Distance in km | 0 | 10 | 20 | 30 | 40 | 50 | 60 |
Time | 10.00 am | 10.10 am | 10.20 am | 10.30 am | 10.40 am | 10.50 am | 11.00 am |
S. No. | Distance | S. No. | Displacement | |
1. | Distance is the length of the actual path traveled by an object in a given interval of time. | 1. | Displacement is the shortest distance between the initial and final positions of a moving object in a particular direction. | |
2. | Distance traveled by an object depends upon the path followed by the particle in going from initial position to the final position. | 2. | The displacement of an object between initial and final positions of the particle does not depend upon the path followed by it. | |
3. | Distance traveled by an object in a given interval of time is always positive. | 3. | Displacement of an object in a given interval of time may be positive, negative or zero. | |
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Introduction
In everyday life we always come across some objects which are at rest and some objects in motion. The branch of Physics, which deals with the behavior of moving objects, is known as mechanics. Mechanics is further divided into two sections namely Kinematics and Dynamics. Kinematics deals with the study of motion without taking into account the cause of motion, while Dynamics is concerned with the cause of motion, namely force. This chapter covers only the different aspects of motion without considering the cause of motion.
Motion
A body is said to be in motion if its position changes continuously, with respect to a stationary object, taken as reference point, with the passage of time.
When we are sitting in a moving bus, we observe the continuous changes of position with respect to stationary object like houses, trees, lamp posts, etc. We say that the bus is moving or that the bus is in motion.
Motion and rest are Relative Terms
Suppose you are travelling in a train, which is in motion.
Observe 1: Is there any change in your position with respect to your co passengers? Answer: No
Observe 2: Is there any change of scene you view through the window?
Answer: Yes
Thus, we can say that an object is in, motion, if it changes its position continuously with respect to its surroundings in a given time. Since the position of desk and benches does not changes with time, hence we can say that they are not in motion, i.e. they are stationary. You must have observed sky at night: the position of stars changes with the passage of time, whereas the position of house and our surrounding remains the same. But in reality the earth is also moving, that means all the objects on the surface of the earth are also in motion. Thus, an object which appears to be at rest, may actually be in motion. Therefore, motion and rest are relative terms. Hence, to describe the motion of an object we have to specify how its position changes with respect to a fixed point called the reference point or origin.
Without frame of reference we cannot specify whether an object is in motion or at rest. A frame of reference is another object or scene with respect to which we compare an object's position.
Types of Motion
Newton's Laws of Motion
Newton's First Law of Motion
Everybody remains in its state of rest or of uniform motion, unless it is compelled to change its state by an unbalanced force, impressed on it.
\[F=m\times a\]
if, \[F=0\]
\[\Rightarrow \,\,a=0\,\,(m\ne 0)\]
(i) We tend to fall forward when a bus suddenly stops. When bus stops suddenly, we tend to resist the change in our state of motion and hence fall forward.
(ii) We tend to fall backward when a bus suddenly starts.
(iii) We tend to get thrown to one side when a car takes sharp turn.
Martin and Robert are arguing in the cafeteria. Martin says that if he throws the burger with a greater speed it will have a greater inertia. Robert argues that inertia does not depend upon speed, but rather upon mass. With whome do you agree?
(a) Martin
(b) Robert
(c) Both are correct
(d) Both are incorrect
(e) None of these
Answer: (b)
Why it is advised to tie the luggage with a rope on the roof of bus?
(a) Luggage might fall in forward direction
(b) Luggage might fall in backward direction
(c) To prevent fall (in both the directions) of the luggage from the roof of the bus.
(d) All of these
(e) None of these
Answer: (c)
Newton's Second Law of Motion
The rate of change of momentum of a body is directly proportional to the unbalanced force applied and takes place in the direction of the force.
Mathematical expression,
\[F=\frac{dp}{dt}=m\times a\]
\[\Rightarrow \,\,F\alpha \frac{\Delta p}{\Delta t}\]
\[\Rightarrow \,\,F=k\frac{\Delta p}{\Delta t}\]
Where, k is the constant of proportionality.
Taking the limit \[\Delta t\to 0,\] the term \[\frac{\Delta p}{\Delta t}\] becomes the derivative and is denoted by,
\[\frac{dp}{dt}=\frac{d(m\times v)}{dt}=\frac{mdv}{dt}=m\times a\]
\[\Rightarrow \,\,F=\frac{dp}{dt}\,\,\,\,\,\,=m\times a\]
(i) A cricketer gradually pulls hand backwards with the moving ball when he catches the ball. While doing so he increases the time during which the high velocity of the ball is reduces to zero and the acceleration of the ball is also reduced drastically and hence the impact of catching the ball is also reduced and it does not hurt the player.
(ii) A judo champ breaks a pile of bricks in one go. His action is too fast. In this case he needs to increase the force with which he can break the pile, so he decreases the time by fasting his action. more...
Inertia
Inertia is the property of a body due to which the body opposes any change in its state of rest or of uniform motion along a straight line. Inertia is the measure of mass. More mass means more inertia.
Inertia of Rest
A body which is at rest will remain at rest unless or until some external force is applied to it.
It is the inability of a body to move on its own.
Activity
Classification of Forces
Forces may be classified into the following two classes:
Introduction
The Greek thinker, Aristotle held the view that if a body is moving, something external -is required to keep it moving. The Aristotelian law of motion may be phrased for our purpose here, : An external force is required to keep a body in motion.
Force
Force is that agency which can change the state of rest or of uniform motion or shape or direction of motion of any object. The unit of force in M.K.S. system is newton. It is a vector quantity.
Effects of force
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