Question

A fish is suspended by a line from a fishing rod. Choose two forces acting on the fish and describe the interaction partner of each. (IMAGE CAN'T COPY)

Short answer

The forces on the fish are gravity (interaction partner: Earth) and tension (interaction partner: fishing rod).

Step-by-step solution

Identifying the Forces

When a fish is suspended by a line, the two primary forces acting on it are the force due to gravity and the tension in the fishing line. Gravity acts downward, while tension acts upward to support the fish.

Describing Gravity Force and its Interaction Partner

The force of gravity acts downward on the fish, pulling it toward the Earth. The interaction partner for this gravitational force is the Earth itself. According to Newton's third law, the fish also exerts an equal and opposite force on the Earth. This is a classic example of gravitational interaction.

Describing Tension Force and its Interaction Partner

The tension force in the line acts upward, counteracting gravity to keep the fish suspended. The interaction partner for the tension force is the fishing rod. The line pulls upward on the fish due to tension, and simultaneously, according to Newton's third law, the fish pulls downward with an equal force on the line.

Key concepts

Understanding Gravity Force

When a fish is hanging suspended in air, the force called "gravity" is one of the main forces acting on it. Gravity is what keeps us all grounded, either on Earth or in this case, pulling the fish downwards. It is a force that attracts two bodies toward each other. The equation used to describe this force of attraction comes from Newton's law of universal gravitation and is given by:\[ F = G \frac{m_1 m_2}{r^2} \]where \( F \) is the gravitational force, \( G \) is the gravitational constant, \( m_1 \) and \( m_2 \) are the masses of the two interacting objects (the fish and the Earth), and \( r \) is the distance between the centers of their masses.
For the fish, this gravitational force is always pointed downward towards the Earth. The Earth, being a massive object, pulls the fish down with a certain force. According to Newton's Third Law, the fish also exerts an equal and opposite force on Earth, although we don't see Earth moving due to its vast mass.
These forces are in perfect balance, demonstrating how gravity plays a vital role in keeping even the heaviest of objects from floating away into space. This is a primary example of Newton's Third Law with gravitational interactions.

What is Tension Force?

While gravity pulls the fish downward, another force called "tension" acts upward when the fish is held by a fishing line. Tension is the force exerted by a string, rope, cable, or in this case, the line, when it is subjected to a force that tries to stretch it. It arises from the fishing line trying to keep the fish from falling due to gravity.
When a fishing rod is used, it creates tension in the line. This tension works to balance the downward pull of gravity. The line's tension force points upward, countering gravity and holding the fish safely in place. The situation here involves:

• Gravity pulling the fish down
• Tension in the line pulling the fish up

This balancing act forms part of a dynamic system where forces compete to hold the fish in equilibrium. Tension keeps the fish suspended, demonstrating Newton's Third Law, where for every action, there is an equal and opposite reaction. Thus, while the line pulls the fish upwards, the fish pulls back with an equal downward force on the line.

Exploring Interaction Partners

In physics, "interaction partners" refer to the pair of forces that influence each other, bound by Newton's Third Law of Motion. Objects exerting force upon each other are consistently paired, as seen when the fish interacts with both gravity and tension.
Looking at the fish suspended in mid-air:

• The Earth and the fish are gravity's interaction partners—as gravity pulls the fish down, the fish exerts an equal force upwards on the Earth.
• The fishing line and rod play their part in tension forces—as the line pulls the fish upwards, the fish responds with a downward force on the line, which transmits this to the rod.

These opposing forces create balance in the system—a push and pull that allows the fish to remain suspended. The key lies in understanding that all forces come in pairs, and interaction partners illustrate these force pairs in action—providing a harmonious balance between all participating forces.