coin at the bottom of a pile comes in a state of motion due to force exerted by the striker on it. However, other coins of pile remain intact due to their inertia of rest. Q. A bullet with a mass of 10 g is fired from a rifle. The bullet takes 0.004 seconds to move through the barrel and leaves it with a velocity of 400 m/s. Calculate the force exerted on the bullet by the rifle. (CBSE 2018, 2022) Q. If the moon attracts the earth, why does the earth not move towards the moon? 1. (a) State the reason why a bullet of small mass fired from a gun can be fatal. (b) A bullet with a mass of 4 g is fired with a velocity of 50 m/s and penetrates a wall to a depth of 10 cm. Calculate the average resistance offered by the wall. (c) How will the depth of penetration into the wall change if a bullet of mass 5 g strikes it with a velocity of 40 m/s? Provide a reason to justify your answer. (2016, 2018, 2022) 2. Name the physical quantities corresponding to the following units: (CBSE 2015, 2016, 2022) 3. An object of mass 50 kg is accelerated uniformly from a velocity of 4 m/s to 8 m/s in 8 seconds. Calculate the initial and final momentum of the object. Also, find the magnitude of the force exerted on the object. (CBSE 2015, 2018, 2023) 4. A cricket player catches a ball of mass 0.1 kg moving with a speed of 10 m/s in 0.1 second. What is the force exerted by him? a) 4 N b) 2 N c) 1 N d) 10 N 5. When an athlete comes running from a distance, he is able to jump longer. Why? (CBSE 2015, 2017, 2022)  "Shake off the inertia of distractions—don’t lose focus until your goals are in motion!" ”