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Forces

Key Concepts

  • A force is a push or pull measured in Newtons (N) which is a substitute for kgm/s².
  • An object cannot apply a force on itself.
  • Forces are not needed to keep an object in motion, only to change motion.
  • Any change in motion is caused by acceleration.
  • F = ma → forces cause acceleration.
    • Where F = force, m = mass, and a = acceleration.

Types of Forces

  • ==Contact forces==: forces applied via physical contact between objects.
    • ^^Applied force^^: the force a person exerts on an object.
    • ^^Friction^^: the resisting force between two objects.
    • Static: resists putting an object at rest into motion
    • Kinetic: resists keeping an object in motion
    • ^^Normal^^: the supporting force a surface exerts on objects in contact with it (to keep the object from falling through).
    • Perpendicular to the surface.
    • Any lifting force on an object will reduce the normal force and vice versa.
    • ^^Tension^^: the pulling force of a cable holding up another object.
    • Always points away from the hanging object.
    • On an ideal pulley, the tension on both sides of the wheel are the same.
  • ==At-a-distance/non-contact forces==: forces that don’t require physical contact
    • ^^Gravitational^^: attractive force between masses.
    • Electrical: attractive force between an electron and a nucleus.
    • Magnetic: attractive force between opposite poles and repelling force between similar poles.
    • Electrical and magnetic forces concern AP Physics 2 students

Newton’s Laws of Motion

  • An object at rest/motion stays that way unless acted upon a net external force.
    • If the net force (sum of all forces acting on an object) = 0, the object’s (lack of) motion will not change.
    • This property is often referred to as inertia.
    • Mass is the measure of inertia and is directly proportional to it. (more mass = more inertia)
  • Acceleration is directly proportional to the net force applied on an object, but is inversely proportional to mass.
    • ΣF = ma → divide both sides by mass → a = ΣF/m
  • Every applied force will have an equal and opposite reaction.
    • These forces don’t cancel out because they get applied on different objects.
    • Example: hand pushes a wall, wall pushes back on hand with equal force

Force Equations

  • Force = F = ma
    • On a ramp of angle θ, F = mgsinθ
  • Normal force = N = W = mg
    • Elevators: N = W + Fₑ = mg + ma = m(g + a)
    • On a ramp of angle θ, N = mgcosθ
  • Friction = ƒ = μN
  • Gravitational force = F₉ = Gm₁m₂/r²
    • r = distance between the two masses

Solving Force Problems

Forces Only

  1. Read the question.
  2. Draw a diagram showing all the forces acting on the object (no components).
  3. Based on the diagram and the situation, decide if the net force equals 0 or not. Then write the statement ΣF = ma or ΣF = 0.
  4. Replace ΣF with all of the given/implied forces involved in the problem.
  5. Expand the forces if possible (e.g w = weight = mg).
  6. Isolate the variable you are solving for.
  7. Input values.

Force-Kinematics Problems

  1. Follow steps 1-4 above.
  2. List all kinematics variables you know based on the question.
  3. Solve for acceleration using forces or kinematics depending on which on you are given enough info for. This is because both the force equation and most kinematics equations contain acceleration.
  4. Substitute into the other equation and solve.