Class 9 Science Chapter 7 Question Answers - Motion

Q1: Define the term velocity. What is its unit? Is it a scalar or vector quantity?
Ans: Velocity is a physical quantity that gives the speed and direction of motion of the body  
Velocity = Displacement  / Time
The S.I. unit of velocity is ms^–1. Velocity is a vector quantity because its description requires both magnitude and direction.

Q2: Define acceleration. Is it a scalar or a vector quantity?
Ans: In non-uniform motion, the velocity of a body changes with time. It has different velocities at different instants of time and at different points of its path. In such a situation, we define a physical quantity called acceleration which is a measure of the change in the velocity of a body per unit time. It is a vector quantity.

Q3: What is the acceleration of a body moving uniform velocity?
Ans: The acceleration of a body moving with uniform velocity is zero.

Q4: “The direction in which an object moves is given by the direction of velocity of the object and not the direction of acceleration.” Give an example to justify this statement.
Ans: When an object is thrown upward, its velocity is directed upward while its acceleration is directed downwards. Thus, the velocity determines the direction of motion.

Q5: Name the different types of graph.
Ans: Bar graphs, straight line graphs, histograms, etc.

Q6: Name the two types of physical quantities.
Ans: The two types of physical quantities are scalar quantity and vector quantity.

Q7: Define a scalar quantity.
Ans: A physical quantity which has only magnitude and no direction is called a scalar quantity.

Q8: Define vector quantity.

Ans: A physical quantity which has both magnitude and direction is called a vector quantity.

Q9: Give two examples of a vector quantity?
Ans: Two examples of a vector quantity are displacement and velocity.

Q10: Give the simplest type of motion.
Ans: The simplest type of motion is the motion along a straight line.

Q11: Give two examples of a scalar quantity.
Ans: Two examples of a scalar quantity are distance and speed.

Q12: What is magnitude?
Ans: The numerical value of a physical quantity is called its magnitude.

Q13: Which type of graph is used to describe motion?
Ans: Line graph are used to describe the motion of an object.

Q14: When is the acceleration of a body positive?
Ans: The acceleration of a body is taken to be positive if it is in the direction of velocity.

Q15: Differentiate acceleration from velocity.
Ans: Velocity is the change in position of a body per unit time in a given direction and acceleration is the change in velocity of a body per unit time.

Q16: What is the Si unit of acceleration?
Ans: The SI unit of acceleration is m/s².

Q17: What is the relationship between distance travelled and the time elapsed for the motion with uniform velocity?
Ans: Distance is directly proportional to time used.
In fact, distance travelled  
= Uniform velocity × Time used.