Class 7 Motion-Time Answer

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State whether True (T) and false (F)
(a) The physical quantity which measures how far an object moves during a fixed period of time is called speed.
(b) Every object moves with a constant speed.
(c) An odometer measures the distance covered in kilometres.
(d) We can tell the time of a day using a water clock or sand clock.
(e) The time-period of a given pendulum is not constant.

State whether True (T) and false (F)
(a)—(F), (b)—(F), c—(T), (d)—T, (e)—(F)

Very Short Answer Type Questions.
1. What is the standard unit of speed?
2. Which unit of speed will you use to express the speed of an aeroplane?
3. Name the instrument which is used to record the distance covered by a vehicle?
4. Name two devices which were used for measuring time in ancient times before pendulum clocks were made.
5. Who gave the law of oscillation of the pendulum.
6. What type of motion is illustrated by the bob of a simple pendulum?
7. Define speed.
8. Write the formula for calculating speed.
9. A man riding a bike covers a distance of 40 km in 120 minutes. Calculate the speed of the man?

Answers:

1. m/s is the SI unit of speed.
2.
3. The odometer is an instrument for measuring the distance travelled by a vehicle.
4. Two-time measuring devices used in ancient times were: sundial, sand clock.
5. Galileo Galilei gave the law of oscillation of the pendulum.
6. The swinging of a pendulum is an example of periodic motion.
7. The speed of an object is defined as the distance travelled by it in unit time. The speed of an object can be calculated by dividing the ‘distance travelled’, by the ‘time taken’.
8. The speed of an object can be calculated by dividing the ‘distance travelled’, by the ‘time taken’.
Speed = Distance travelled / Time taken
9. The distance travelled = 40 km
The time taken = 120 minutes
= 120/60 = 2 hr.
We know that,
Speed = Distance travelled / Time taken
So, speed of the bike = 40 km / 2 hr
= 20 km/hr

Short Answer Type Questions.
1. What is motion? What are the two types motion?
2. Classify the following as rectilinear motion, circular motion or periodic motion (oscillatory motion):
(a) The motion of your hands while running.
(b) The motion of a child on a see-saw.
(c) The motion of a horse pulling a cart on a straight road.
(d) The motion of a train on a straight bridge.
(e) The motion of the hammer of an electric bell.
3. What do you mean by the uniform and non-uniform motion of a body?
4. Draw the distance-time graph:
(a) for an object moving at a constant speed
(b) for a stationary object (which is not moving)
5. A bus travelled 160 km in 4 hours, another bus travelled 175 km in 5 hours, which bus moved faster?
6. A student riding a bicycle reaches his school in 20 minutes. The school is 2 km far from his house. Calculate the
speed.
7. A car travelling at a speed of 50 km/h takes 25 minutes to reach its destination. What distance has the car
travelled?
8. A train is running at the speed of 60 km/h. Calculate the speed of the train in m/s.
9. The speed of a racing car is 60 m/s. What will this speed be in km/h?
10. Define amplitude and time period of a pendulum.
11. A simple pendulum takes 36 seconds to complete 20 oscillations. What is the time period of this pendulum?

Answers:

1. Motion is a change in position of an object with respect to time.
There are two types of motion:-
(i) Uniform motion — If an object is moving along a straight line path is said to have a uniform motion, if its speed
remains constant.
(ii) Non-uniform motion — An object moving along a straight line path is said to have non-uniform motion if its speed keeps changing (it does not remain constant).
2. (a) Motion of your hands while running – periodic motion.
(b) The motion of a child on a see-saw- periodic motion.
(c) The motion of a horse pulling a cart on a straight road- rectilinear motion.
(d) The motion of a train on a straight bridge rectilinear motion.
(e) The motion of the hammer of an electric bell – periodic motion.
3. 1. Uniform motion—If an object is moving along a straight line path is said to have a uniform motion, if its speed
remains constant.
2. Non-uniform motion—An object moving along a straight line path is said to have non-uniform motion if its speed
keeps changing (it does not remain constant).

5. For the first Bus

Distance travelled = 160 km
Time taken = 4 hr.
Speed of the first bus = Distance travelled/Time taken
= 160 km / 4 hr
= 40 km/ hr
For second Bus
Distance travelled = 175 km
Time taken = 5 hr.
Speed of the first bus = Distance travelled /Time taken
= 175 km / 5 hr
= 35 km/ hr
So, the first bus is moving faster.
6. Distance travelled = 2 km
Time taken = 20 minutes
= 20/60 = .33hr.
Speed of the first bus = Distance travelled /Time taken
= 2 km / .33 hr
= 6.06 km/ hr
7. Distance travelled = ?
Time taken = 25 minutes
= 25/60 = .41hr.
Speed of the travelling = Distance travelled /car Time taken
50 km/hr = Distance travelled /.41 hr
So, Distance travelled = Speed of the
by the car travelling car × Time taken
= 50 km/hr* .41 hr
= 122 km
8. 60 km/hr = 60×1000 m / 60×60 sec
= 16.6 m/sec
9. The speed of a racing car is 60 m/s. What
will this speed be in km/h.
60 m = 60/1000 = 0.06 km
1 sec = 1/60×60 = .000277 hr
60 m / s = 0.06 km / .000277 hr
= 216.60 km/ hr.
10. Amplitude of pendulum: As the pendulum oscillates to-and-fro, the maximum displacement of the bob from its mean position on either side is called the amplitude of pendulum.
The time period of the pendulum: The time was taken by the bob of a pendulum to complete one oscillation is called the time period of the pendulum.
11. Time is taken for 20 oscillations
= 36 seconds.
So, Time taken for 1 oscillations = 36/ 20
=1.8 seconds
So, Time period of this pendulum is 1.8 seconds.

HOTS-Higher Order Thinking Skills:
1. How will you reduce the time period of a pendulum you have made by tying a stone to a
string?
2. Why do we need to measure time?
3. Justify the statement that a car moving along a straight line covers the equal distance in equal intervals of time, has uniform speed.

 


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