Which quantity represents the turning effect of a force about a given point?

Which quantity represents the turning effect of a force about a given point?

• A. Work
• B. Energy
• C. Torque
• D. Momentum

Answer: (C)

Turning effect about a point is described by torque, the moment of a force. It depends on how far the force is from the point (the lever arm) and on how the force is directed relative to that lever arm. The basic idea is that a force tends to rotate a body about the chosen point, and the strength of that tendency grows with a longer lever arm and with a larger perpendicular component of the force. Mathematically, torque is the cross product τ = r × F, so its magnitude is |τ| = r F sin θ, where r is the distance from the point to where the force is applied and θ is the angle between r and F. When the force is perpendicular to the lever arm, sin θ = 1 and you get the maximum turning effect (τ = rF). If the force is aligned with the lever arm, θ = 0 and there is no turning effect. This concept is distinct from work or energy (which involve displacement along the force) and from linear momentum (which concerns straight-line motion). The turning effect is the rotational analogue of momentum, and torque is what causes or tends to cause angular acceleration about the point.

Turning effect about a point is described by torque, the moment of a force. It depends on how far the force is from the point (the lever arm) and on how the force is directed relative to that lever arm. The basic idea is that a force tends to rotate a body about the chosen point, and the strength of that tendency grows with a longer lever arm and with a larger perpendicular component of the force. Mathematically, torque is the cross product τ = r × F, so its magnitude is |τ| = r F sin θ, where r is the distance from the point to where the force is applied and θ is the angle between r and F. When the force is perpendicular to the lever arm, sin θ = 1 and you get the maximum turning effect (τ = rF). If the force is aligned with the lever arm, θ = 0 and there is no turning effect. This concept is distinct from work or energy (which involve displacement along the force) and from linear momentum (which concerns straight-line motion). The turning effect is the rotational analogue of momentum, and torque is what causes or tends to cause angular acceleration about the point.