Force and Motion — Class 7 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 7 Science, Term 1 — Chapter 2. Speed, velocity, acceleration and motion graphs.
1. About this chapter
This chapter covers speed, velocity and acceleration, uniform circular motion, positive and negative acceleration, and the graphical representation of motion.
2. Speed, velocity and acceleration
- Speed = distance ÷ time (magnitude only). Velocity = displacement ÷ time — speed in a stated direction; it tells how fast the position changes. Velocity is measured in metre/second (m/s).
- Acceleration = (change in velocity) ÷ time — it tells how fast the velocity changes; it is measured in metre/second² (m/s²).
- If velocity increases with time, the object has positive acceleration; if it decreases, it has negative acceleration (deceleration).
- The analogy: displacement/time : velocity :: change of velocity/time : acceleration. (The speed of an aeroplane is measured in knots.)
3. Uniform circular motion
- In uniform circular motion an object moves around a circle at constant speed, but because its direction keeps changing, its velocity changes — so it is in accelerated motion. Examples: a merry-go-round, a roller coaster, planets orbiting the Sun.
- After half a circle of radius r, the displacement is 2r (the diameter), even though the distance travelled is πr.
4. Graphical representation of motion
- In a velocity–time graph, a straight slanting line shows uniform acceleration; the slope gives the acceleration.
- In a distance–time graph, a horizontal line means the object is at rest and a slanting line means uniform speed.
5. Worked examples
Example 1. A car travels 150 m in 10 s. Find its speed. 150 ÷ 10 = 15 m/s.
Example 2. A body's velocity rises from 4 m/s to 20 m/s in 8 s. Find the acceleration. (20 − 4) ÷ 8 = 2 m/s² (positive acceleration).
Example 3. Find the displacement after half a circle of radius 7 m. 2r = 14 m (the diameter).
6. Book-back questions (Samacheer Kalvi)
I. Choose the correct answer
- The displacement of a particle moving in a circular path of radius r after half a circle is — (a) πr / (b) 2r. Ans: (b) 2r.
- A boy on a merry-go-round moving with constant speed is in — (a) rest / (b) accelerated motion. Ans: (b) accelerated motion.
- A straight slanting line in a velocity–time graph shows — (a) uniform acceleration / (b) rest. Ans: (a) uniform acceleration.
II. Fill in the blanks 4. Velocity is measured in metre/second and acceleration in metre/second². 5. The speed of an aeroplane is measured in knots. 6. If the velocity of an object increases with time, the object has positive acceleration.
III. Answer briefly 7. Differentiate velocity and acceleration. — Velocity tells how fast the position changes; acceleration tells how fast the velocity changes. 8. Why is an object in uniform circular motion said to be accelerating? — Because its direction (and hence velocity) keeps changing, even though its speed is constant.
7. Common mistakes
- Mistake: Saying uniform circular motion has no acceleration. Fix: The direction changes, so the velocity changes — it is accelerated motion.
- Mistake: Taking the distance (πr) as the displacement for half a circle. Fix: The displacement is 2r (the straight-line diameter).
- Mistake: Confusing the units of velocity and acceleration. Fix: Velocity is m/s; acceleration is m/s².
8. Quick revision
- Term 1 · Ch 2 · force and motion.
- Speed (distance/time); velocity (displacement/time, m/s); acceleration (change of velocity/time, m/s²).
- Uniform circular motion: constant speed but changing velocity → accelerated; half-circle displacement = 2r.
- Velocity–time slanting line = uniform acceleration (slope = acceleration); distance–time horizontal = rest.
