Chapter:2 Kinematics (9th physics new book)

Chapter: 2 Kinematics

Q1.            Is it possible that displacement is zero but not the distance? Under what condition displacement is equal to the distance?

Statement:

Displacement is zero but not the distance and displacement is equal to the distance.

Main idea:

                  Definition of displacement and distance.

Conclusion:

Yes it is possible that displacement is zero but not the distance. When an object moves in a circle and complete one round trip its displacement become zero while the distance covered is not zero and is equal to 2πr.

The displacement and distance covered is equal if and only if the body is moving in a straight line.

                                               A                 B

Q2.            Does speedometer measure a car’s speed or its velocity?

Statement:

                  Speedometer measure a car’s speed or its velocity.

Main idea:

                  Definition of speed and velocity.

Conclusion:

The speedometer of a car measures only magnitude of velocity not the direction. Therefore, we can say that speedometer of the car does not measure it velocity.

Q3.            Is it possible for an object to be accelerating and at rest at the same time? Explain with example.

Statement:

                  An object be accelerating and at rest at the same time.

Main idea:

                  Gravitational acceleration (free fall motion).

Conclusion:

Yes it possible for an object to be accelerating and at rest at the same time.

Example:

When a body is thrown upward then the velocity decreases and reaches to zero but the body is still under acceleration due to gravity (g = - 9.8 m/s²)

Q4.            Can an object have zero acceleration and non-zero velocity at the same time? Give example.

Statement:

                  An object have zero acceleration and non-zero velocity at the same time.

Main idea:

Definition of acceleration and velocity.

Conclusion:

Yes an object have zero acceleration and non-zero velocity at the same time.

Example:

Suppose a body is moving with a uniform velocity of 5m/s from point A to B. So the initial velocity of the body v_i = 5m/s at point A and final velocity is v_f = 5 m/s at point B. Now acceleration is

a =   v_f  - v_i =  5m/s  - 5m/s = 0 m/s²

            t                t

Hence acceleration a = 0 m/s² for non-zero velocity of 5 m/s.

Q5.            A person standing on the roof of a building throws a rubber ball down with a velocity of 8 m/s. What is the acceleration (magnitude and direction) of the ball?

Statement:

A person standing on the roof of a building throws a rubber ball down with a velocity of 8 m/s and acceleration of the ball.

Main idea:

                  Gravitational acceleration (free fall motion).

Conclusion:

According to famous scientist Galileo all bodies falling toward earth with a constant acceleration of g = 9.8 m/s². Now a person standing on the roof a building throws a rubber ball down with a velocity of 8 m/s. The acceleration of the ball will be g = 9.8 m/s² directed toward the earth.

Q6.            Describe a situation in which the speed of an object is constant while the velocity is not.

Statement:

                        A situation in which the speed of an object is constant while the velocity is not.

Main idea:

                  Definition of uniform speed and uniform velocity.

Conclusion:

A situation in which the speed of an object is constant while the velocity is not constant, may be that of circular motion. Suppose a body is moving with a constant speed of 5m/s in a circle. Then the speed of the body will be uniform while velocity of the body is changing at each point.

a

 

                                                                                                     V

                                                                                                                                      

Q7.            Can an object have northward velocity and a southward acceleration? Explain.

Statement:

                   An object have northward velocity and a southward acceleration.

Main idea:

            Negative acceleration (deceleration).

Conclusion:    

       Yes an object have northward velocity and a southward acceleration when a body is moving towards north and gradually its velocity decreases. Now the acceleration (deceleration) produced will be in southward direction.

                                                S                                                           N

                                                                      -a  

Q8.            As a freely falling object speeds up, what is happening to its acceleration – does it increases, decreases, or stay the same?

Statement:

                   A freely falling object speeds up and acceleration.        

Main idea:

                  Gravitational acceleration (free fall motion).

Conclusion:

                  According to famous scientist Galileo all bodies falling toward earth with a constant acceleration of g = 9.8 m/s². The acceleration will be g = 9.8 m/s² and it does not increase, decrease but stay the same.

Q9.            A ball is thrown vertically upward with an initial speed of 5m/s. What will its speed be when it returns to its starting point?

Statement:

                         A ball is thrown vertically upward with an initial speed of 5m/s and its speed when it returns to its starting point.

Main idea:

                  Gravitational acceleration (free fall motion).

Conclusion:

                  The body returns to its starting point with the same speed of 5m/s in the absence of air resistance.