You Are Moving and Sitting Still at the Same Time

Yes. Sitting in a moving train you are at rest with respect to the seat beside you, and moving at $60km/h$ with respect to the platform. Both statements are correct because they use different observers.

That is why physics never says "moving" without saying moving compared with what. This page covers everything in the ICSE Class 7 Physics chapter on motion: rest and motion as relative terms, the types of motion, speed and its units, and average speed.

Because a body is described as at rest or in motion only with respect to a chosen observer, called the frame of reference.

A body is at rest if its position does not change with time relative to its surroundings, and in motion if its position does change.

So a passenger in a moving bus is at rest relative to the other passengers and the bus itself, while in motion relative to a shopkeeper on the roadside. Neither observer is wrong; they have simply chosen different references.

The trees seen from a train window make the same point vividly. They appear to rush backwards, because the frame you are judging from is the moving train.

And a book on your table, which seems the clearest case of rest, is carried along with the rotating Earth, so it is in motion with respect to the Sun.

That is why the phrase "absolutely at rest" has no meaning in physics. Every statement about motion is silently attached to a frame, and answers should name that frame: at rest with respect to the bus.

Motion is classified by the path a body follows and by whether the motion repeats.

Translational — the whole body moves from one place to another. It has two forms: rectilinear, along a straight line, such as a car on a straight road; and curvilinear, along a curved path, such as a car going round a bend.

Rotational — the body spins about a fixed axis without moving away, as a ceiling fan or a potter's wheel does.

Circular — the body moves along a circular path, such as a stone whirled on a string or the hands of a clock.

Oscillatory (vibratory) — the body moves to and fro about a fixed position, as a swing or the string of a sitar does.

Periodic — any motion that repeats itself in equal intervals of time, such as the pendulum of a wall clock or the Earth going round the Sun.

The categories overlap, and that is the part students find confusing. A pendulum is oscillatory and periodic, and the Earth's revolution is circular and periodic. These are descriptions of different features, not a list of boxes where each motion fits exactly one.

A moving bicycle shows two at once: the wheels rotate while the whole bicycle moves translationally down the road.

A body has uniform speed if it covers equal distances in equal intervals of time, however small those intervals are. Otherwise its speed is non-uniform or variable.

Real journeys are almost never uniform — a bus in city traffic speeds up, slows at signals and stops — so we describe them with a single representative figure:

$$\text{Average speed}=\frac{\text{total distance travelled}}{\text{total time taken}}$$

Worked example. A car covers $120km$ in $1.5h$, then $60km$ in $1h$. Total distance is $180km$ and total time is $2.5h$, so

$$\text{average speed}=\frac{180}{2.5}=72km/h$$

Here is the trap, and it is examined often: the average speed is not the average of the two speeds. Those legs were run at $\frac{120}{1.5}=80km/h$ and $\frac{60}{1}=60km/h$, whose average would be $70km/h$ — not the correct $72km/h$. Only total distance over total time is right, because the car spent longer at the higher speed.

And stopped time counts. A journey of $100km$ taking $2h$ of driving plus a $30$-minute halt has an average speed of $\frac{100}{2.5}=40km/h$, not $50km/h$.