Question

Math question please help

Answer 1

All you really have to know for this one is that 1 watt means 1 joule/second.  Now let's look at it:

The blender takes 400 watts of electrical power from the wall outlet.  That's 400 joules of electrical energy/second.

It's only 85% efficient though, so it can only provide (0.85 x 400) = 340 J/sec to whip the smoothie.

In 30 seconds:

-- It takes (30 s x 400 J/s) = 12,000 joules of electrical energy from the outlet

and

-- It whips (30 s x 340 J/s) = 10,200 Joules of energy into the smoothie.

The other 2,000 Joules of energy that comes out of the wall but doesn't go into the smoothie is what heats up the blender motor.  

Answer 2

$400 watts * \frac{1 J/sec}{1 watt} * 30 sec * \frac{85}{100}efficiency = 10,200 J$

Answer: D