## Physics 8 Lesson 14: Law of work

## 1. Theoretical summary

### 1.1. Experiment

– Tools: Straight ruler, dynamometer, weight, pulley, test stand.

– Conduct:

- Hook the weight on the dynamometer and pull it slowly so that the lifting force F
_{first}= P_{qn}, Read the value of F1, length S_{first} - Using the movable pulley pull the object up the same length S
_{first}, so that the reading of the dynamometer remains constant. Read dynamometer readings and measure the distance traveled_{2} - Complete 14.1 . panel

**C1:** F_{first} > F_{2}

**C2:** S_{first} < WILL_{2}

**C3: **A_{first} = A_{2}

**C4:** Using a movable pulley gains twice the force and loses twice in terms of travel. That means there is no benefit to the public

### 1.2. Laws of Public

There is no simple machine that gives us any advantage in work. How many times you gain in terms of force, how many times do you lose on the way and vice versa.

### 1.3. Common types of simple mechanical machines

- Fixed Pulley: Only change the direction of the force, not change the magnitude of the force.

- Dynamic Pulley: When using a movable pulley, we gain twice in force and lose twice in the path.

- Inclined plane: Advantage in force, loss in path.

- Leverage: Gain in force, lose on path or vice versa.

### 1.4. Performance of simple machines

In fact, in simple machines there is always friction. Therefore the work done must be used to overcome friction and lift the object. This work is called total work, the work of lifting objects is useful work. Work to overcome friction is wasted work.

– Total work = Useful work + Wasted work

– Ratio of useful work (A_{first}) and total work (A_{2}) is called the performance of the machine:

- Recipe: \(H = \frac{{{A_1}}}{{{A_2}}}.100\% \)

## 2. Illustrated exercise

### 2.1. Form 1: Find the pulling force and the height to bring the object up

In order to lift an object of weight P = 420N vertically up with a moving pulley, according to Figure 13.3, the worker must pull the end of the rope a distance of 8m. Ignore friction. Calculate the pull force and the height to lift the object.

**Solution guide**

Pulling an object up by a movable pulley, the pulling force is only half the weight of the object:

F = 1/2.P = 420/2 = 210 N.

Using a movable pulley has a double advantage in force, so it has to lose twice in terms of the path (according to the law of work), that is, to raise the object to a height h, it must pull the end of the rope a distance l = 2h.

l = 2.h = 8 m ⇒ h = 8/2 = 4 m

### 2.2. Form 2: Determine the work of the pulling force

Evenly pull two cargo boxes, each weighing 5000N onto the floor of the car 1 m above the ground with an inclined plank (negligible friction). Calculate the work force of pulling the crate along the inclined plane on the floor of the car.

**Solution guide**

The work of pulling the cargo box along the inclined plane on the car is also the same as the work of pulling the cargo box directly vertically on the car: A = P. h = 500.1 = 500 J.

## 3. Practice

### 3.1. Essay exercises

**Question 1:** Using a movable pulley and a fixed pulley to lift an object 20 m high, one must pull the end of the rope with a force F = 450 N. Calculate:

a) Work must be done to lift the object.

b) The mass of the object. Know the magnitude of the 30 N resistance.

**Verse 2:** Pull a 100 kg object 25 m high with a hoist consisting of 2 movable pulleys and 2 fixed pulleys. The efficiency of the hoist is 80%. Calculate:

a) Work required to lift the object.

b) The pull force on the end of the rope.

**Question 3:** An inclined plane is used to pull an object of mass 50 kg to a height of 2 m. If there is no friction then the pulling force is 125 N. The friction and the dynamometer are actually 175 N. What is the efficiency of the upper inclined plane?

**Question 4:** To lift an object of weight P = 500 N up with a moving pulley, the rope must be pulled a distance of 8 m. What is the pulling force, the height to lift the object, and the work to lift the object?

### 3.2. Multiple choice exercises

**Question 1:** Evenly pull two cargo boxes, each weighing 500 N, onto the floor of the car above the ground with an inclined plank (negligible friction). Pull the first box using a 4m plank, and drag the second box using a 2m plank. Which of the following comparisons is true regarding the work done in the two cases?

A. In the first case, the work of the pulling force is less and less than twice.

B. In both cases the work done by the pulling force is equal.

C. In the first case, the work force of the pulling force is greater and greater than 4 times.

D. In the second case, the work of the pulling force is 4 times smaller and smaller.

**Verse 2:** A cyclist rides steadily from the bottom of the slope to the top of a 5 m steep slope. The slope is 40 m long, the friction force preventing the car from moving on the road is 20 N and both people and the car have a mass of 37.5 kg. What is the total public that the person is born with?

A. 3800 J B. 4200 J C. 4000 J D. 2675 J

**Question 3: **A weight is raised to a height h in two ways:

Method 1: Pull the object directly up in the vertical direction.

Method 2: Drag the object along an inclined plane whose length is twice the height h. Neglect friction in inclined planes. Compare work done in two ways. Which statements below is correct?

A. The work done in the 2 way is greater because the path is twice as large.

B. The work done 2 way is smaller because the drag force on the inclined plane is smaller.

C. The work done in the 1st way is greater because of the larger pulling force.

D. The work done in both ways is the same.

**Question 4:** How many types of simple machines are there?

A. 1 B. 2 C. 3 D. 4

## 4. Conclusion

Through this lesson, students will be familiar with the knowledge related to the Law of Work along with related exercises at many levels from easy to difficult…, they need to understand:

- Understand the law of public.
- Solve problems related to work.

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