Lucy needs to lift a crate to a shelf 20 m high. She does 12,000 J of work on a machine to help her. The machine exerts a force of 500 N to lift the crate.

83.34%.

Explanation:

Given that,

Lucy does 12,000 J of work on a machine to help her

The machine exerts a force of 500 N to lift the crate.

Height of the shelf = 20 m

We need to find the efficiency of the machine. The ratio of output work to the input work is called the efficiency of any machine. It is given by :

So, the efficiency of the machine is 83.34%.

b) phosphorus pentoxide

so easy. just google the electromagnetic waves and it’ll pop up.