Grade 2 → Measurement and Data → Time ↓
Calculating elapsed time in hours
Time is an integral part of our daily lives, and it is important to understand how to measure and interpret it. One of the fundamental aspects of dealing with time is calculating elapsed time. In this lesson, we will explore how to count elapsed time in hours, focusing on understandable methods for grade 2 math.
Understanding time
Before we start calculating the elapsed time, let's ensure a basic understanding of time. In our daily lives, time is measured in units such as seconds, minutes, and hours. A 24-hour day is divided into smaller segments:
- 1 hour = 60 minutes
- 1 minute = 60 seconds
For Grade 2, we will focus on hours and minutes, which are most commonly used in their everyday experiences. Let's start with an example.
What is the elapsed time?
Elapsed time is the time that passes from the start of an event to its end. For example, if you start watching a movie at 4:00 PM and it ends at 6:00 PM, the elapsed time is the period between 4:00 PM and 6:00 PM.
See the time on the clock
The easiest way to measure elapsed time is to use a clock. Let's look at an example:
Consider the clock above. Suppose the short needle (hour hand) points to 3 and the long needle (minute hand) points to 12. This indicates that the time is 3 PM. If your program starts at 3:00, we can use this visualization to understand when it ends by moving the hour hand.
Simple elapsed time example
Let us calculate the elapsed time if a task starts at 2 PM and ends at 5 PM.
From a start time of 2 PM, the clock's hour hand will point to 2. At an end time of 5 PM, it will point to 5. To find the elapsed time, count the number of steps per hour taken by the hour hand to go from 2 to 5. Here, it is:
5 - 2 = 3 hours
This means that 3 hours have passed.
Time in half hours and hours
Let's consider a situation where we deal with half an hour. What if an event starts at 3:30 PM and ends at 5:00 PM?
The starting time, 3:30 PM, means that the minute hand is at 6 on the clock face, indicating the 30th minute of the hour. The ending time is straightforward. The steps involve counting the hours from 3:30 PM to 5:00 PM. Since the time from 3:30 PM to 4:00 PM is half an hour and the time from 4:00 PM to 5:00 PM is one hour, the elapsed time is:
0.5 + 1 = 1.5 hours
Understanding AM and PM
It's important to note that time is usually divided into two parts: AM and PM. AM stands for ante meridiem, which means before noon, and PM stands for post meridiem, which means after noon. Be sure to differentiate between these when calculating elapsed time, as this can significantly change the result.
A comprehensive example
Let's look at a scenario with time intervals of morning and afternoon. Imagine you start a picnic at 10:00 am and end it at 3:00 pm. To find the total time spent on the picnic:
Calculate the hours from 10 AM to 3 PM:
3 - 10 = 5 hours
The elapsed time is 5 hours.
Practice exercises
Exercise 1
If your school starts at 8 a.m. and ends at 2 p.m., how many hours are you in school?
6 hours
Exercise 2
A football match starts at 4:00 pm and ends at 7:00 pm. Calculate the duration of the football game.
3 hours
Exercise 3
Your family travels from 9:30 a.m. to 1:00 p.m. How long was the trip?
3.5 hours
Conclusion
Counting elapsed time in hours is a basic skill that helps us manage our daily schedule. It helps children understand the concept of time and its measurement, which they can apply in real-world situations. By practicing various examples and using visual representations such as clocks, learners can understand these concepts effectively, laying the groundwork for more advanced time-related calculations in the future.