Torque Calculations to Know for AP Physics 1 (2025)

Torque calculations are essential for understanding rotational motion in physics. By exploring the torque formula, its units, and how to calculate net torque, we can analyze how forces affect objects' rotation and achieve equilibrium in various scenarios.

1. Torque formula: τ = r × F (cross product of lever arm and force)

   • Torque (τ) is a vector quantity that represents the rotational effect of a force.
   • The lever arm (r) is the distance from the pivot point to the point where the force is applied.
   • The force (F) must be applied perpendicular to the lever arm for maximum torque.

2. Scalar form of torque equation: τ = r F sin θ

   • This equation calculates torque using the angle (θ) between the lever arm and the force vector.
   • The sine function accounts for the effective component of the force that contributes to rotation.
   • Maximum torque occurs when θ = 90°, where sin(90°) = 1.

3. Units of torque: Newton-meters (N·m) or foot-pounds (ft·lb)

   • Torque is measured in Newton-meters in the SI system, indicating the force applied at a distance.
   • Foot-pounds is a common unit in the imperial system, used primarily in the United States.
   • Both units reflect the product of force and distance, emphasizing the rotational effect.

4. Net torque calculation for multiple forces

   • The net torque is the vector sum of all individual torques acting on an object.
   • Consider both the magnitude and direction of each torque when calculating the net effect.
   • Positive and negative torques can cancel each other out, affecting the overall rotation.

5. Torque direction using right-hand rule

   • The right-hand rule helps determine the direction of the torque vector.
   • Point your fingers in the direction of the lever arm (r) and curl them toward the force (F).
   • Your thumb will point in the direction of the torque vector (τ).

6. Equilibrium condition: Σ τ = 0 (sum of all torques equals zero)

   • An object is in rotational equilibrium when the sum of all torques acting on it is zero.
   • This condition means there is no net rotational acceleration, and the object remains at rest or moves with constant angular velocity.
   • Analyzing torques around a pivot point is essential for solving equilibrium problems.

7. Relationship between torque and angular acceleration: τ = I α

   • Torque (τ) is directly proportional to angular acceleration (α) and the moment of inertia (I) of the object.
   • Moment of inertia represents the distribution of mass relative to the axis of rotation.
   • This relationship is fundamental in understanding how forces cause rotational motion.

8. Calculating torque for a wrench or lever

   • To calculate torque, measure the length of the wrench or lever arm (r) and the force applied (F).
   • Use the angle between the force and the lever arm to determine the effective torque using τ = r F sin θ.
   • Ensure the force is applied at the correct angle for maximum efficiency.

9. Torque in rotational equilibrium problems

   • In problems involving rotational equilibrium, identify all forces and their respective lever arms.
   • Set up the equation Σ τ = 0 to find unknown forces or distances.
   • Consider both clockwise and counterclockwise torques to ensure a balanced system.

10. Center of mass and its effect on torque calculations

    • The center of mass is the point where the mass of an object is concentrated for rotational motion.
    • Torque calculations are simplified when forces are applied at or through the center of mass.
    • Understanding the center of mass helps predict how an object will rotate under applied forces.