91Ó°ÊÓ

• The net force acting on the system = 1900 N.
• The sum of the boat's mass and the mass of the trailer = 700 kg.
• m_c is the mass of the car = 1100 kg.
• a is the acceleration =1.20 m/s².

Apply Newton’s second law of motion as:

$F_{net}=Ma$

$F−f=\left(m_b+m_t+m_c\right)a$ …………. (i)

Here, F_net is the net force acting on the system, m_b is the mass of the boat, m_tis the mass of the trailer, m_c is the mass of the car, a is the acceleration, F is the force exerted by the car, and f is the resistance force.

Substitute 700 kg for (m_b+m_t), 1100 kg for m_c, 1900 N for F, and 1.20 m/s² for a in equation (i), and we get,

$\begin{array}{c}1900\text{ N}−f=\left(700+1100\right)\text{ k²µ}×{\text{1.20​â¶Ä„m/²õ}}^2\\ 1900\text{ N}−f=1800\text{ k²µ}×{\text{1.20 m/²õ}}^2\\ f=\left(1900−2160\right)\text{ N}\\ f=−260\text{ N}\end{array}$

Hence, the total resistance force is -260 N.

The result is unreasonable because the force of friction can never be in the direction of motion. It's always in the opposite direction.

The unreasonable premise is that an acceleration of 1.20 m/s² can be produced.