Page 1/6

Transcription

1 FORCE AND LAWS OF MOTION Force An agent that change or try to change the state of an object is called force.the force applied on a body can bring about the following changes: It can change the state of rest of a body or change its position. It can change the speed of the body. It can change the direction of motion of a body. Balanced Forces Forces which do not cause any change in state of rest or of uniform motion along a straight line are called balanced forces. The total balanced forces is always equal to zero (because the forces are equal and opposite) Unbalanced Forces The forces acting on a body produce any change in state of rest or motion, then the forces are said to be unbalanced Force. Unbalanced force acting on an object changes its speed or direction. The total unbalanced force is always greater than zero. Inertia Tendency of an object to resist any change in its state of rest or of uniform motion is called inertia. Or, Inertia can be defined as the property of matter by virtue of which it opposes any change in its state of rest or of uniform motion along a straight line. Inertia is classified as: Inertia Of Rest, Egs: A passenger standing in a bus leans backwards when the bus starts all of a sudden. Fruits fall down when the branches of a tree are shaken Dust particles on a carpet falls when we beat the carpet with a stick Inertia of Motion, Egs: A passenger standing in a moving bus leans forward when the bus stops all of a sudden A man carelessly alighting from a moving train leans forward Inertia of Direction, Egs: The water particles sticking to the cycle tyre are found to fly off tangentially. The passengers thrown to opposite side when a bus takes a sudden turn. edurash@gmail.com Page 1/6

2 Inertia of a body depends upon its mass. That is, massive objects possess more inertia than lighter ones. Mass is the measure of inertia. Q: - Explain why some of the leaves may get detached from a tree if we vigorously shake its branch. Answer: The answer of this cause lies behind the Newton s First Law of Motion. Initially, leaves and tree both are in rest. But when the tree is shaken vigorously, tree comes in motion while leaves have tendency to be in rest. Thus, because of remaining in the position of rest some of the leaves may get detached from a tree if we vigorously shake its branch. Q:- 4 - Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest? Answer: In a moving bus, passengers are in motion along with bus. When brakes are applied to stop a moving bus, bus comes in the position of rest. But because of tendency to be in the motion a person falls in forward direction. Similarly, when a bus is accelerated from rest, the tendency to be in rest, a person in the bus falls backwards. Newton's First Law of Motion A body continues to be in a state of rest or of uniform motion along a straight line unless an external force acts upon it. This means that every object has a tendency to resist any change in its state of rest or motion. This law is also known as law of inertia. Momentum The momentum of an object is defined as the product of its mass and its velocity. Momentum is a vector quantity and its direction will be same as that of velocity. It is represented by p. p = mv Where, m is the mass of the object, v is its velocity. SI unit of momentum is kg m/s. Newton's Second Law of Motion It states that rate of change of momentum is directly proportional to applied force and takes place in the same direction as the applied force Explanation Consider a body of mass m, having an initial velocity u. Let the body be acted upon by some force F for time t, such that its final velocity is v. Initial momentum = m u Final momentum = m v edurash@gmail.com Page 2/6

3 Change in momentum in time = m(v - u) Change in momentum in unit time = According to Newton's second law, Rate of change of momentum is directly proportional to applied force. F α F = ma (since, a = ) F = K m a (K is the constant of proportionality) Or, F = ma ( K = 1) Or, Force = mass * acceleration Unit of Force We know that SI unit of mass is kg and acceleration is m/s 2. SI unit of force is kg m/s 2. But 1kgm/s 2 is defined as 1 Newton(N) in honor of Sir Issac Newton. 1 N = 1 kg m/s 2 One Newton force is that force which produces an acceleration of 1 m/s 2 on an object of mass 1 kg. Applications of Newton's Second Law of Motion : In a cricket match a fielder moves his arms back while trying to catch a cricket ball because if he tries to stop the fast moving ball suddenly then the speed decreases to zero in a very short time. Therefore the fielder has to apply a larger force to stop the ball. Thus, he may get hurt as the ball exerts a great pressure on the hands. But if he tries to stop it gradually by moving his arms back then the velocity decreases gradually in a longer interval of time and hence the force exerted by ball on the hand decreases and the fielder does not get hurt. A karate expert can break a pile of tiles or slab of ice with a single blow. He strike the slab with his hand as fast as he can. Here, by decreasing the time of action entire momentum of the hand is reduced to zero and so force delivered will be enough to break the slab. edurash@gmail.com Page 3/6

4 Newton's Third Law of Motion: To every action there is an equal and opposite reaction". Explanation: The two forces involved are called the "action force" and the "reaction force." They are equal in magnitude and are always act in opposite directions. Forces always come in pairs. Newton's third law of motion applied to collisions between two objects. In a collision between two objects, both objects experience forces which are equal in magnitude and opposite in direction. Such forces cause one object to speed up (gain momentum) and the other object to slow down (lose momentum). Applications: Walking: When we walk on the ground, then our foot pushes the ground backward (action force) and the ground in turn exerts a force on the foot (reaction force) pushing the foot forward. Recoil of a Gun: When a bullet is fired from a gun, a force is exerted on the bullet in the forward direction. The bullet is also exerting an equal force in backward direction. This backward movement of the gun is called the recoil of the gun. Flying Birds: The birds, while flying, push the air downwards with the help of their wings (action force) and the air in turn exerts a force on the bird in the upward direction (reaction force) Swimming: A swimmer pushes the water in the backward direction (action force) and the water exerts a force on the swimmer (reaction force) which pushes him forward. Rowing a boat: While rowing a boat the man pushes the water backward and the water exerts an equal and opposite push on the boat, which makes the boat moves forward. Jumping out of a boat: When a man jumps out of a boat, the man pushes the boat with his foot in the backward direction. The boat also exerts an equal force on the man in the forward direction. Q. If action is always equal to reaction, explain how a horse can pull cart? Ans: The horse pulls the cart with a force (action) in the forward direction. Since every action has an equal and opposite reaction so, the cart also pulls the horse with an equal force (reaction) in the backward direction. As a result of which the two forces get balanced. But while pulling the cart the horse also pushes the ground with its feet in the backward direction. The reaction of the earth of the earth makes it forward direction along with the cart. This is how the horse applies force and pulls the cart. edurash@gmail.com Page 4/6

5 Q. Explain why it is difficult for a fireman to hold a hose, which ejects large amount of water at a high velocity? Ans: Water is ejected with a large forward force (action). As we know by Newton s third law of motion that every action has an equal and opposite reaction so, because of this action fireman experiences a large backward force or reaction. That is why he feels difficulty in holding the hose. Law of Conservation of Momentum: If two objects are exerting force on each other(colliding with each other), their total momentum remains conserved before and after the collision provided there is no external force acting on them. Two bodies A and B of masses m 1 and m 2 are moving in the same direction with initial velocities u 1 and u 2. They make a direct collision. After collision, let v 1 and v 2 are the velocities of the bodies A and B respectively. The force exerted on A, F 1 = m 1 a 1 F 1 = F 1 = The force exerted on B, F 2 = m 2 a 2 F 2 = F 2 = According to Newton's third law of motion, these two forces are equal and opposite. i.e., F 1 = - F 2 = - = - m 1 v 1 - m 1 u 1 = - m 2 v 2 + m 2 u 2 m 1 v 1 + m 2 v 2 1 = m 1 u 1 + m 2 u 2 i.e., total momentum before collision is equal to the total momentum after collision, which is nothing but law of conservation of momentum. edurash@gmail.com Page 5/6

6 Q:- An object experiences a net zero external unbalanced force. Is it possible for the object to be travelling with a non-zero velocity? If yes, state the conditions that must be placed on the magnitude and direction of the velocity. If no, provide a reason. Answer: When a net zero external unbalanced force is applied on the body, it is possible for the object to be travelling with a non-zero velocity. In fact, once an object comes into motion and there is a condition in which its motion is unopposed by any external force; the object will continue to remain in motion. It is necessary that the object moves at a constant velocity and in a particular direction. Q :- When a carpet is beaten with a stick, dust comes out of it. Explain. Answer: Beating of a carpet with a stick; makes the carpet come in motion suddenly, while dust particles trapped within the pores of carpet have tendency to remain in rest, and in order to maintain the position of rest they come out of carpet. This happens because of the application of Newton s First Law of Motion which states that any object remains in its state unless any external force is applied over it. Q :- Why is it advised to tie any luggage kept on the roof of a bus with a rope? Answer:- Luggage kept on the roof of a bus has the tendency to maintain its state of rest when bus is in rest and to maintain the state of motion when bus is in motion according to Newton s First Law of Motion. When bus will come in motion from its state of rest, in order to maintain the position of rest, luggage kept over its roof may fall down. Similarly, when a moving bus will come in the state of rest or there is any sudden change in velocity because of applying of brake, luggage may fall down because of its tendency to remain in the state of motion. This is the cause that it is advised to tie any luggage kept on the roof a bus with a rope so that luggage can be prevented from falling down. edurash@gmail.com Page 6/6