"Power" Mechanical and Power PPT Courseware

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"Power" Mechanical and Power PPT Courseware

Observe and think

Which one does the work faster, man or crane? How did you compare?

Doing the same amount of work takes different amounts of time.

In the same time, the amount of work done is different.

How to compare work done quickly and slowly

Method 1: When W A = W B, the shorter the time, the faster the work will be done;

Method 2: When tA = tB, the one who does more work will do it faster.

If time and work are not the same, how to compare the speed of work?

1. Power

1. The ratio of work to time is called power P

Numerically equal to the work done per unit time.

2. expression

Power=work/time P=W/t

3. International unit: watt (watt, W)

Commonly used units:

1 kW = 103 W

1 MW = 106 W

2. Understand the size of power

The long-term power of a bicycle is about 70W; the short-term power of elite athletes can reach 1kW.

The new BMW X5 has a maximum output of 235kW

The Yangtze Three Gorges Water Conservancy Project is the largest water conservancy project in the world today and is listed as one of the global super projects. The total installed capacity is - 18.2 million kilowatts

Example question analysis

An electric motor takes 2 minutes to transport a high-altitude cable car to the top of the mountain. It takes about 20 minutes for a 1.2 x 106 kg person to climb from the bottom of the mountain to the top. What is the approximate power of this person to overcome gravity? (The height difference from the foot of the mountain to the top is less than 200 m.)

Solution: The work done by the motor W1=1.2 x 106J, and the time taken

t1=2 x 60s=120 s.

Motor power P1=W1/t1=1.2 x 106/120s=1 x 104W

The work done by a person overcoming gravity during climbing is W2 = mgh, and the time taken

t2=20 x 60s=1200s.

Human power P2=W2/t2=60kg x 9.8N/kg x 200 m/1200s=98W

Answer: The power of the motor is 1 x 104W, and the power used by people climbing mountains is about 98W.

Consolidation exercises

Example 1 When using machinery to do work, the following statement is correct ( ).

A． A machine with high power must do more work than a machine with low power

B． A machine with high power must take less time to do work than a machine with low power.

C. A machine with low power will always work slower than a machine with high power.

D. None of the above statements are correct

Example 2 The mass of the boulder is 6 t. The crane's hook lifts the boulder 1 m at a constant speed in 15 s. What is the power of the crane in lifting the boulder?

Solution: Because the pulling force of the crane on the big stone at a constant speed is equal to the gravity on the big stone, that is

F = G = mg = 6×103 kg×10 N/kg = 6×103 N

The distance the stone moves in the direction of tension s =1m

The work done by the pulling force is: W = Fs = 6×103 N ×1 m = 6×104 J

The power of the crane to lift the cement slab is

P=W/t=6 x 104J/15s=4 x 10³w

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For more information about the PPT courseware "Power Machinery and Work", please click the Power ppt Machinery and Work ppt tab.

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Update Time: 2024-11-25

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