- 283

- 18

**1. The problem statement, all variables and given/known data**

The man pushes the weightless stick, and so exerts a force with a magnitude of 240 N to the bottom of the lake. Hypothesize that the stick is in the same vertical as the boat's keel. At some point in time, the stick creates a 35 degree angle with the vertical and the water exerts an horizontal force of 47.5 N to the boat, with a direction opossite to the velocity's, which faces towards the front of boat, and has a magnitude of 0.857 m/s. The mass of the boat, the cargo and the man is 370 kg.

a) The water exerts an upthurst force, which faces vertically upwards. Find the magnitude.

b) Consider that these forces stay constant for very short time spans. With that info, find the velocity of the boat 0.450 s later.

**2. Relevant equations**

ΣF = m*a

Vf = Vi + a*t

**3. The attempt at a solution**

The thing is, I'm not sure how the stick, the boat and the bottom of the lake connect. When he moves the stick, he exerts a force directly at the bottom, correct? So, since he's moving forwards (let's say forwards is east), he exerts a 240 N force at the bottom, which faces to the east. Then, since the stick is weightless, according to Newton's 3rd Law, where does the bottom exert the opposite force to?

When the stick is at an angle, then this new force crates a net force, with both horizontal and vertical vectors. At first, I figured I'd find the vertical force, I'd subtract it from the weight, and I'd be done, but that's not the correct result. I'm obviously missing something (I figure the whole "370 kg is the mass of the boat, the man and the cargo" play some role later?), so I'd appreciate the help.