# Class 7 Science Chapter 13 Important Questions

Class 7 Science Chapter 13 Important Questions Answers and Main Keywords of Motion and Time free to use for new academic session 2020-2021. Class 7 Science Chapter 13 Extra Questions answers contain all the topics given in NCERT Textbook 2020-21.

Practice with these questions to prepare your unit test or periodic exams. NCERT Solutions and Important Extra Questions collectively provide the full preparation of exam for a student of standard 7 Science.## Class 7 Science Chapter 13 Important Questions for 2020-2021

Class: 7 | Science |

Chapter: 13 | Motion and Time |

Contents: | Important Questions with Answers |

#### Class 7 Science Chapter 13 Important Questions Set – 1

##### What do you mean by “Motion and Time”? Give examples.

The movement of an objects is called motion. The movement or duration in which things occurs is called time. An object is said to be in motion or moving when its positions changes with time. For examples, when the position of car changes with, we say that car is moving or that the car is in motion. Our school starts at particular time.

##### Define “Rectilinear and Circular motion”.

When an object moves along a straight line it is called rectilinear motion. The motion of a bullock cart moving on a straight road is a rectilinear motion or motion along a straight line.

When an object moves along a circular path it is called circular motion. For example, the earth moves around the sun in a circular path or circular orbit, so the motion of the earth around the sun is circular motion.

##### “The distance moved by objects in a given interval of time can help us to decide which one is faster or slower” – How?

The distance moved by objects in a given interval of time can help us to decide which one is faster or slower. For example, imagine that you have gone to see off your friend at the bus stand. Suppose you start pedalling your bicycle at the same time as the bus begins to move. The distance covered by you after 5 minutes would be much smaller than that covered by the bus.

We often say that the faster vehicle has a higher speed. In a 100-metre race it is easy to decide whose speed is the highest. One who takes shortest time to cover the distance of 100 metres has the highest speed.

##### What is a sand clock?

Sand clock is a clock used by the Romans to measure a constant time called hour. It consists of two chambers fitted over one another filled with an amount of sand and it takes exactly one hour to empty the chamber. Once the upper chamber is completely emptied, the chamber is turned upside down to record the time again. It is also called “Hour glass”.

**How would you explain the term “speed of the object”?**

The most convenient way to find out which of the two or more objects is moving faster is to compare the distances moved by them in a unit time. Thus, if we know the distance covered by two buses in one hour, we can tell which one is slower. Remember, a slow-moving object is said to have a slow speed. On the other hand, a fast-moving object is said to have high speed. A higher speed indicates that a given distance has been covered in a shorter time. We call the distance covered by an object in a unit time as the speed of the object.

#### Class 7 Science Chapter 13 Important Questions Set – 2

##### “We use the term speed for average speed” – How? Explain with example.

When we say that a car is moving with a speed of 50 kilometres per hour, it implies that it will cover a distance of 50 kilometres in one hour. However, a car seldom moves with a constant speed for one hour. In fact, it starts moving slowly and then picks up speed. So, when we say that the car has a speed of 50 kilometres per hour, we usually consider only the total distance covered by it in one hour. We do not bother whether the car has been moving with a constant speed or not during that hour. The speed calculated here is actually the average speed of the car. In this book we shall use the term speed for average speed. So, for us the speed is the total distance covered divided by the total time taken. Thus,

Speed = Total distance covered / Total time taken

##### Differentiate between uniform and nonuniform motion.

When a body moves with a constant speed i.e., it covers equal distance in equal intervals of time, the motion is said to be uniform. If a car is running a constant speed of 10 metres per second, it will cover equal distances of 10 metres in every 1 second time interval. This is called uniform motion.

Non uniform motion is the different distances covered in equal intervals of time. Such a body moves with a variable speed. That means, it does not remain constant. The motion of a train starting from a railway station is an example of non-uniform motion.

##### Falcon is a bird which flies with a speed of 270 km per hour. Calculate its speed in meter per second.

We know that, 1 kilometer = 1000 meter

270 km = 270000 meters

1 hour = 3600 seconds (60 x 60)

Now,

Speed = Distance travelled / Time taken

= 270000 meter / 3600

= 2700/36

= 75 m/s

##### What is a sundial?

A sundial consists of triangular metallic plate called gnomon fixed vertically at the centre of a circular plate. The device is placed in an open in such a way that gnomon points in the north south direction. It works on the principle that as position of sun in the sky changes, the position and length of shadow also changes. These are calibrated with time.

**What is a simple pendulum? What is the time period of a pendulum?**

A simple pendulum has a small metallic ball called bob suspended from rigid stand by a thread. The time taken by the pendulum to complete one oscillation is called time period.

One oscillation of a pendulum is completed when the bob moves from position O (centre) to A (any side) and then from A to B (B is opposite site of A) and then back to O or from A to B to A. The time taken by the pendulum to complete one oscillation is called its time period.

#### Class 7 Science Chapter 13 Important Questions Set – 3

##### Write something on graph and its origin?

A simple graph can give the relation between two variables example distance and time and the origin of a graph is the intersection point of two axis.

##### On what factors does the time period of Pendulum depend?

Period of a pendulum depends upon – (i) length of the pendulum and (ii) acceleration due to gravity.

##### How does a bob complete one oscillation movement?

A simple pendulum consists of a small metallic ball or a piece of stone suspended from a rigid stand by a thread. The metallic ball is called the bob of the pendulum. It shows the pendulum at rest in its mean position. When the bob of the pendulum is released after taking it slightly to one side, it begins to move to and fro. The to and fro motion of a simple pendulum is an example of a periodic or an oscillatory motion.

**With the help of an activity, show how do you find out one oscillation movement or time period of a pendulum clock.**

Set up a simple pendulum with a thread or string of length nearly one metre. Switch off any fans nearby. Let the bob of the pendulum come to rest at its mean position. Mark the mean position of the bob on the floor below it or on the wall behind it. To measure the time period of the pendulum we will need a stopwatch. However, if a stopwatch is not available, a table clock or a wristwatch can be used.

To set the pendulum in motion, gently hold the bob and move it slightly to one side. Make sure that the string attached to the bob is taut while you displace it. Now release the bob from its displaced position. Remember that the bob is not to be pushed when it is released. Note the time on the clock when the bob is at its mean position. Instead of the mean position you may note the time when the bob is at one of its extreme positions. Measure the time the pendulum takes to complete 20 oscillations.

Your observations could be different from this. Repeat this activity a few times and record your observations. By dividing the time taken for 20 oscillations by 20, get the time taken for one oscillation, or the time period of the pendulum.

##### What do you mean by “speedometer and odometer”?

The panel of instruments and controls facing the driver of a vehicle, when he sits in the vehicle, is called dashboard. An instrument on the vehicle’s dashboard which indicates the speed of the vehicle when it is running, is called speedometer. The speedometer has the symbol “km/h” written on its dial. Speedometer tells us the speed of a running vehicle at that instant of time.

Odometer is an instrument for measuring the distance travelled by a vehicle. Odometer record the distance travelled by a vehicle in kilometres.

#### Class 7 Science Chapter 13 Important Questions Set – 4

##### “Different units of time are used depending on the need” – Comment.

Different units of time are used depending on the need. For example, it is convenient to express your age in years rather than in days or hours. Similarly, it will not be wise to express in years the time taken by you to cover the distance between your home and your school.

##### A car moves with a speed of 40 kilometer per hour for 15 minutes and then with a speed of 60 kilometer per hour for the next 15 minutes. What is the total distance covered by the car?

We should first convert the time of 15 minutes into hours by dividing it by 60

= 15 /60 hours = 0.25 hour

i)In the 1st case:

Distance travelled = Speed x time

= 40 x 0.25 km

= 10 km

ii)In the 2nd case:

Distance travelled = Speed x time

= 60 x 0.25 km

= 15 km

Total distance: 10 km +15 km

= 25 km

##### If Boojho covers a certain distance in 1 hour and Paheli covers the same distance in 2 hours, who travels in a higher speed?

Boojho travels with a higher speed, as he has covered the same distance in lesser time then Paheli.

**Write the differences between speed and velocity.**

- (i) Speed is the distance covered by a body per unit time in any direction whereas velocity is the distance travelled by a body in a given direction.
- (ii) Speed is a scalar quantity whereas velocity is a vector quantity.
- (iii) Speed is always positive or zero but never negative whereas velocity may be positive or negative or zero.

**Write the main differences between distance and displacement.**

- (i) Distance is the length of the actual path covered by a body irrespective of its direction of motion whereas displacement is the shortest distance between the initial and final position of a body in a given direction.
- (ii) Distance between two given points may be same or different for different paths chosen whereas displacement between two given points is always same.
- (iii) Distance covered is always positive or zero whereas displacement covered may be positive, negative or zero.
- (iv) Distance is a scalar quantity whereas displacement is vector quantity.

#### Class 7 Science Chapter 13 Important Questions Set – 5

##### Paheli and Bujho have to cover different distances to reach their school but they take the same time to reach the school. What can you say about their speed?

They do not have equal speed because they cover unequal distances in equal intervals of time. One of them has higher speed, whom has to cover larger distance with respect to other.

##### The average age of children of class VII is 12 years and 3 months. Express this age in seconds.

Given the average age of children is 12 years and 3 months:

1 year = 365 days = 365 x 24 h

= 365 x 24 x 3600 s

= 31536000 s

Therefore,

12 years = 31536000 s x 12

= 378432000 s

And 3 months = 3 x 30 days = 30 x 3 x 24 h = 30 x 3 x 24 x 3600 s = 7776000 s

Total age in second = (378432000 + 7776000) seconds

= 386208000 s

##### Salman takes 25 minutes from his house to reach his school on a bicycle. If the bicycle has a speed of 2 meter per second, calculate the distance between his house and the school.

In this problem, the speed is given in meter per second and the time is given in minutes. So, before putting in the formula for speed, we will have to convert time from minutes into seconds by multiplying the minutes by 60.

Time taken = 25 minutes

= 25 x 60 seconds = 1500 s

Speed = 2 m / s

Distance travelled? (To be calculate)

Now,

Speed = Distance travelled / Time taken

2 = Distance travelled / 1500

By cross multiple:

Distance travelled = 2 x 1500 m

= 3000 m

= 3 km

###### How can we choose a suitable scale to draw a distance time graph?

We should keep the following points in mind while choosing the most suitable scale for drawing a graph:

- (i) Difference between the highest and lowest values of each quantity.
- (ii) The intermediate values of each quantities so that with the scale chosen it is convenient to mark the values on the graph.
- (iii) To utilise the maximum part of the upper paper on which the graph is to be drawn.

**Write down the conclusion of the distance time graph for moving or stationary object**.

- If the distance time graph of an object is a straight line, then it is moving with a constant speed (the greater the slope of distance time graph, the greater the speed).
- If the distance time graph of an object is not a straight line or curved line, then the speed of an object is not constant. The speed is changing.
- If the distance time graph of an object is a horizontal line parallel to the time axis, then the speed of object is zero and the object is not moving.