Force Laws of Motion of Class 9
Frequently Asked Questions
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/s2
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.
Correct 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=f2
D: T=f3
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.
