Length for Class 4

In this chapter, we explore the wonderful world of length! You have fun comparing lengths and using length in art. Have fun with your new knowledge and keep on measuring!

Length

Length is all about measuring how long or short things are. It helps us figure out if something is big or small. Everything we see around us like cars, buses, books, pencils and more, has a length.

To measure length, we use different tools like measuring tape, rulers, our hands (using our hand span) and even sticks. These tools help us compare the length of things with the tool itself so we can say how long something is.

Units of Length

When we measure length, we use units. Units are like special words that tell us how long something is. For example, Adam's pencil is 4 centimetres long. Here, centimetre is the unit to measure the length.

In this way, measuring length helps us understand and describe how big or small things are.

Metric Units of Length

Imagine you are going on a journey to explore the world of length, where we measure how long or short things are. To do this, we use special units called metric units. These units make it easy for people all over the world to talk about length in the same way.

Some important metric units of length are as follows:

1. Millimetre (mm)
2. Centimetre (cm)
3. Decimetre (dm)
4. Metre (m)
5. Decametre (dam)
6. Hectometre (hm)
7. Kilometre (km)

Conversions of units of Length

When you want to change from a smaller unit to a bigger unit, you multiply the number associated with the smaller unit of length by 10.

When you want to change from a bigger unit to a smaller unit, you divide the number associated with the bigger unit of length by 10.

The conversion of one unit to another is shown as:

Here are some common units of length and how to convert between them:

Kilometres (km) to Metres (m)

→ 1 km = 1000 m
→ To convert from km to m, multiply by 1000.
→ Example: 5 km = 5 × 1000 = 5000 m

Metres (m) to Millimetres (mm)

→ 1 m = 1000 mm
→ To convert from m to mm, multiply by 1000.
→ Example: 5 m = 5 × 1000 = 5000 mm

Centimetres (cm) to Millimetres (mm)

→ 1 cm = 10 mm
→ To convert from cm to mm, multiply by 10.
→ Example: 8 cm = 8 × 10 = 80 mm

Metres (m) to Centimetres (cm)

→ 1 m = 100 cm
→ To convert from m to cm, multiply by 100.
→ Example: 50 m = 50 × 100 = 5000 cm

Millimetres (mm) to Centimetres (cm)

→ 1 mm = 1/10 cm 
→ To convert from mm to cm, divide by 10.
→ Example: 500 mm = 500/10 = 50 cm

Centimetres (cm) to Metres (m)

→ 1 cm = 1/100 m
→ To convert from cm to m, divide by 100.
→ Example: 500 cm = 500/100 = 5 m

Metres (m) to Kilometres (km)

→ 1 m = 1/1000 km
→ To convert from m to km, divide by 1000.
→ Example: 5000 m = 5000/1000 = 5 km

Feet (ft) to Inches (in)

→ 1 foot (ft) = 12 inches (in)

→ To convert from ft to in, multiply by 12.
→ Example: 5 ft = 5 × 12 = 60 in

Note: Foot is a singular form and feet is a plural form.

Inches (in) to Feet (ft)

→ 1 inch (in) = 1/12 foot (ft)
→ To convert from ft to in, divide by 12.
→ Example: 600 in = 600/12 = 50 ft

Let us take some examples and understand them better.

Example 1: In a sports event at St. Mary's School, the heights of all kids are measured to participate in high jumping. What is the height of a kid marked by his teacher in millimetres, as shown?

a) 105 mm
b) 1050 mm
c) 110 mm
d) 1100 mm

Answer: b) 1050 mm

Explanation: Height of a kid marked by his teacher = 105 cm
Height of a kid marked by his teacher in mm = 105 × 10 
                                                                        = 1050 mm

Example 2: A delivery van covers a distance of 1680 km in a day. How much distance in metres does it cover in 7 hours?

a) 420000 m
b) 460000 m
c) 490000 m
d) 520000 m

Answer: c) 490000 m

Explanation: Distance covered by a van in a day (24 hours) = 1680 km
Distance covered by a van in 1 hour = 1680 ÷ 24 
                                                           = 70 km

Distance covered by a van in 7 hours = 70 × 7
                                                            = 490 km

Distance in metres covered by a van in 7 hours = 490 × 1000 m
                                                                            = 490000 m