# Friction and Efficiency

Hi, welcome to this video tutorial about Power, Friction, and Inefficiency.

We know that whenever we work, forces act against us. For example, when I push a box across the floor, there is thing called friction that makes it more difficult than simply pushing the weight of the box, because I’m pushing the weight of the box AND the resistance caused by friction on the floor. This means I’m not as efficient as I could be. The same is true for engines and other things, that’s why we add oil and lubricants to make the friction decrease and our efficiency increase.

Sometimes questions about friction and efficiency show up on your mechanical comprehension exam, so let’s try a couple practice problems to get ready.

Imagine you’re using a pulley system to lift a box that weighs 100 lbs, but the pulleys are only 50% efficient. How many lbs of force are you actually putting into moving the box?

Here is how to solve one of these:

     Just divide Power Output by Efficiency = PO/E
Here our PO is 100 and E is .50 thus PO/E = 100/.50
Then we have our initial force, which is 200lbs


Make sense?

Try one by yourself:

An engine produces 90hp, and is 90% efficient. So, how much horsepower is being lost because of inefficiency?

Remember, the output is placed over the efficiency. 90hp/.90 = 100hp is what the engine would be on total efficiency. But the question is how much is being lost due to inefficiency of the engine. So, we subtract the 90hp output from the 100hp which would be 100% efficiency, and end up with 10hp… 10hp is being lost due to inefficiency.

I hope that helps, thanks for watching this video tutorial.

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by Mometrix Test Preparation | Last Updated: June 15, 2020