Two kids play on a seesaw that has mass 20 kg, length 3 m, and its fulcrum at its mid-point. The seesaw is originally horizontal, when the kids sit at the edge of opposite ends (m,LEFT = 25 kg, m,RIGHT = 30 kg). Calculate the Net Torque from the 3 weights acting on the seesaw, immediately after the kids sit (simultaneously) on their respective places.
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14. Torque & Rotational Dynamics
Torque Due to Weight
Problem 4
Textbook Question
What is the mass of the diver in Fig. 12–52 if she exerts a torque of 1800 m · N on the board, relative to the left (A) support post?
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Identify the key concept: Torque is the rotational equivalent of force and is calculated using the formula: , where is the torque, is the distance from the pivot point, is the force, and is the angle between the force and the lever arm.
Determine the pivot point: The problem specifies that the torque is calculated relative to the left support post (point A). This means the distance is measured from point A to the diver's position on the board.
Relate the force to the diver's weight: The force exerted by the diver is due to her weight, which is given by , where is the diver's mass and is the acceleration due to gravity (approximately ).
Set up the torque equation: Substitute the known values into the torque formula. The torque is given as 1800 N·m, and the force is . The distance is the distance from point A to the diver's position. Assuming the force is perpendicular to the board, is 1. The equation becomes: .
Solve for the diver's mass: Rearrange the equation to isolate : . Substitute the given torque (1800 N·m), the distance (from the diagram, if provided), and the value of to calculate the mass.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Torque
Torque is a measure of the rotational force applied to an object, calculated as the product of the force and the distance from the pivot point (lever arm). It is expressed in Newton-meters (N·m) and determines how effectively a force can cause an object to rotate around an axis. In this context, the diver's torque on the board is crucial for understanding how her weight and position affect the board's rotation about the support post.
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Equilibrium
Equilibrium in physics refers to a state where the sum of forces and the sum of torques acting on an object are both zero, resulting in no net motion. For the diver and the board system, achieving equilibrium means that the torque exerted by the diver must balance the torque due to the weight of the board and the diver's mass. This concept is essential for solving the problem as it allows us to set up equations based on the torques involved.
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Center of Mass
The center of mass is the point in an object or system where its mass is evenly distributed and balanced. For a diver on a board, the position of the center of mass affects how the diver's weight contributes to the torque about the support post. Understanding the center of mass is vital for calculating the diver's mass, as it influences the lever arm distance and the resulting torque exerted on the board.
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Intro to Center of Mass
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