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A child pulls a wagon along a grassy field. Define the system, the pairs of forces at work, and the results.
Solution Video

# OpenStax College Physics for AP® Courses Solution, Chapter 4, Problem 18 (Test Prep for AP® Courses) (2:57)

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This is College Physics Answers with Shaun Dychko. A child is pulling a wagon on a grassy field. We have to identify the Newton's third law pairs of forces involved in this situation. There's going to be a force on the wagon due to the child. The Newton's third law counterpart will be an equal magnitude force but in the opposite direction on the child due to the wagon. Now the child is going to apply a force of magnitude sufficient to keep the wagon going at constant speed because if they pulled too hard, then the wagon will approach the child and accelerate towards the child. They don't want that, they just want to keep it in the same relative place as it is here as they both move along together. This force will be controlled and tuned such that it just balances the force friction, which is the next one we should talk about. There's a force applied on the earth due to the wagon. Then the wagon, in turn, exerts a force, or-- Sorry. The earth and turn exerts a force on the wagon. It says the friction force, in the opposite direction to its velocity, the velocity being this way. This force on the wagon due to the child will be adjusted by the child such that it equals his force on the wagon due to the earth and to go at constant speed anyhow. Now, there's also a force on the earth due to the child. This is the force on the earth due to the child. Then the child experiences of force forwards due to the earth. This is because of friction between the child's shoes and the grass. These are most of the forces, I guess we can talk about the gravity as well. There's a force of gravity downwards on the wagon and then there's a corresponding ormal force upwards exerted by the ground on the wagon. These are not Newton's third law pairs. That's a tricky question. They are equal magnitude because there's no vertical acceleration but the Newton's third law counterpart to gravity is the force on the earth through the wagon. The force of gravity exerted by the wagon on the earth in other words. Anyhow, we'll talk about just the force is on the wagon here, and the force on the child, normal force upwards. Gravity downwards again, but this is on the child in this case. There we go. Those are all the forces we need to be concerned with.