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Explanation

The normal stress in the bar is calculated as the applied force divided by the cross-sectional area of the bar.

$$\sigma = \frac{F}{A}$$

The strain in the longitudinal direction can then be calculated using Hooke's law:

$$\varepsilon_{long} = \frac{\sigma}{E} = \frac{F/A}{E}$$

Due to the Poisson effect, the strain in the longitudinal direction will cause the bar to get thinner in the lateral direction. Poissons ratio is defined as:

$$\nu = \frac{- \varepsilon_{lat}}{\varepsilon_{long}}$$

We can then calculate the lateral strain:

$$\varepsilon_{lat} = - \nu \varepsilon_{long} = - \nu \frac{F/A}{E} = - 0.27 \% $$