## Physics 8 Lesson 15: Capacity

## 1. Theoretical summary

### 1.1. Wattage

- To find out which person or machine works harder (does work faster), one compares the work done per unit of time.
- The work done in a unit of time is called power.

### 1.2. Formula for calculating power

– Formula: \(P=\frac{A}{t}\)

In there:

- P: Power
- A: mechanical work
- t: Time to perform work

### 1.3. Power unit

– The unit of power is Jun/second (J/s) called watt, symbol is W.

- 1W = 1 J/s
- 1KW = 1000 W
- 1MW = 1000 KW

**Note:**

– We cannot use only the size of the work or only the time it takes to compare the work done quickly or slowly. But to know which machine works stronger or performs work faster, we have to compare the work done in a unit of time called power.

– Power unit in addition to watt (W) also has horsepower (horsepower)

- French horsepower (symbol CV): 1 CV 736 W
- British Horsepower (symbol HP): 1 HP 746 W

## 2. Illustrated exercise

**Question 1:** A horse pulls a cart at a constant speed of 9 km/hr. Horse pulling force is 200N. Calculate horse power.

**Solution guide**

In 1 hour (3600s) the horse pulling the cart travels a distance s = 9 km = 9000m.

The work done by the horse’s drag on the track s is:

A = F. s = 200 . 9 000 = 1 800 000 J

Horse power:

P = A/t=1800000/3600= 500W.

**Verse 2:** To plow a pole of land, people use buffalo plow, it takes 2 hours, but if using Bong Sen plow, it only takes 20 minutes. Ask buffalo or plow with a larger capacity and how many times larger?

**Solution guide**

Plowing the same pole means that the work done by the buffalo and the plow is the same

- Plowing buffalo takes time t
_{first}= 2 hours = 120 minutes. - The plow takes time t
_{2}= 20 minutes. - Capacity when using buffalo is: \({P_1} = \frac{{{A_1}}}{{{t_1}}}\)
- The capacity when using the machine is: \({P_2} = \frac{{{A_2}}}{{{t_2}}}\)

Set up the ratio P_{first}/P_{2}, we get:

\(\frac{{{P_1}}}{{{P_2}}} = \frac{{{A_1}}}{{{A_2}}}.\frac{{{t_2}}}{{t_1} }} = \frac{{{t_2}}}{{{t_1}}} = \frac{{20}}{{120}} = \frac{1}{6}\)

⇒ P_{2 }= 6P_{first}

So, using a plow has a capacity of 6 times greater than using a buffalo.

## 3. Practice

### 3.1. Essay exercises

**Question 1:** What is the power of a walker if in 1 hour 30 minutes he walks 750 steps, each step requires 45 J work?

**Verse 2:** The horse pulling the cart moves at a constant speed of 9 km/h. The pulling force is 200 N. How much power can the horse get?

**Question 3:** To plow a pole of land, if using buffalo plow, it takes 2 hours, if using a plow, it takes 20 minutes. Ask buffalo or plow with a larger capacity and how many times larger?

**Question 4:** Two friends Nam and Hung pull water from the well. Nam pulls the bucket of water twice as heavy, and Hung’s time to pull the bucket up is only half of Nam’s time. Compare the average capacity of Nam and Hung.

### 3.2. Multiple choice exercises

**Question 1:** The unit of power is

A. Watt (W)

B. Kilowatts (kW)

C. Jun per second (J/s)

D. All three units above

**Verse 2:** Which of the following is true about capacity?

A. Power is defined as the work done in a unit of time.

B. Power is determined by the force applied in 1 second.

C. Power is determined by the formula P = At

D. Power is determined by the work done when the object is moved 1 meter.

**Question 3: **The expression for power is:

A. P = At B. P = A/t

C. P = t/A D. P = A^{t}

**Question 4:** Capacity is:

A. Work done in one second.

B. Work done in one day.

C. Work done in one hour.

D. Work done in a unit of time.

## 4. Conclusion

Through this lesson, students will be acquainted with the knowledge related to Power along with related exercises at many levels from easy to difficult, etc. They need to understand:

- Understand the concept of power.
- Know the formula, the unit of power.
- Understand and solve power problems.

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