Question

A physicist is investigating the effect that different conditions have on the force of friction. The material used is an ordinary brick, with a mass of 1.8 kg. It is pulled across the surface of a wooden table. Friction is measured by pulling the brick with a string attached to a spring scale, calibrated in newtons (N). When the brick is pulled at constant speed, the reading on the scale is equal to the force of friction between the brick and the table top.
Experiment 1 : The brick is placed on the table in three different positions. First, it is allowed to rest on its broad face (area $=180c{m}^2$), then on its side (area $=130c{m}^2$ ), and finally on its end (area $=56c{m}^2$ )
Table 1
$\begin{array}{cccc}Area\left(c{m}^2\right)& 180& 130& 56\\ Friction\left(N\right)& 7.1& 7.3& 7.2\end{array}$

Experiment 2: A wooden block of mass 0.6 kg is made to the same dimensions as the brick, and the experiment is repeated.
Table 2
$\begin{array}{cccc}Area\left(c{m}^2\right)& 180& 130& 56\\ Friction\left(N\right)& 1.2& 1.1& 1.2\end{array}$

Experiment 3: The time, the wooden block is loaded by adding 1.2 kg of extra mass on top of it, to give it the same weight as the brick.
Table 3
$\begin{array}{cccc}Area\left(c{m}^2\right)& 180& 130& 56\\ Friction\left(N\right)& 3.5& 3.6& 3.7\end{array}$
Which combination of experiments shows that the amount of friction depends on the weight of the object?

• A. Experiment 1 and Experiment 2
• B. Experiment 1 and Experiment 3
• C. Experiment 2 and Experiment 3
• D. Experiment 1 Experiment 2 and Experiment 3

Answer 1

The correct option is C

Experiment 2 and Experiment 3

In these two experiments, both the surface area of contact and the kind of materials in contact are the same, and the only difference is in the weight. The other three choices all include Experiment 1, in which a different kind of material is in contact with the surface and this might be the reason for the difference in results.