Motion — Important Questions

Speed = Distance ÷ Time

Speed tells us how fast a body is moving. It is defined as the distance travelled per unit time. So the formula is: Speed (v) = Distance (d) ÷ Time (t). Multiplying distance and time, or adding them, has no physical meaning. Dividing time by distance gives a different quantity (slowness), not speed.

50 km/h

Step 1: Write down the given values. Distance = 150 km, Time = 3 h. Step 2: Use the formula: Speed = Distance ÷ Time. Step 3: Speed = 150 km ÷ 3 h = 50 km/h. Option A (450 km/h) is wrong — that comes from multiplying distance and time. Options C and D come from adding or subtracting time from distance, which is incorrect.

m/s

The S.I. (International System of Units) unit of speed is metre per second, written as m/s or m s⁻¹. This is because the S.I. unit of distance is metre (m) and the S.I. unit of time is second (s). km/h is a commonly used unit but is NOT the S.I. unit. cm/s and km/min are also not S.I. units.

Uniform motion

When a moving body covers equal distances in equal time intervals, its speed remains constant throughout the journey. This is called uniform motion. For example, a car moving 30 km every hour is in uniform motion. Non-uniform motion means unequal distances in equal times. Oscillatory is to-and-fro motion. Random motion is irregular zig-zag motion.