**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
= $\frac{\text{distancetravelled}}{\text{timetaken}}$

$=\frac{120}{2}=$ 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 $\u2013$ A parade

Circular
motion: Motion of an object along a circular path is called a circular motion.
Example $\u2013$ A spinning top

Oscillatory
motion: The to and fro motion of an object is called an oscillatory motion.
Example $\u2013$ A simple pendulum

Periodic
motion: The motion of an object that is repeated at regular interval is called
a periodic motion. Example $\u2013$ 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

$=30\text{}\text{km}\xf7\frac{1}{3}=30\times \frac{3}{1}=90\text{\hspace{0.17em}}\text{km/h}$

**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 $=20\times 4=\text{\hspace{0.17em}}80\text{\hspace{0.17em}}\mathrm{sec}$

**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 = $\frac{\text{Distancetravelled}}{\text{timetaken}}$ $=\frac{2}{8}=$ 0.25 m/s