A moment and a stretch of time

The idea

An instant of time is a single reading of a clock, whereas a time interval is the duration between two instants, that is, between two clock readings.

One reading or two

A stopwatch shows 4 s when the athlete passes a mark, and 10 s when she passes the next mark.

Which of these is a time interval?

• The 6 s between the two readings
• The single reading 4 s
• The single reading 10 s

Readings on a line

Press Next step to place the readings and find the interval.

Point and stretch

A single clock reading, such as 4 s, is a point on the time line: an instant. The 6 s between 4 s and 10 s is a stretch of time: an interval. To find the interval we take the difference of the two readings.

An instant of time is a single reading of a clock, whereas a time interval is the duration between two instants, that is, between two clock readings.

Notes

An instant is one clock reading. A time interval is the duration between two clock readings.

Check yourself

Which of these is an instant of time?

A clock reads 4 s at the start and 10 s at the end. What is the time interval?

Answer: 6 s

10 s − 4 s = 6 s.

What is a time interval?

An athlete starts at the clock reading 0 s and stops at 16 s. The time interval for her run is…

• From 4 s to 10 s. That is a duration between two clock readings, so it is a time interval.
• The clock reads 10 s — correct. Yes!
• The time the race took. How long the race took is a duration, so it is a time interval.
• A single reading of a clock. That is an instant of time, not an interval.
• The same thing as an instant. An instant is one reading of a clock. An interval is the duration between two readings, so they are different.
• The duration between two instants, that is, between two clock readings — correct. Yes!
• 16 s — correct. Yes!
• 0 s. 0 s is the first clock reading, an instant. The interval is the duration between the two readings.
• 8 s. 8 s is half of the run. The interval between 0 s and 16 s is 16 s.