Class 7 Science Chapter 9 Question Answers - Motion and Time

Q1. If a car travels 216 kilometres in 4 hours, calculate its speed.
Ans: Given,
Total distance travelled = 216 km
Total time taken = 4 hours
As we know, speed is the distance travelled per unit time. So,
Speed = distance/time = 216/4
= 54 km/hr

Q2. If a pendulum completes 40 oscillations in 10 seconds, calculate its time period.
Ans: Given,
Total time taken = 10 s
Number of oscillations = 40
As it is known, Time period of a simple pendulum is defined as the time taken for a single oscillation.
Hence, Time period = total time taken/total number of oscillations
= 10/40
=0.25s

Q3. Explain oscillatory motion.
Ans: Oscillatory motion is a form of periodic motion in which an object performs the same motion repeatedly. Oscillatory motion can go on indefinitely in the absence of friction. Oscillatory motion is also known as periodic motion. In this type of motion the object moves between two extreme ends and the centre of these extreme positions is called mean position. Example: Simple Pendulum.

Q4. Explain non-uniform motion.
Ans: In non-uniform motion the graph between distance and time is not a straight line i.e the object is not travelling equal distance in equal interval of time. Velocity throughout the journey is not constant.
E.g. Motion of a car on a traffic jammed road.

Q5. Explain uniform motion.
Ans: When an object travels equal distance in equal time in a straight line it is called uniform motion. Velocity of the object remains the same throughout the journey. There is zero acceleration in this kind of motion. The distance-time graph makes a straight line.
E.g. Moving blades of a ceiling fan.

Q6. Consider the following distance time graph and answer the questions that follow.

(a) What was Sian’s speed for the first 30 minutes of her journey? 
Ans: In the first 30 minutes Sian had travelled 20km.
Distance = 20km
Time = 30 min
Speed = distance/ time
= 20km/30min
= 20 km/0.5 hr
= 40km/hr 

(b) 15 minutes after starting the journey how far had Sian reached
Ans: 15 minutes into her journey Sian had travelled 10 km. 

(c) What is plotted in the X- axis and what is plotted in the Y- axis?
Ans: On the X-axis, time is plotted in minutes and on Y axis distance is plotted in kilometres.

Q7. Consider the following distance time graph and answer the questions that follow.

(a) At what time did Anil leave his home?
Ans:  Anil left his home at 9:05 AM.

(b) How many minutes did Anil wait in the park?
Ans: Anil waited 10 minutes in the park, from 9:35-9:45 AM 

(c) What is the distance from Anil’s home to the park?
Ans: The distance from Anil’s home to the park is 7 kilometres.

(d) What was Anil’s speed in his journey to the park?
Ans: Speed = distance/time and given that time taken is 30 min (from the graph)
Speed (to the park) = 7km/30 min
= 7km/0.5 hr
= 14km/hr 

(e) What was Anil’s speed in the return journey?
Ans: As it is known,
Speed = distance/time
And, Time taken for the return journey = 20 min.
Hence, Speed for the return journey = 7km/20min
= 21 km/hr