# 1st Year Physics Notes Chapter 3 - 11th Class Notes pdf

Looking for the notes of Physics class 11 chapter 3 Motion and Force. Here we have uploaded the 1st Year Physics Notes Chapter 3 - 11th Class Notes pdf download or read online Topics notes, Short and long questions and exercise questions.

Q.1 Define and explain the following terms:
(1) Displacement
(2) Velocity
(3) Average velocity
(4)1 Instantaneous velocity
(5) Acceleration
(6) Average acceleration
(7) Instantaneous acceleration
(1) Displacement
The change in position of body from its initial position to its final position is called displacement It tells us that how far and in what direction the body has displacedUnit The SI unit of displacement is meter (m)
Magnitude: Its magnitude is equal to the length of line It gives the distance between initial and final position
(2) Velocity
The time rate of change of displacement of a body is called velocity
(3) Average velocity
The ratio of the total displacement to the total time taken to cover this displacement is called as average velocity
Limitation of average velocity Average velocity does not tell us that
The motion is steady or variable
The path is straight or curved
Unit
Si unit is m/sec (i n ms)
It is a vector quantity
Its direction of velocity is along the direction of displacement
Q. Under what condition the body is said to be moving with uniform velocity?
If the average and instantaneous velocity of a body are equal the body is moving with uniform velocity
OR
If the instantaneous velocity does not change the body is said to be moving with uniform velocity
(5) Acceleration
The time rate of change of velocity of a body is called acceleration Unit
Si unit of acceleration is m/sec
The dimensions of acceleration are
Direction
Acceleration is also a vector quantity as velocity is a vector quantity The direction of acceleration is along the direction of force
Positive Acceleration
If the velocity of a body is increasing
Its acceleration is positive
Negative Acceleration
If the velocity of a body is decreasing its acceleration is negative The negative acceleration is also called retardation or deceleration

•  a body is moving with constant velocity then acceleration is zero Uniform acceleration
•  A body has uniform acceleration if it has equal changes in velocity in equal intervals of time nowever short the interval maybe

Acceleration of a body is positive if its velocity increases with time. The direction of this acceleration is the same in which the body is moving without change in its direction.
Acceleration of a body is negative if velocity of the body decreases The direction of negative acceleration is opposite to the direction in which the body is moving Negative acceleration is also called deceleration or retardation Condition under which the body is said to be moving with uniform acceleration.
For a body moving with uniform acceleration, its average and instantaneous acceleration are equal
Q. What are Displacement-time graphs? Discuss.
distance-time graph
It is useful to represent the motion of objects using graphs The terms distance and displacement are used interchangeably when the motion is in a straight line Similarly if the motion is in a straight line then speed and velocity are also used interchangeably in a distance time graph, time is taken along horizontal axis while vertical axis shows the distance covered by the object
(1) Object at rest In the graph
the distance moved by the object with time is zero That is, the object is at rest Thus, a horizontal line parallel to time axis on a distance-time graph shows that speed of the object is zero.
Object moving with constant speed:
The speed of an object is sad to be constant if it covers equal distances in equal intervals of time The distance-time graph is a straight line Its slope gives the speed of the object. Consider two points A and B on the graph
Object moving with variable speed: When an object does not cover equal distances in equal intervals of time then its speed is not constant. In this case the distance-time graph is not a straight line. The slope of the curve at any point can be found from the slope of the tangent at that point.
Q. Discuss Newton's law of motion briely.
Sir Isaac Newton published his empirical laws of motion in his famous book principia in 1687 These laws hold good for bodies moving with smaller speed compared to the speed of light.
Newton's First Law of Motion / Law of Inertia Statement
A body at rest will remain at rest and a body moving with uniform velocity will continue to do so unless unbalanced external force acts on it This is known as first law of inertia
Inertial frame of reference
The frame of reference in which Newton s first law holds and acceleration is zero is called internal frame of reference
Example Earth is approximately an inertial frame of reference Newton's Second Law of Motion
Statement
When a force is applied on a body, it produces the acceleration in its own direction The magnitude of acceleration is directly proportional to applied force and inversely proportional to its mass
Mathematical form F= ma
Newton's Third Law of Motion
Action and reaction are equal in magnitude and opposite in direction
Explanation
When two bodies interact with each other then Action and reaction forces act for the same length of time Action and reaction forces never act on the same body Action and reaction forces act on the different body

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