Motion Measurement – Long Answer Questions (Speed and Velocity)
Medium Level (Application & Explanation)
Q1. Explain the difference between speed and velocity with suitable examples and units.
Answer:
Speed is the distance covered per unit time. It has only magnitude.
Velocity is speed in a specific direction. It has magnitude and direction.
SI unit for both is m/s. Other common units are km/h and cm/s.
A car moving at 60 km/h is describing speed.
A car moving at 60 km/h north is describing velocity.
If direction changes, velocity changes, even if speed stays the same.
Q2. Describe average speed. Why is it useful in daily travel where the speed keeps changing?
Answer:
Average speed = Total distance / Total time.
In real life, speed is often not constant due to stops and traffic.
Average speed helps us summarize the whole journey with one value.
It tells us how fast we moved overall, not at each moment.
Example: If you travel 100 km in 2 hours, average speed is 50 km/h.
It is useful for planning, scheduling, and comparing trips.
Q3. A runner completes 400 m in 50 s, rests for 10 s, and then runs 200 m in 30 s. Find the average speed for the whole activity.
Answer:
Total distance = 400 m + 200 m = 600 m.
Total time = 50 s + 10 s + 30 s = 90 s.
Average speed = Total distance / Total time.
So, Average speed = 600 m / 90 s = 6.67 m/s (approx.).
Rest time is included because time keeps running.
This shows average speed depends on both movement and pauses.
Q4. When can the average velocity of a journey be zero even if the average speed is not zero? Explain with an example.
Answer:
Average velocity depends on displacement, not total distance.
If you return to the starting point, displacement is zero.
Then average velocity = 0 m/s, even if you moved a lot.
Example: Run 500 m forward and 500 m back.
Total distance = 1000 m, so average speed is not zero.
But displacement = 0, so average velocity is 0 m/s.
Q5. What does uniform velocity mean? How is it different from variable velocity?
Answer:
Uniform velocity means constant speed in the same direction.
Both magnitude and direction stay unchanged.
Example: A plane flying straight at 200 m/s east.
Variable velocity changes in speed, direction, or both.
A car turning a corner changes direction, so velocity changes.
A car speeding up or slowing down changes magnitude, so velocity changes.
High Complexity (Analysis & Scenario-Based)
Q6. A student cycles 3 km east in 0.5 h, then 4 km west in 0.5 h. Calculate average speed and average velocity for the whole trip.
Answer:
Total distance = 3 km + 4 km = 7 km.
Total time = 0.5 h + 0.5 h = 1 h.
Average speed = Total distance / Total time = 7 km/h.
Displacement = 3 km east − 4 km west = 1 km west.
Average velocity = Displacement / Time = 1 km west / 1 h = 1 km/h west.
So, average speed is 7 km/h, but average velocity is 1 km/h west due to direction.
Q7. Two cars travel for 2 hours. Car A moves straight north at 60 km/h. Car B moves at 60 km/h but changes direction several times. Compare their average speeds and average velocities.
Answer:
Car A: Speed = 60 km/h, direction is constant north.
Car B: Speed = 60 km/h, but direction changes.
Average speed for both may be 60 km/h if distance and time match.
Car A’s displacement is 120 km north, so average velocity is 60 km/h north.
Car B’s displacement may be less due to turns, so average velocity is lower.
Same average speed can still give different average velocities.
Q8. A car’s speed stays at 40 km/h, but it takes a U-turn and comes back. Explain how its velocity changes and what this means for average velocity.
Answer:
Speed remains 40 km/h throughout.
But direction reverses during the U-turn.
So, velocity changes because velocity depends on direction.
Over the full out-and-back trip, displacement can be small or zero.
If it returns to the start, average velocity = 0 km/h.
This shows speed can be constant while velocity changes.
Q9. A vehicle accelerates from 20 m/s to 60 m/s. Using the given relation, find the average velocity and explain its meaning.
Answer:
Given: Initial velocity u = 20 m/s, Final velocity v = 60 m/s.
Average velocity = (u + v) / 2 = (20 + 60) / 2 = 40 m/s.
This is the mean value of the starting and ending velocities.
It applies when we discuss velocity over the change from u to v.
It tells us the typical velocity during the period of change.
It is useful for quick estimates of overall motion in one direction.
Q10. A tourist walks 2 km east in 30 min, 1 km north in 15 min, and 1 km east in 15 min. Find average speed and average velocity. Explain the difference clearly.
Answer:
Total distance = 2 + 1 + 1 = 4 km.
Total time = 30 + 15 + 15 = 60 min = 1 h.
Average speed = Total distance / Total time = 4 km/h.
Net displacement: East = 2 + 1 = 3 km east, North = 1 km north.
