Motion of Class 9

A man throws the ball with the same speed vertically upwards one

after the other at an interval of 2 seconds. What should be the speed of the throw so that more than two balls are in the sky at any time? (Given g = 9.8 m/s^2)

A: More than 19.6 m/s

B: At least 9.8 m/s

C: Any speed less than 19.6 m/s

D: Only with speed 19.6 m/s

Solution:

Explanation

Given:t =2s ;g =9.8 m/s ²

When the ball is thrown upward the final velocity become zero.i.e v =0

Now, Time taken by ball to reach maximum height

v =u - gt

O =u - gt

u = gt

On substituting the given, we get

u =9.8×2

u = 19.6 m/s

As a result, when he throws a ball at a minimum speed of in19.6 m/s a 2s time period, two balls can be in the air at the same moment.

Final Answer

The correct answer is A i.e More than 19.6 m/s.

The strength of force is expressed by

A: Direction

B: Magnitude

C: Both

D: None

Solution:

Explanation:

The strength of the force can be expressed by the magnitude because force can be smaller and larger in magnitude.

C orrect option is (B)

Final Answer:

The strength of the force can be expressed by the magnitude.

The relation between frequency f and time period T is given by

A: f = T

B: f =  1/T

C: T=f ²

D: T=f ³

Solution:

Answer:

Explanation:

Frequency can be defined as a number of complete oscillation in one second

And the time period is the time which is required to complete one complete oscillation.

Final Answer:

The frequency and the time period is reciprocal to each other.Hence option(B) is correct.

What is time period Write its S.I unit

Solution:

Explanation:

The time Period can be defined as the time taken to complete one oscillation in the dense medium.

The S.I unit of time period is second.

Final Answer:

The time taken by a particle of medium to complete its one vibration is called the time period of the wave.

The S.I unit will be second.

A sonar device on a submarine sends out a signal and receives an echo 5 s later

Calculate the speed of sound in water if the distance of the object from the submarine is 3625 m.

Solution:

Ans. Given,

Time taken = 5 seconds

The time taken by ultrasound signal to travel from the submarine to the object will be half of this time:

=5/2

= 2.5 seconds

We know that,

Speed = Distance/Time

= 3625/2.5m

= 1450 m/s

Therefore, the speed of sound is 1450 m/s.