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(1) Displacement

(2) Velocity

(3) Average velocity

(4)1 Instantaneous velocity

(5) Acceleration

(6) Average acceleration

(7) Instantaneous acceleration

Answer

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

The time rate of change of displacement of a body is called 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

It is a vector quantity

Its direction of velocity is along the direction of displacement

Answer

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

The time rate of change of velocity of a body is called acceleration Unit

The dimensions of acceleration are

Acceleration is also a vector quantity as velocity is a vector quantity The direction of acceleration is along the direction of force

If the velocity of a body is increasing

Its acceleration is positive

If the velocity of a body is decreasing its acceleration is negative The negative acceleration is also called retardation or deceleration

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

Answer

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

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.

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

Answer

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.

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

The frame of reference in which Newton s first law holds and acceleration is zero is called internal frame of reference

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

Action and reaction are equal in magnitude and opposite in direction

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

**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

Answer

**(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?**Answer

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.**Answer

**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.**Answer

**About Newton's Law of Motion**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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