Motion
ICSE Class 7 · Physics · Chapter 2
What this chapter covers
- Motion and rest
- Types of motion
- Repetitive motion: periodic and non-periodic
- Distance
- Uniform and non-uniform motion
- Speed
- Average speed
- Mass and weight
Key points
- Motion is the change in the position of an object with time; rest is when position does not change with time.
- Rest and motion depend on the reference point: the same object can be at rest compared with one thing and in motion compared with another.
- By path, motion can be translatory, circular (circulatory), oscillatory or random; an object can show more than one type at once.
- Repetitive motion repeats the same movement again and again. It is periodic when it repeats after equal intervals of time and non-periodic when it repeats after unequal intervals. The time for one repetition of a periodic motion is the time period.
- Distance is the length of the actual path travelled; its SI unit is the metre.
- Uniform motion covers equal distances in equal intervals of time; non-uniform motion covers unequal distances in equal intervals.
- Speed = distance ÷ time; its SI unit is m/s, and km/h is used for vehicles. To change km/h to m/s, multiply by (5)/(18).
- Average speed = total distance ÷ total time, including any time spent stopped.
- Mass is the quantity of matter in an object, measured in kg with a beam balance, and stays the same everywhere.
- Weight is the force with which the Earth pulls an object, measured in newtons with a spring balance; on Earth each kilogram weighs about 10 N, and on the Moon an object weighs about one-sixth of its Earth weight.
Questions answered
A passenger sits in a moving bus. Is the passenger at rest or in motion? Explain with reference to two different objects.
The passenger is at rest with respect to the seat and the bus, because the distance from them does not change. The passenger is in motion with respect to a tree by the roadside, because the bus carries the passenger past it. Rest and motion depend on the reference point chosen.
Name the type of motion in each case: (a) a lift going up, (b) the tip of a ceiling fan's blade, (c) a swinging pendulum, (d) a mosquito in a room.
(a) Translatory, (b) circular, (c) oscillatory, (d) random.
What is repetitive motion? How is periodic motion different from non-periodic motion? Give one example of each.
Repetitive motion is motion in which an object repeats the same movement again and again. Periodic motion repeats after equal intervals of time, such as the hands of a clock. Non-periodic motion repeats after unequal intervals of time, such as a swing pushed by a friend at irregular moments.
Why does the wheel of a moving bicycle show two types of motion?
The wheel turns in a circle about its axle, which is circular motion, and it also moves forward along the road with the bicycle, which is translatory motion.
A car covers 20 m in every second for one whole minute. Is its motion uniform or non-uniform? Give a reason.
Uniform, because it covers equal distances in equal intervals of time.
A girl walks 1200 m in 10 minutes. Find her speed in metres per minute and in metres per second.
Speed = (1200 m)/(10 min) = 120 m per minute. Since 10 minutes is 600 s, speed = (1200 m)/(600 s) = 2 m/s.
Convert 72 km/h into m/s.
72 × (5)/(18) = 20 m/s.
A cyclist travels 15 km in the first hour and 25 km in the next 1.5 hours. Find the average speed.
Total distance = 40 km and total time = 2.5 h, so average speed = (40)/(2.5) = 16 km/h.