Force and Acceleration

Newton's Second Law of Motion

Learning Targets

  1. I can interpret and construct a force (free-body) diagram.
  2. I can diagram and calculate the components of force acting in two dimensions.
  3. I can describe the relationship between net force, mass, and acceleration.
  4. I can calculate the net force, mass, or acceleration of an object.
  5. I can explain the difference between mass and weight given a changing gravitational field.
  6. I can calculate apparent weight given a situation of vertical acceleration.
  7. I can evaluate and develop a mathematical model for elastic (spring) forces.
  8. I can predict the force required to slide an object relating to the coefficient of friction.
  9. I can predict an object's motion and the associated forces of objects moving in a circle.

Formulas

  • Fnet = m a
  • a = Δv / Δt
  • Δx= ½ at2 (if v0 = 0)
  • v12 = v02 + 2a (Δx)
  • Felastic = k Δx
  • Fk = μkFN
  • ac = v2 / r
  • Fc = mv2 / r

Vocabulary

  1. circular-have the form of a circle
  2. centripetal-a force that acts on a body moving in a circular path and is directed towards the center around which the body is moving.

  1. centrifugal-moving or tending to move toward a center
  2. free-body diagram-diagrams used to show the relative magnitude and direction of all forces acting upon object in a given situation.
  3. friction-the resistance that one surface or object encounters when moving over another.
  4. kinetic-of, relating to, or resulting from motion
  5. static-lacking in movement, action, or change, especially in a way viewed as undesirable or uninteresting
  6. linear-arranged in or extending along a straight or nearly straight line
  7. normal force-the component, perpendicular to the surface of contact, of the contact force exerted on an object by, for example, the surface of a floor or wall, preventing the object from falling
  8. net force-the overall force acting on an object
  9. tangent-a straight line or plane that touches a curve or curved surface at a point, but if extended does not cross it at that point
  10. torque-a twisting force that tends to cause rotation
  11. weight-a relative mass or the quantity of matter contained by it, giving rise to a downward force
  12. apparent-clearly visible or understood, obvious
  13. weightlessness-having little weight or lacking apparent gravitational pull

Practice Problem #1

What is the estimated acceleration due to the gravitational force on the moon if a person on Earth weighs 550 N- but only 75 N on the Moon?

Practice Problem #2

A rider on a motorcycle with a combined mass of 300 kg are going down the road at 40 m/s. The rider wants to stop and applies the brakes until coming to rest. What force will the brakes need to apply in order for the rider to stop in a distance of 150 m?

Practice Problem #3

Suppose a net force of 34 N is applied horizontally while pushing a desk weighing 545 N. Assuming there is no friction, what is the displacement of the desk after 10 seconds?

Main Points

This unit covers:

  • problems involving gravitational forces, elastic forces, and frictional forces.
  • calculating the weight using force diagrams and Newton's laws of motion.
  • the difference between mass and weight.
  • kinetic and friction forces
  • net forces acting in two dimensions with vector addition trigonometry or diagrams.
  • solving for net force, final velocity, time , displacement, and acceleration for objects sliding down different types of inclines.
  • calculating centripetal force by using a separate equation.