Navigation
Learn why a car's speedometer changes and how to calculate the speed of an entire trip.
Why does your GPS say you'll arrive in 20 minutes even when you're currently standing still at a red light?
Instantaneous speed is the speed of an object at a specific, single moment in time. Think of it as a 'snapshot' of motion. If you take a photo of a car's speedometer while driving, the number you see is the instantaneous speed. It tells you how fast you are going right now, but it doesn't tell you anything about where you started or how long you've been driving. In physics, we often use this to understand how forces are acting on an object at a precise second.
Quick Check
If a police officer uses a radar gun to catch a speeder, are they measuring average or instantaneous speed?
Answer
Instantaneous speed.
A family drives to a park. The drive takes of moving time, but they stopped for to eat lunch. What was their average speed for the whole trip?
A common mistake is trying to find the average speed by simply averaging two different speeds. Don't do this! If you walk for one minute and for one minute, the average is . But if the distances or times are different, that shortcut fails. You must always return to the golden rule: Total Distance divided by Total Time. This ensures that the time spent at each speed is weighted correctly.
A drone flies at a speed of , then flies another at a speed of . What is its average speed?
1. Find time for Part 1: . 2. Find time for Part 2: . 3. Find total distance: . 4. Find total time: . 5. Calculate: .
Quick Check
True or False: If you stop at a red light for 30 seconds, your instantaneous speed is 0 m/s, but your average speed for the trip decreases.
Answer
True
A bus travels at . It then stops at a station for (). Finally, it travels at . Calculate the average speed for the entire operation.
1. Time for Leg 1: . 2. Time for Stop: . 3. Time for Leg 2: . 4. Total Distance: . 5. Total Time: . 6. Average Speed: .
Which of these is an example of instantaneous speed?
If you travel in and then in , what is your average speed?
Average speed can be higher than the highest instantaneous speed reached during a trip.
Review Tomorrow
In 24 hours, try to explain to a friend why your average speed on a walk is lower if you stop to tie your shoe.
Practice Activity
Next time you are in a car, note the distance of the trip and the total time. Calculate the average speed and compare it to the speeds you saw on the speedometer.