4. After adding another tug to the one pushing the barge, they started pushing the barge with double the force. How will the power spent on movement change if the water resistance is proportional to the first power of the barge's speed?
Solution. Since the barge is moving at a constant speed, the traction force of the tugboats is balanced by the resistance force. When the traction force is doubled, the resistance force also increases by the same factor, meaning the speed of the barge has increased by 2 times. Power is the product of force and speed, so the power has increased by 4 times.

Answer: increased by 4 times.

Criteria: 20 points - complete and correct solution, 10 points - the speed of the barge when towed by two tugboats is determined and justified, 0 points - all other cases.