When the total product starts falling, then the marginal product is

Analyzing the question

The question asks what happens to marginal product (MP) at the point where total product (TP) starts to fall as more units of a variable factor are added.

The concept

Marginal product is the extra output gained from adding one more unit of a variable input:

MP = change in TP ÷ change in input = ΔTP / ΔL

So MP tells you the slope of the total product curve:

• When TP is rising, MP is positive.
• When TP is at its maximum (the peak), MP = 0.
• When TP is falling, MP is negative.

Step-by-step solution

1. When TP is rising, each extra worker adds to total output, so MP > 0.
2. At the peak of TP, an extra worker adds nothing, so MP = 0.
3. Once TP starts to fall, an extra worker reduces total output, so ΔTP < 0.
4. Since MP = ΔTP / ΔL and ΔTP is negative, MP is negative.

A quick example

| Workers | TP | MP | |---|---|---| | 1 | 10 | 10 | | 2 | 24 | 14 | | 3 | 36 | 12 | | 4 | 40 | 4 | | 5 | 40 | 0 | | 6 | 36 | -4 |

At 5 workers TP is at its maximum and MP = 0. At 6 workers TP has dropped from 40 to 36, and MP = -4, which is negative.

Checking the options

• A. Increasing: wrong. MP has been falling because of diminishing returns.
• B. Maximum: wrong. MP is at its maximum much earlier, where TP rises fastest.
• C. Zero: this is when TP is at its peak and not yet falling. It is not the answer for when TP is falling.
• D. Negative: correct. A falling TP means each additional unit of input subtracts from output.

Exam tip

Remember the link: TP rising → MP positive; TP maximum → MP zero; TP falling → MP negative. Also, MP cuts through the AP curve at AP's maximum point. JAMB often tests these relationships.