eUnit 5: Forces and Newton’s Laws

Building on knowledge from previous units, students connect forces with motion by utilizing a bowling ball and a broom. By investigating forces through a variety of hands-on activities and labs, students develop a mathematical relationship between force of gravity and mass, and between the elastic force and the stretch of a spring. Students then conduct labs to investigate the concept of inertia (Newton’s first law), develop the relationship between mass and acceleration (Newton’s second law) and conduct qualitative and quantitative labs to investigate the “equal magnitude but opposite direction” nature of action and reaction forces (Newton’s third law). By the end of the unit, students will be able to answer the following questions:

  • What is a force?
  • How can one represent a force?
  • How does the “support” or normal force act?
  • How are gravitational force and mass related?
  • How does the elastic force in a spring depend on its stretch?
  • How does a force diagram look like?
  • What is inertia?
  • How are force and acceleration related?
  • When two objects are in contact how do the forces between them compare to each other?
  • What is the connection between force and motion?

Timeline for teaching Unit 5 (draft)

Big Ideas

  1. Forces represent interactions between objects.
  2. For a force to exist there must be an agent to apply it.
  3. Forces have magnitude and direction.
  4. When the net force is zero, the object is stationary or moving with constant velocity.
  5. When the net force is not zero, the object accelerates.
  6. Net force is proportional to mass and acceleration.
  7. Weight and mass are related but are not the same quantity.
  8. Forces come in pairs.

Learning Goals and Objectives


By the end of this unit, the students should be able to:

  1. Investigate the types and characteristics of forces. (DOK3).
    a) Identify different forces in a given physical situation.
    b) Measure the strength of a force using a spring scale.
    c) Identify the direction of a force.
    d) Distinguish between contact forces and field forces.
    e) Identify an agent, a receiver, an effect, and the direction of a force.
    f) Predict the outcome on a force by changing various parameters (e.g., roughness of a surface changes friction; size of an object changes air resistance).
  2. Determine the strength of force of gravity and elastic force. (DOK3)
    a) Use technology (i.e., force probes) to quantitatively measure and analyze forces.
    b) Use data to construct a graph of force of gravity vs. mass.
    c) Determine the strength of the force of gravity from the slope of a force vs. mass graph.
    d) Develop a mathematical relationship between force of gravity (weight) and mass.
    e) Compare the strength of gravity on different planets and satellites of planets.
    f) Use data to construct a graph of elastic force vs. extension.
    g) Determine the strength of an elastic force from the slope of a force vs. extension graph.
    h) Develop a mathematical relationship between elastic force and extension of a stretchable object.
    i) Compare the strength of elastic force for different stretchable objects.
  3. Analyze the forces acting on an object using multiple representations (i.e., force diagrams, verbal descriptions, graphs, pictures, mathematical models, etc.). (DOK4)
    a) Construct and label a force diagram for a given physical situation.
    b) Verbally describe the effect of forces acting on an object.
    c) Draw a picture from a verbal description of forces acting on an object.
    d) Determine the strength of force of gravity using the equation: .
    e) Determine the strength of elastic force using the equation: .
    f) Convert between verbal descriptions, force diagrams, graphs and mathematical models.
    g) Use the concept of balanced forces to calculate a specific force acting on a stationary object.
    h) Calculate the net force using Newton’s Second Law.
  4. Determine the effect of the net force on the motion of an object. (DOK3)
    a) Identify that a force is required to start and to stop an object, to change velocity, and to change the direction of motion, but not to sustain motion (inertia).
    b) Determine if the forces acting on an object are unbalanced or balanced (situations where net force is zero, Newton’s First Law).
    c) Relate the mass of an object to its inertia.
  5. Identify and relate pairs of forces for interacting objects (Newton’s Third Law). (DOK3)
    a) Predict the relationship between action and reaction forces for interacting objects and then test the predictions, using technology (i.e., force probes).
    b) Identify the pairs of action and reaction forces for interacting objects.
    c) Construct a separate force diagram for each object in an interacting pair, and label the action and reaction forces.
    d) Draw the action-reaction pair of forces as equal in magnitude and in opposite direction.
  6. Investigate the relationship between force, mass, and acceleration. (DOK2)
    a) Predict the relationship between force, mass, and acceleration.
    b) Show that force is related to acceleration (but not velocity).
    c) Demonstrate that a net force applied in (against) the direction of motion of an object will speed up (slow down) the object.
    d) Develop and use the mathematical relationship between force, mass, and acceleration, F = ma (Newton’s Second Law).
  7. Design and conduct an experiment to explain the relationship between, force, mass and acceleration. (DOK4) (optional)
    a) Predict the relationship between force, mass, and acceleration.
    b) Investigate the relationship between force, mass, and acceleration, (using technology, e.g., motion detectors, force probes).
    c) Show that force is related to acceleration (but not velocity).
    d) Identify the system for a given problem in which Newton’s Second Law is applied.
    e) Show that the directions of net force and acceleration are the same.
    f) Plot and interpret a graph of force vs. acceleration, a graph of force vs. mass and a graph of mass vs. acceleration.
    g) Develop the mathematical relationship between force, mass, and acceleration.
    h) Determine the net force in situations where acceleration is known (Newton’s Second Law).
  8. Design and conduct experiments to study forces and their effects.(DOK4).
    a) Make observations.
    b) Operationally define variables.
    c) Generate a hypothesis.
    d) List the steps of the procedure used to collect the data.
    e) Collect the data and organize it into a table.
    f) Convert metric units as needed.
    g) Graph and interpret the results.
    h) Develop conclusions based on results.