Physics class 9
chapter No 02
Conceptual questions
Question No 02
Does a speedometer measure a car's speed or velocity?
Statement: Yes a speedometer measure a car's speed but not the velocity.
Reason: The reason is that velocity is the time rate of change in displacement it is a vector quantity which require both magnitude and direction.
Conclusion :
Hence the speedometer cannot tell us about the direction and only measure a car's speed.
There for a speedometer measures only speed but not velocity.
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Question No 03
Is it possible for an object to be accelerating at rest at the same time? Explain with an example.
Answer
Statement: No it is not possible for an object to be accelerating and at rest at the same time.
Reason:The reason is that acceleration refers to a change in an object's velocity, which includes both the speed and direction of motion. If an object is at rest, its velocity is zero, and there is no change in velocity, meaning there is no acceleration .
This is impossible for an object to be at rest in one reference frame while accelerating in another frame.
Example: For example a ball is resting on a flat surface. If the ball is not moving and there are no external forces acting on it, it is considered to be at rest. Since there is no change in its velocity, the acceleration is zero. Therefore, the ball cannot be accelerating while being at rest.
Conclusion: Hence it is concluded that this is impossible for an object to be accelerating and at rest at the same time .
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Question No 04
Question No 4
Can an object have Zero acceleration and non zero velocity at the same time? give example.
Answer
Statement: yes, an object can have zero acceleration and non zero velocity at the same time.
Example: consider a simple pendulum when it starts oscillation between the two extreme positions A and A' at point A and A' the direction of velocity is changing. The magnitude of velocity is 0 at extreme positions A and A' while acceleration is maximum.
When it starts motion from point A the velocity increases and as the pendulum reaches the mean position the acceleration goes to zero while the velocity remains constant .........
Look at the picture below .
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Question No 05 other
A person standing on the roof of a building throws A rubber but down with a velocity of 8.0m/s*2 . What is the acceleration( magnitude and direction) of the ball?
Answer
When a person standing on the roof of a building throws a rubber ball with the velocity of 8.0 m/sec*2 .
That rubber ball falls the word the earth due to the force of gravity.
If we neglect the air resistance then the ball will fall freely with an acceleration to gravity "g".
Magnitude of acceleration: the magnitude of acceleration due to gravity g= 9.8m/sec*2
Direction: the acceleration due to gravity will be directed towards the center of earth.
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Question No 06
Describe a situation and in which the speed of an object is constant while the velocity is not.
Answer:
When an object is moving along a circular path have a constant speed and non constant velocity because the direction of velocity changes continuously during circular motion.
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Question No 07
Can an object have northward velocity and Southward acceleration explain?
Answer
Statement; yes it is possible for an object to have northward velocity and Southward acceleration.
Explaination: for example a decelerating body has velocity and acceleration in opposite direction.
If a body moves towards North while decreasing then it's acceleration will be directed towards South.
Conclusion: therefore an object can have are northward velocity and a southward acceleration.
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Question No 8
As a free falling object speed up what is happening to its acceleration does it increase, decrease or stay the same?
Statement: As a free falling object speed up it's acceleration remains constant.
Explaination: When a freely falling object speeds up, its acceleration remains constant. This acceleration is typically denoted by the letter "g" and is equal to approximately 9.8 meters per second squared on Earth. Regardless of the speed of the falling object, its acceleration remains constant as long as there are no other forces (such as air resistance) acting upon it
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Question No 9
A ball is thrown upward with an initial speed of 5 m/sec*2 . What will be it's speed when it return to the starting point
Answer.
When the ball is thrown upward, it will experience a constant deceleration due to gravity. The speed of the ball will gradually decrease until it reaches its highest point and then starts to fall back downwards. At its highest point, the ball momentarily stops moving upwards.
To find the speed of the ball when it returns to the starting point, we can use the concept that the speed at any point in the projectile motion is equal to the initial speed plus the acceleration multiplied by the time taken.
In this case, the initial speed is 5 m/s upwards, and the acceleration due to gravity is approximately -9.8 m/s^2 (negative due to its direction being downwards). Since the ball will take the same amount of time to reach its highest point as it will take to return to the starting point, we can consider the total time of flight.
The time taken for the entire journey can be found by dividing twice the initial speed by the acceleration:
The time taken for the entire journey, from throwing the ball upward to when it returns to the starting point, is approximately 1.02 seconds.
Now, using this total time, we can find the speed of the ball when it returns to the starting point:
Speed = initial speed + (acceleration * time taken)
= 5 m/s + (-9.8 m/s^2) * 1.02 s
≈ 5 m/s - 10 m/s
= -5 m/s (downward)
So, when the ball returns to the starting point, its speed will be 5 m/s downward.
