Lesson: Motion and Time

Define motion.

The change in position of a body with respect to time is called motion.

Write S.I. unit of speed and time.

S.I. unit of speed is metre per second (m/s) and S.I. unit of time is second (s).

What is a speedometer?

It is a device that is used to measure the speed of the vehicle in km/h.

Define time period of a simple pendulum.

The time taken by a simple pendulum to complete one oscillation is called its time period.

Name any two ancient clocks that are used to measure time?

Sand clock and water clock

Define distance and speed.

Distance: The distance covered by a moving object is the actual length of the path followed by the object.

Speed: Speed is the total distance covered divided by the total time taken.

Define periodic motion and give an example.

When an object repeats its motion after an equal interval of time, this type of motion is called periodic motion.

Example: Motion of a simple pendulum

What is odometer? What is its function?

It is a device that records total distance covered by the vehicle. It is used in vehicles to calculate the distance travelled in a particular journey.

Define oscillation? On which factor does the time period of a pendulum depend?

The pendulum completes one oscillation when its bob moves from one extreme position A to the other extreme position B and comes back to A.

The time period of a simple pendulum depends on the length of the pendulum.

Express 72 km/h in m/s.

72 km/h = $\frac{\text{ 72 }\times 1000}{60\times 60}=$ 20 m/s

The distance between two stations is 120 km. A train takes 2 hours to cover this distance. Calculate the speed of the train.

Distance travelled= 120 km

Time taken = 2 h

Speed = $\frac{\text{distance travelled}}{\text{time taken}}$  

$=\frac{120}{2}=$ 60 km/h

What are the different types of motion?

Rectilinear motion: Motion of an object along a straight line is called a rectilinear motion. Example $–$ A parade

Circular motion: Motion of an object along a circular path is called a circular motion. Example $–$ A spinning top

Oscillatory motion: The to and fro motion of an object is called an oscillatory motion. Example $–$ A simple pendulum

Periodic motion: The motion of an object that is repeated at regular interval is called a periodic motion. Example $–$ Revolution of Earth around the Sun

The odometer of a car reads 5500.0 km when the clock shows the time 06:30 p.m. What is the distance moved by the car, if at 06:50 p.m., the odometer reading has changed to 5530.0 km? Calculate the speed of the car in km/h for this duration.

Distance travelled by the car between 06:30 p.m. and 06:50 p.m.

 = 5530.0 km − 5500.0 km

= 30 km

Time taken by the car to cover that distance

= 06:50 p.m. − 06:30 p.m.  

= 20 m

= 20/60 h

= 1/3 /h

Speed of the car in km/min

$=\frac{30}{20}=1.5\text{ km}/\text{min}$

Speed of the car in km/h

$=30\text{km}÷\frac{1}{3}=30\times \frac{3}{1}=90\text{km/h}$

A simple pendulum takes 20 seconds to complete 5 oscillations. Calculate the time taken by the pendulum to complete 20 oscillations.

The time taken by a pendulum to complete 5 oscillations = 20sec

The time taken by the pendulum to complete 1 oscillation $=\frac{25}{5}=$ 4 sec

Therefore, the time taken by the pendulum to complete 20 oscillations $=20\times 4=80\sec$

The given figure represents distance-time graph. Answer the following questions.

(a)               What is the distance covered after 8 sec?

(b)               Calculate the speed of the object between 12 s and 20 s.

 

(a)               The distance covered by the object after 8 second is 4 m.

(b)               Distance travelled between 12^th sec and 20^th sec

$=8-6=2\text{ m}$.

Time interval $20-12=8\text{ sec}$

Speed = $\frac{\text{Distance travelled}}{\text{time taken}}$ $=\frac{2}{8}=$ 0.25 m/s