# Class 11 Physics Chapter 3 Important Questions Motion in a Straight Line

Q.1. Are rest and motion absolute or relative terms? Ans. Both rest and motion are relative terms. Q.2. Under what condition will the distance and

It is important for the students that all the concepts should be very clear for better marks in future. Here, we are providing important conceptual questions and answers for class 11 physics chapter 3 Motion in a Straight Line. In this lesson, students will learn about Motion in a Straight Line. This will not only help the students to know the important questions but will also help them during revision.

Q.1. Are rest and motion absolute or relative terms?
Ans. Both rest and motion are relative terms.

Q.2. Under what condition will the distance and displacement of a moving object will have the same magnitude?
Ans. When the object moves along a straight line and always in the same direction.

Q.3. Can the speed of a body change if its velocity is constant?
Ans. No, it is because speed of the body is equal to magnitude of its velocity.

Q.4. Can a particle in one dimensional motion with zero speed may have non zero velocity?
Ans. No, it is because speed of a particle is equal to magnitude of its velocity.

Q.5. Is it possible for a body to be accelerated without speeding up or slowing down. If it is so give an example.
Ans. Yes, When a body is rotated along a circular path with uniform angular speed, its speed is constant but it has a varying velocity. The change in velocity occurs due to a continuous change in its direction of motion.

Q.6. What will be the nature of position-time graph for uniform motion?
Ans. It will be a straight line, inclined with time axis.

Q.7. Can position-time graph be a straight line parallel to time-axis?
Ans. No, It is because, the position-time graph parallel to time-axis indicates that the object is at rest.

Q.8. What is the nature of position time graph for a stationary object?
Ans. The position-time graph for a stationary object is parallel to time-axis.

Q.9. Can position-time graph be a straight line parallel to position-axis?
Ans. No, It is because the position-time graph parallel to position-axis indicates that the position of the object is changing at a given instant of time.

Q.10. What does slope of position-time graph represent?
Ans. Slope of position-time graph represents velocity of the object.

Q.11. Can position-time graph have a negative slope?
Ans. Yes, It happens, when the velocity of the object is negative.

Q.12. What does slope of velocity-time graph represents?
Ans.The slope of velocity-time graph represents acceleration.

Q.13. What does area under velocity-time graph represents?
Ans. The area under velocity-time graph represents displacement.

Q.14. Can a particle in one dimensional motion with zero speed at any instant may have a non-zero acceleration at that instant?
Ans. Yes for example consider that a body is thrown vertically up at the highest point the body possesses zero velocity and finite acceleration (g = 9.8 ms-2).

Q.15. If the acceleration of a particle is constant in magnitude but not in direction, what type of path the particle follow?
Ans. If the acceleration of a particle is constant in magnitude but not in direction, then the particle will follow a non-linear path.

Q.16. What does the speedometer of a car measure?
Ans. The speedometer of a car measures instantaneous speed.

Q.17. How can the distance travelled be calculated from velocity-time graph?
Ans. The area under velocity-time graph for a given time interval is numerically equal to the distance covered by the body in that time interval.

Q.18. Consider that the acceleration of a moving body varies with time. What does the area under acceleration-time graph for any time interval represent?
Ans. The area under acceleration-time graph for any time interval represents the change of speed of the body during that time interval.

Q.19. A ball is thrown straight up. What is its velocity and acceleration at the top?
Ans. Velocity at the top = 0 and acceleration at the top = 9.8 ms-2 (downwards)

Q.20. Two balls of different masses (one lighter and other heavier) are thrown vertically upward with same initial speed. Which one will rise to the greater height?
Ans. Both the ball will rise to the same height. It is because, for a body moving with given initial velocity and acceleration, the distance covered by the body does not depend on the mass of the body.

Q.21. When will the relative velocity of two moving bodies be zero?
Ans. When the two bodies are moving with equal velocities (i.e. same speed and in the same direction).

Q.22. Rest and motion are relative terms. Explain
Ans. Absolute rest and absolute motion have got no meaning as such. For example, two persons sitting in a moving bus at rest w.r.t each other but are in motion w.r.t. a person standing on the roadside. Further, trees, buildings, etc. on the surface of the earth appears to be at rest. In fact they are in motion as the Earth revolves around the sun. Thus, there is no object which can be considered to be at absolute rest. Hence, rest and motion are relative terms.

Q.23. Given below are some examples of motion. State in each case, if the motion is one, two or three dimensional: (a) a kite flying on a windy day (b) a speeding car on a long straight highway (c) an insect crawling on a globe (d) a carrom coin rebounding from the side of the board (e) a planet revolving around its star.
Ans.

• (a) three dimensional
• (b) one dimensional
• (c) three dimensional
• (d) two dimensional
• (e) two dimensional

Q.24. What are the characteristics of displacement?
Ans. Following are the main characteristics of displacement :

• (a) The displacement has units of length.
• (b) The displacement of an object in a given time interval can be positive, zero or negative.
• (c) The actual distance travelled by an object in a given time interval is either equal to or greater than the magnitude of the displacement. Thus, the displacement of an object between two points tells the shortest distance between these two points.
• (d) The displacement of an object between two points does not tell exactly how the object actually moved between those two points.

Q.25. Distinguish between distance and displacement.
Ans. Following are the points of difference between the displacement and distance :

• (a) The displacement is a vector quantity, whereas the distance covered is a scalar quantity.
• (b) The displacement of an object in a given time interval can be positive, zero or negative whereas the distance covered is always positive.
• (c) The actual distance travelled by an object in a given time interval is either equal to or greater than the magnitude of displacement.
• (d) The displacement of an object between two points does not tell exactly how the object actually moved between those two points.

Q.26. What are the characteristics of uniform motion?
Ans. Following are the main characteristics of uniform motion :

• (a) Generally, the displacement may or may not be equal to the actual distance covered by an object. However, when the uniform motion takes place along a straight line in a given direction the magnitude of the displacement is equal to the actual distance covered by the object.
• (b) For an object to be in uniform motion, no cause or effort i.e. no force is required.
• (c) Since the velocity during uniform motion is same at each point of the path or at each instant, the average and instantaneous velocity is in a uniform motion always equal.
• (d) The velocity-time graph for an object having uniform motion is a straight line parallel to time-axis.

Q.27. Distinguish between speed and velocity.
Ans. Following are the points of difference between speed and velocity :

• (a) Speed is a scalar quantity, whereas the velocity is vector quantity.
• (b) The speed of an object in a particular direction is called velocity of the object.
• (c) Whereas the velocity of an object can be positive, zero or negative, the speed of an object can only be positive or zero.
• (d) Speed of an object gives only quantitative knowledge of 'how fast object is moving'. It tells nothing about the direction of motion of the object. On the other hand, the velocity tells both about how fast the object is moving and its direction of motion.

Q.28. A particle in one dimensional motion with positive value of acceleration must be speeding up. Is it true? Explain.
Ans. The answer depends on the choice of the positive direction of position axis.

To explain, let us consider the motion of a body falling freely under gravity and the positive direction of position to be a vertical line in downward direction. It follows that when a positive direction of position-axis is along the direction of motion, the body speeds up.

On the other hand, when a body is projected vertically upwards and the positive direction of position-axis is regarded as to be opposite the direction of motion, the body slows down, instead of speeding up.

Q.29. " The direction in which an object moves is given by the direction of velocity of the object and not by the direction of acceleration". Explain the above statement with some suitable example.
Ans. When object is thrown up, the direction of motion of the object and its velocity are along vertical in upward direction. As it moves up, it is always attracted in downward direction i.e. acceleration is along vertical in downward direction. Therefore, the direction of motion of the object is that of the velocity and not that of acceleration.

Q.30. Can a body be at rest as well as in motion at the same time? Explain.
Ans. Rest and motion are relative terms. A body at rest with respect to another body may be in motion with respect to some other body. For example, an electric pole is at rest with respect to a tree on the surface of the earth. But the same electric pole is in motion with respect to an observer in a running train.

Q.31. For a particle in one dimensional motion, the instantaneous speed is always equal to the magnitude of instantaneous velocity. Why?
Ans. In an accelerated motion, the velocity of an object always keeps on changing. Therefore, one has to measure the instantaneous velocity. However, when accelerated motion takes place along a straight line, the velocity of the body changes only due to the change in magnitude of velocity. Therefore, the instantaneous speed is always equal to the magnitude of instantaneous velocity of the particle in one dimensional motion.

Q.32. Is the rate of change of acceleration with time important to mechanics? Comment.
Ans. The rate of change of acceleration with time can be defined but it is not important to mechanics. It is because the basic laws of motion involve only acceleration and the quantity -- the rate of change of acceleration is not required at all. Galileo and Newton discovered that to understand and explain motion, it is enough to define velocity (rate of change of position) and acceleration (rate of change of velocity).

Q.33. A man standing on the edge of a cliff throws a stone straight up with initial speed u and then throws another stone straight down with same initial speed u and from the same position. Find the ratio of speeds, the two stones would attain, when they hit the ground at the base of the cliff.
Ans. When the stone is thrown up with speed u, it will rise up, till its speed becomes zero and then will start falling down. It can be proved that when the stone reaches the thrower, it will acquire the same speed u, with which it was thrown up.

Since both the stones are falling under gravity with the same initial speed u, the two stones will head the ground with the same speed and hence the ratio of their speed is 1.

Q.34. What condition should be satisfied for vectors to be equal?
Ans. Two vectors are said to be equal, if they have (i) the same magnitude and (ii) the same direction.

Q.35. Does vector addition hold for any two vectors?
Ans. No. The vectors addition holds for two vectors of same kind i.e. vectors representing two physical quantities of same dimensions.

Q.36. Can we add velocity vector to the displacement vector?
Ans. No, The vectors having the same nature only can be added.

Q.37. Is it possible to add vectors representing force of 5 newton and 200 dyne?
Ans. Yes. It is because both the vectors represent force. However, the given two vectors representing force have to be expressed in the same system of units i e. either in SI or CGS system.

Q.38. What is a resultant vector?
Ans. The resultant of two (or more) vectors is a single vector, which produces the same effect as the individual vectors together produce.

Q.39. Can the flight of a bird be an example of composition of vectors?
Ans. Yes. When a bird has to fly, it presses air with its two wings along the oblique directions. The resultant of the reaction on the two wings of the bird enables it to fly up in the sky.

Q.40. Is A quantity which has a magnitude and direction always a vector? Give examples.
Ans. No. A quantity which possesses both magnitude and direction may not be a vector. For example, electric current, finite angular displacement, etc. For a quantity having magnitude and direction to be a vector, it must be the commutative law for addition.

Q.41. Can a vector be multiplied with both dimensional and non dimensional scalars?
Ans. Yes. When a vector is multiplied with the dimensionless scalar, it will be a vector having dimensions as that of the given vector. On the other hand, If a vector is multiplied with a dimensional scalar, the resultant vector will have different dimensions. For example, when a velocity vector is multiplied with mass (a dimensional scalar), the resultant vector has the dimensions of momentum.

Ans. In scalar addition, one has to add the magnitudes of scalars. Since the vectors possess direction in addition to their magnitudes, the scalar addition cannot be used to add two vectors. For this reason, the vector addition is different from the scalar addition.

Q.43. What is the essential condition for adding the vectors?
Ans. The essential condition for adding the vectors is that the quantity is represented by them should be of the same physical nature. For example, all the vectors to be added should be displacements or velocities or forces. Forces cannot be added to velocities.

Q.44. Can three vectors not in one plane give a zero resultant? Can four vectors do?
Ans. Three vectors not in one place cannot give zero resultant. It is because, the resultant of any two vectors lies in their own plane and therefore it cannot cancel the effect of a third vector; which does not lie in the plane of the other two.

The resultant of two vectors not in one plane may be zero.

Q.45. Can two vectors of different magnitudes be combined to give zero resultant? Can three vectors do?
Ans. Two vectors of different magnitudes cannot give zero resultant.

Three vectors lying in a plane can give zero resultant.

Q.46. A man moving in rain holds his umbrella inclined to the vertical even though the rain drops are falling vertically downwards. Why?
Ans. When the man moves in the rain falling vertically downwards, the rain drops appear to fall 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 apparent velocity of the rain with respect to himself.