KINEMATICS: PHYSICS SHORT ANSWER QUESTIONS
Basically, there are two ways to do a study of motion, Kinematics, and Dynamic. Kinematics is the branch of mechanics and study of the motion of the objects, without making reference to the forces that cause motion. Similarly, the dynamic is also the same but it gives the reference to the force that creates motion. A few examples of Kinematics are: train moving, Bus moving, moving water in a river, etc.
Ans: It is possible only when the object moves on a straight line in the same direction.
2. Can a body have a constant speed but a varying velocity?
Ans: Yes, it is possible when the body is moving in a circular path with a uniform speed. At every point of its path, the speed is the same but the velocity is directed tangentially.
3. Can a body have a constant velocity but a varying speed?
Ans: No, it is not possible because the modulus of the velocity or a magnitude is the speed.
4. What does the speedometer of a car measure?
Ans: It measures the instantaneous speed of a car.
5. While firing, one has to aim a little above the target. Why?
Ans: It is because due to gravity, the bullet moves in the parabolic path. So it falls vertically and that is why it should be aimed a little above the target.
6. Why is time repeated twice in the unit of acceleration?
Ans: Velocity is the rate of change of displacement and acceleration is the rate of change of velocity. So acceleration is the rate of a rate of change of displacement. That is why time is repeated twice.
7. Can speed be negative?
Ans: No, it is not possible for speed to have a negative value because it is the magnitude of velocity.
8. In a long jump, which factors determine the span of a jump?
Ans: The span of the jump depends upon (i) initial velocity of projection and (ii) the angle of projection.
9. If the magnitude and direction of the acceleration are constant, is it necessary to be a straight line path?
Ans: Not necessary. Because a projectile has constant acceleration both in magnitude and direction, it still has a parabolic path.
10. A person sitting in a moving bus throws a stone vertically upward. How does the ball appear to move to an observer inside the bus? What for the outside observer?
Ans: To observe inside the bus it appears moving vertically upward. However, for an observer outside the bus, the motion of the stone is parabolic.
11. What are horizontal and vertical components of the acceleration of a body thrown horizontally with uniform speed?
Ans: When a body is thrown horizontally with uniform speed, then its horizontal acceleration is zero and vertical acceleration is g.
12. Can you give the example such that velocity is zero but acceleration is non-zero?
Ans: When an object is thrown up, then at the highest point velocity is zero but the acceleration due to gravity is g.
13. Is the value of g positive or negative?
Ans: Acceleration due to gravity is always vertically downward. So the value of g will be negative or positive depending on the choice that upward direction is taken positive or negative.
14. Can a body be regarded in a state of rest as well as in motion at the same time? Give an example.
Ans: Yes, a body can be regarded in a state of rest as well as in motion at the same time. It is due to relative motion between the source and surroundings. For example, suppose a car is moving. Here the car driver feels at rest with respect to the car but he feels in motion with respect to it's surrounding.
15. When is an object said to be accelerating?
Ans: Whenever the velocity (magnitude or direction or both) of an object changes, that the object is accelerating. However, an object moving at a constant speed in a straight line is not accelerating because its velocity is not changing ie., magnitude, as well as the direction of velocity, is the same.
16. Two balls of different masses (one lighter and other heavier) are thrown vertically upwards at the same speed. Which one will pass through the point of projection in their downward direction with greater speed?
Ans: Since the acceleration of both balls is the same ie. g. the speed with which a body returns back is always equal to the speed with which it is thrown up. Since expression for final speed does not involve mass, both the balls will acquire the same speed.
17. In one dimensional motion, can a particle have zero speed at an instant but non zero acceleration at that instant?
Ans: Yes. For example, when a body starts falling freely from rest, its speed is zero but acceleration is 9.8 m/s2.
18. Can a body have zero velocity and finite acceleration?
Ans: Yes. When a body is thrown vertically upward, the body possesses zero velocity and finite.
19. A man moving in rain holds his umbrella inclined to the vertical even though the raindrops are falling vertically downwards. Why?
Ans: When the man moves through the rain falling vertically downwards, the raindrops appear to fall acceleration (g = 9.8 m/s2) at the highest point in a direction inclined to the vertical, So as to protect himself from the rain, he holds the umbrella inclined to the vertical in the direction of relative (or apparent) velocity of the rain with respect to himself.
20. Can the relative velocity of two bodies be greater than the absolute velocity of either?
Ans: Yes. When the bodies move in opposite directions, the relative velocity is greater than the absolute velocity of either.
21. Can you use the equations of kinematics to find the height attained by an object projected upward with any velocity? Why?
Ans: No, because equations of kinematics are applicable only when an object moves with constant acceleration. The acceleration due to gravity is constant only near the surface of the earth. When initial velocity is larger the body attains sufficient height and the results obtained from kinematics equations
are will be wrong.
22. Why do we not consider the rate of change of acceleration in the study of linear motion?
Ans: We do not consider the rate of change of acceleration in the study of linear motion, instead, we only consider acceleration because the basic laws of motion involve only acceleration but not the rate of change of acceleration.
23. Can an object with constant acceleration reverse its direction? Explain.
Ans: Yes, an object with constant acceleration reverses its direction. For example: when a body is thrown vertically upwards, the direction of velocity is clearly vertically upwards and the direction of acceleration due to gravity (g) is vertically downwards. When it falls after attaining the maximum height, the direction of both velocity and acceleration 'g is vertically downwards. So, the direction of velocity reverses while its acceleration is constant.
24. When a rifle is fired at a distant target, the barrel is not lined up exactly on the target. Why not? Does the angle of correction depend on the distance of the target?
Ans: When a rifle is fired at a distant target keeping the barrel lined up exactly on the target, the bullet will not hit the target at the point at which it was aimed. It will hit a little below as it follows a parabolic path and falls through some vertical distance due to the effect of gravity. Hence, the barrel is not lined up
exactly on the target. Yes, the angle of correction also depends upon the distance of the target.
25. A projectile acceleration is perpendicular to the velocity? Explain.
Ans: If a projectile moves in a parabolic path without air resistance at the highest point of its path the vertical component of its velocity becomes zero. Only the horizontal component is active which is perpendicular to its acceleration that is always acting downward i.e. acceleration due to gravity.
26. Can you have a zero acceleration but non-zero velocity? Explain with
the help of a graph?
Ans: Yes, a body moving with a certain constant velocity along a particular direction has a zero acceleration but non zero velocity. The velocity-time graph for a body moving with zero acceleration.
27. A bomb is to be dropped from a moving helicopter on a target on the ground. Explain how it can hit the target.
Ans: When a bomb is dropped from a moving helicopter the bomb does not follow straight but follows the parabolic path due to the effect of gravity. As a result, the bomb will not hit the target at the point, if it is thrown straight to the target. Therefore, so as to hit the target exactly, one has to aim a little forward that point on the target. But if the helicopter is lifting vertically upwards, the bomb dropped from the helicopter hit the target if it is dropped straight to the target.
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all questions are well answered. thanks
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