Think It Over — the box at the s - How Forces Affect Moti

The chapter opens with a picture of canoes and two questions. We can answer both of them now.

Why does a canoe move forward when the canoeist pushes water backwards with their paddle, and why does it move faster when they push harder?

The paddle pushes the water backwards.
By Newton's third law, the water pushes the paddle forwards with an equal force.
The two forces act on two different objects, the water and the paddle, so they do not cancel each other.
The force on the paddle carries the paddle and the canoe forward.
When the canoeist pushes harder, the forward force on the paddle is larger.
A larger force gives a larger acceleration, so the canoe picks up a greater velocity.

GIF Watch: the paddle pushes the water backwards

The same paddle force is used in two canoes, one empty and one carrying another passenger. In which case will the canoe move faster?

The empty canoe will move faster.
The force applied is the same in both cases, but the empty canoe has a smaller mass.
By Newton's second law, a = F / m, so a smaller mass gets a larger acceleration.
A larger acceleration means the empty canoe picks up speed faster.
Suggested answer. The book does not print an answer for this question.

GIF Watch: the empty canoe will move faster

Class 6