# Class 11 Physics Chapter 4 Important Questions Motion in a Plane

Q.1. What is projectile? Ans. Any object projected into space or air touch that it moves under the effect of gravity alone is called a projectile.

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 4 Motion in a Plane. In this lesson, students will learn about Motion in a Plane. This will not only help the students to know the important questions but will also help them during revision.

Q.1. What is projectile?
Ans. Any object projected into space or air touch that it moves under the effect of gravity alone is called a projectile.

Q.2. What do we mean by trajectory?
Ans. The path followed by the projectile is called its trajectory.

Q.3. Define angular displacement.
Ans. The angular displacement of an object in a given time interval be defined as the angle which its position vectors sweeps out in that time interval.

Q.4. Define unit vector.
Ans. A vector divided by its magnitude is called a unit vector along the direction of the vector.

Q.5. What is meant by resolution of a vector?
Ans. The process of splitting up a vector into two or more vectors is known as the resolution of a vector.

Q.6. Can a vector be zero if one of its components is not zero?
Ans. No. For example, consider a vector along Z-axis. Its components along x-axis and y-axis will be zero but it itself will not be equal to zero.

Q.7. Can the walking on a road be an example of resolution of vectors?
Ans. Yes, when a man walks on the road he presses the road along an oblique direction the horizontal component of the reaction helps the man to walk on the road.

Q.8. Can any of the components of a given vector have a greater magnitude than that of the vector itself?
Ans. Yes, provided the components are not rectangular.

Q.9. Give a few examples of motion in two dimensions.
Ans. A bag dropped from an aircraft flying horizontally, a gun shot fired at some angle with the horizontal, etc.

Q.10. What are horizontal and vertical components of acceleration of a body thrown horizontally with uniform speed?
Ans. When a body is thrown horizontally with uniform speed, then

acceleration along the horizontal = 0

and acceleration along the vertical = g

Q.11. What is the angle of projection for a projectile thrown parallel to horizontal?
Ans. It is 0°.

Q.12. Velocity of a projectile is 10 metre per second. At what angle to the horizontal should it be projected so that it covers maximum horizontal distance?
Ans. At an angle of 45° to the horizontal.

Q.13. Two balls are released simultaneously from certain height, one is allowed to fall freely and other thrown with some horizontal velocity. Will they hit the ground together? Substantiate your answer with proper reasoning.
Ans. Both the balls will hit the ground together. It is because the two balls fall through the same height and the initial velocity of both the balls along the vertical is zero.

Q.14. A hunter aims his gun and fires directly at a monkey sitting in a tree. The moment the bullet leaves the barrel of the gun the monkey falls. Will the bullet hit the monkey?
Ans. Yes, the bullet will hit the monkey. It is because of the fact that both the monkey and the bullet fall through the same height and the initial velocity of both of them along the vertical is zero.

Q.15. Give two examples of circular motion with constant speed.
Ans.

• (a) The motion of the hour or the minute hand of watch.
• (b) The motion of the blades on the fan when the electric fan has attained constant speed.

Q.16. What is the direction of velocity vector of a particle in circular motion?
Ans. At every point, the velocity vector of a particle in circular motion acts along the tangent to the circular path.

Q.17. Can the velocity of a particle change even if its speed is constant? Give example.
Ans. Yes. For example, when a body moves along a circular path with constant speed, it possesses centripetal acceleration due to continuous change in its direction of motion.

Q.18. A body is dropped freely from the window of a train. Will the time of the free fall be equal, if the train is stationary, the train moves with a constant velocity, the train moves with an acceleration?
Ans. The motion of the train only affects the magnitude of the horizontal component of velocity and the acceleration of the body along the horizontal, if any. It does not affect the nature of the motion along the vertical. Therefore, in all the three cases, the time of free fall of the body will be equal.

Q.19. A person sitting in a moving train throws a ball vertically upwards how does the ball appear to move to an observer (a) inside the train (b)outside the train?
Ans.

• (1) The observer inside the train the ball will appear to move straight vertically upwards and then downwards.
• (2) To the observer outside the train, the ball will appear to move along the parabolic path.

Q.20. While firing, one has to aim little above the target and not exactly on the target. Why?
Ans. During the time interval, the bullet moves up to the target, it falls through some vertical distance under the effect of gravity. As a result, it will not hit the target at the point, it was aimed at. It will hit a little below. Therefore, so as to hit the target exactly at that point, one has to am a little above that point on the target.

Q.21. Which is greater -- the angular velocity of the hour hand of a watch or angular velocity of earth around its own axis?
Ans. The angular velocity of the hour hand is greater than that of the earth.

Q.22. Justify the statement that a uniform circular motion is an accelerated motion.
Ans. In uniform circular motion, the speed of the body remains the same but the direction of motion of the body changes continuously. Since velocity is a vector quantity the continuous change in the direction of motion of the body implies continuous change in the velocity of the body. As the rate of change of velocity acceleration, a uniform circular motion is an accelerated motion.

Q.23. How can we say that the object is accelerating?
Ans. An object is said to be accelerating if its speed is changing or its direction of motion is changing or both.

Q.24. Write dimensional formula of angular velocity.
Ans. The dimensional formula of angular velocity is [M0 L0 T-1].

Q.25. Write dimensional formula of angular acceleration.
Ans. The dimensional formula of angular acceleration is [M0 L0 T-2].

Q.26. What is the dimensional formula of angular displacement?
Ans. Angular displacement is a dimensionless quantity.

Q.27. Write the characteristics of uniform circular motion.
Ans. Following are the characteristics of the uniform circular motion:

1. Angular speed of the object is constant.
2. Angular acceleration of the object is zero.
3. Linear velocity of the object has constant magnitude but its direction keeps on changing continuously.
4. Uniform circular motion is an accelerated motion, in which acceleration is directed along the radius of the circle and towards its centre.
5. It is a periodic motion having a definite time period and frequency.

Q.28. Define time period in uniform circular motion.
Ans. The time period of an object in uniform circular motion is defined as the time taken to complete one revolution. It is denoted by T.

Q.29. Give few examples of the projectile.
Ans. Following are a few examples of the projectile:

1. A bullet fired from the gun.
2. A shell fired from a Canon.
3. At javelin or hammer thrown by an athlete.
4. A stone thrown from the top of a hill or a tower, etc.

Q.30. What is resolution of a vector?
Ans. The process of splitting up a vector into two or more vectors is known as the resolution of a vector the vectors into which a given vector is plated are called component vectors.