How Do You Calculate Normal Force?

To calculate the normal force on an object, draw a free body diagram, determine the surface’s angle, factor in the other present forces, and solve for the normal force. Note that the normal force is perpendicular to the surface the object sits on.

  1. Draw a free body diagram

    Draw a free body diagram of the object in question, along with all the forces acting on it. For example, if a box is sitting on the ground, the only two forces present are the force of gravity and the normal force. These forces act in opposite directions, with gravity acting downward and the normal force acting upward. If a person were pushing the box, this would be an additional force. These forces are typically shown as arrows to indicate the direction of the force. Recall that normal force is always perpendicular to the surface on which an object sits.

  2. Determine the angle of the surface

    Determine the angle of the surface the object is in contact with. If the surface is flat, there is no angle. If the surface is slanted, use the trigonometric angles of sine, cosine and tangent.

  3. Calculate the other forces present

    Calculate the other forces that are acting on the object. The force of gravity is the mass of the object multiplied by the gravitational constant of 9.81 meters per second squared (m/s^2). Other forces, such as the force generated by a push or pull, may be given to you in the question.

  4. Solve for the normal force

    Solve for the normal force by summing the other forces. If an object is motionless on the ground, the normal force is equal to the force of gravity. Pay attention to the direction in which the force travels. If it moves in the same direction as the normal force, then this force must be subtracted from the gravitational force (Normal = Force of Gravity – Force). If the force travels opposite the normal force, it is added to the gravitational force (Normal = Force of Gravity + Force).