Speed

1.1. Knowledge to remember

a) Problem 1: A car travels a distance of 170 km in 4 hours. On average, how many kilometers does the car travel per hour?

Solution:

The average hour that the car can travel is:

170 : 4 = 42.5 (km)

Answer: 42.5km.

Comment: On average, cars travel 42.5km per hour. We say the average speed or in short the speed of the car is forty-two point five kilometers per hour, abbreviated as 42.5 km/h.

So the speed of the car is:

170 : 4 = 42.5 (km/h)

To calculate speed, divide the distance by the time.

v=s:t

$v=S:t$b) Problem 2: A person runs 60m in 10 seconds. Calculate the person’s running speed.

Solution:

His running speed is:

60 : 10 = 6 (m/sec).

Answer: 6m/sec.

### 1.2. Solve textbook exercises page 139

**Lesson 1 Textbook page 139**

A motorcyclist travels 105 km in 3 hours. Calculate the speed of the motorcyclist.

*Solution guide:*

The speed of the motorcyclist is:

105 : 3 = 35 (km/h)

Answer: 35km/h.

**Lesson 2 Textbook page 139**

An airplane flies 1800km in 2.5 hours. Calculate the speed of the plane.

*Solution guide:*

The speed of the plane is:

1800 : 2.5 = 720 (km/h)

Answer: 720km/h.

**Lesson 3 Textbook page 139**

A person runs 400m in 1 minute 20 seconds. Calculate the person’s running speed in m/s.

*Solution guide:*

Change: 1 minute 20 seconds = 80 seconds.

His running speed is:

400 : 80 = 5 (m/sec)

Answer: 5m/sec.

### 1.3. Solving exercises in the Practice Textbook Pages 139, 140

**Lesson 1 Textbook page 139**

An ostrich can run 5250m in 5 minutes when needed. Calculate the running speed of the ostrich.

__Solution guide:__

The running speed of the ostrich is:

5250 : 5 = 1050 (m/min)

Answer: 1050m/min.

Lesson 2 Textbook page 140

Write in the blank (according to the form):

S | 130km | |||

t | 4 hours | |||

v | 32.5km/h |

__Solution guide:__

S | 130km | 147km | 210km | 1014km |

t | 4 hours | 3 o’clock | 6 seconds | 13 minutes |

v | 32.5km/h | 49 km/h | 35 m/sec | 78 m/min |

Lesson 3 Textbook page 140

Distance AB is 25km long. On the way from A to B, a person walks 5km and continues driving for half an hour to arrive at B. Calculate the speed of the car.

__Solution guide:__

We have: half an hour = \(\frac{1}{2}\) hours = 0.5 hours.

The distance traveled by the car is:

25 – 5 = 20 (km)

The speed of the car is:

20 : 0.5 = 40 (km/h)

(or 20 : \(\frac{1}{2}\ = 40 (km/hr)

Answer: 40km/h.

Lesson 4 Textbook page 140

A canoe travels from 6:30 to 7:45 a.m. for a distance of 30km. Calculate the speed of the canoe.

__Solution guide:__

The travel time of the canoe is:

7 hours 45 minutes – 6 hours 30 minutes = 1 hour 15 minutes

1 hour 15 minutes = 1.25 hours

The speed of the canoe is:

30 : 1.25 = 24 (km/h)

Answer: 24km/h.

.

=====

## Leave a Reply