Final answer:
When a machine with 70% efficiency undergoes 20,000 J of work, the work done by the machine is 14,000 joules, calculated by multiplying the total energy input by the machine's efficiency.
Step-by-step explanation:
When a machine with 70% efficiency undergoes 20,000 J of work, the actual work done by the machine is found by multiplying the total energy used by the machine's efficiency. The efficiency of a machine is defined as the ratio of useful work output to the total energy input, and is expressed as a percentage or a decimal. In this case, the efficiency is given as 70%, which is 0.70 in decimal form.
Thus, the work done by the machine is:
Work done = Efficiency × Total energy input
Work done = 0.70 × 20,000 J
Work done = 14,000 J
Therefore, the machine performs 14,000 joules of work with the energy provided.