7th Class Science Time and Motion Time

       Time

Time is used to measure the duration in which things occur, and interval between two occurrences. Time give us the idea of the duration of the occurrence of a certain event. For example, repetition of the day, night and seasons, phases of the Moon, appearance of certain star in the sky etc.

In ancient time, people observed stars and constellations for such estimations. Appearance of the certain star in the sky was the indicator of new season. But this way of estimation was very inconvenient as stars and constellations appear only at night and could not be seen at cloudy nights. Therefore, they started to develop new systems to measure the day and night durations.

   Sand Clock

Sand clock is one of the instruments, which was used earlier to measure the time duration. Sand clock consists of two glass bulbs placed one above the other and are connected with the help of a narrow glass tube. Sand flows from the top to the bottom. When the top bulb gets empty, clock is inverted so that lower glass tube comes up and sand clock keeps on working. The sand takes a fixed duration of time to flow from one glass tube to other. In this way, earlier people used to measure time duration of an event.

In the modern time we use clocks to know the duration of time. Clock uses second as a unit; 60 second is equal to 1 minute and 60 minute to one hour.

Modern basis of measuring time duration

60 seconds = 1 minute

60 minutes = 1 hour

24 hours = 1 day

365 days = 1 year

10 years = 1 decade

10 decades = 1 century

10 century’s = 1 millennium

            Velocity

Velocity is a physical quantity that is used to show the direction in which object is moving, as well as speed of the object. Thus velocity gives more close information about the position of the object. Velocity is a vector quantity as it has both direction and magnitude.

          Uniform Velocity

When an object moves on a straight path with a constant speed and does not change direction, the object is said to be in the uniform velocity. When an object moves on a circular path, it might have uniform speed but not uniform velocity, as an object has to change its direction continuously to move on a circular path.

In the above figure, the car is moving in east direction with a constant speed. Therefore, it is said that the car is moving with uniform velocity.

            Non-uniform velocity

If an object changes either its direction or speed or both while moving, the object is said to be in non-uniform velocity.

In the above figure, the car is moving in a fixed direction but speed of the car varies. Thus, the car is moving with non-uniform velocity.

            Acceleration

Rate of change in the velocity of an object is called acceleration. Acceleration is also vector quantity. Therefore, if an object changes only direction not speed however the object is said to be accelerated.

           Uniform Acceleration

When velocity of an object changes with a constant rate, the object is said to be in the uniform acceleration.

In the above figure, speed of the car is increases by 10 km/h after every 30 minute. Therefore the car has uniform acceleration.

          Non-Uniform Acceleration

If rate of the acceleration is not constant, the object is said to be in non- uniform acceleration.

In the above figure, in first 30 minutes speed of the car has increased by 15 km/h and in the next 30 minutes speed of the car has increased by 5 km/h.

Thus, the car is not in uniform acceleration.

       How many years are there in a millennium?

(a) 10

(b) 100

(c) 1000

(d) 10000

(e) None of these

Answer: (c)

        Look at the graph given below carefully and find whether the body is in uniform acceleration?

(a) Yes                                                                 

(b) No

(c) The graph does not give the information that the body is in uniform acceleration or not

(d) All of these

(e) None of these

Answer: (b)

A body is in motion if its position changes with the time.

A body is in rest if its position does not change with the time.

If a body covers equal distances in equal interval of time, the body is in uniform motion.

If a body covers unequal distances in equal interval of time, the body is in non-uniform motion.

Rate of change in the velocity of a body is called acceleration.

Distance is the total length of the path on which an object moves.

Displacement is the distance between initial point from where an object starts to move and the final point where the object stops

Time is a physical quantity, which is used to measure the duration between two events.