0Lesson Content:
Topic: Why Do We Feel Lighter or Heavier in an Elevator?
Question: A car starts from rest and, while speeding up steadily, travels 200 metres in 10 seconds. What is its final speed?
Step-by-step
Start speed: The car begins from rest, so its speed at the start is zero.
- What “uniform acceleration” means (in plain words): The car’s speed increases at a steady rate — it doesn’t jump or wobble. Because the increase is steady, the speed in the middle of the trip is exactly halfway between the starting speed and the final speed. In other words, the average speed over the whole trip sits halfway between the initial and final speeds.
- Find the average speed: Average speed = the total distance travelled divided by the total time taken. Divide 200 metres by 10 seconds → the average speed during the 10 s is 20 metres per second.
- Relate average to final speed: Since the car started from zero and speed increased uniformly, the average speed (20 m/s) is exactly half of the final speed. So the final speed must be twice the average speed.
- Final speed: Twice 20 m/s → 40 metres per second.
Answer
Final speed = 40 m/s
Key points to remember
- When acceleration is uniform, speed changes steadily; average speed sits midway between start and end speeds.
- Average speed = total distance ÷ total time (an easy factual step you can always use).
- If the vehicle starts from rest, the final speed is simply twice the average speed for that interval.
