Lesson Plan: Introduction to Newton’s Second Law of Motion

[7 min] Do Now

Review from 1st Law, introduce 2nd Law:

In which of these cases do we have balanced forces? Explain why.

  • A cat is moving with constant velocity towards his date.
  • A car is moving with constant acceleration to pick up more physics homework.
  • A cow is at rest, taking a nap.
  • An apple is hanging from a tree.

Share out and discuss. Bridge the transition between Newton’s First Law and the idea of net force into Newton’s Second Law.

[1 min] Making Clear the Objective

Objective: You will derive the relationship between force and acceleration from simulated experimental data.
Criteria for Success: Graphs of data will show proof of Newton’s 2nd Law of Motion.

[12 min] Simulation: Newton’s Second Law

We will be using the simulation of Newton’s 2nd Law located at: http://phet.colorado.edu/en/simulation/forces-1d

Set: show horizontal force, show total force.
Turn friction off.
Turn on graphs for acceleration and velocity.

Use students to run simulation and call out the data for their classmates to record.

We will be using a simulation. For each trial, record the following:

  • mass of the object
  • force applied to the object
  • acceleration of the object

Run the simulation for the dog (25 kg) with three forces: 50 N, 100 N, 200 N. Ask the students to make a prediction before the last one. Make sure to reset the simulation and graphs before each trial.

Run the simulation  for the textbook (10 kg) with the same three forces.

 

[15 min] Graphing the Data

Turn and Talk:
What was the independent variable and why?
What was the dependent variable and why?
What was the main control variable and why?

What do we put on the y-axis? What do we put on the x-axis?
The independent variable of our experiment always goes on the x-axis (Force). The dependent variable of our experiment always goes on the y-axis (Acceleration).

Work with your partner:
Draw 2 graphs. Don’t forget units and labels!

  • Acceleration vs force variable for the dog
  • Acceleration vs force variable for textbook

 

[15 min] Analyzing the Data

We seem to have found a correlation between two variables, force and acceleration. Let’s see if we can define a relationship between them.

Find the slope of each graph and write it next to the plot.
Find the inverse of the slope for each graph and write it next to the plot.

Think-Pair-Share:
Do we see any patterns? Does the slope look like a variable we recognize? How would I write the equation of this line?

a = 1/m F   →   F = m a

[2 min] Summarize Findings

Newton’s 2nd Law of Motion:
The acceleration of an object is directly proportional to the net force acting on the object. The acceleration will be in the same direction as the net force. The acceleration is resisted by the mass of the object.

F = m a

Estimated Instructional Time: 52 min

 

[6 min] Exit Ticket


The catapult on an aircraft carrier can can accelerate a fighter jet from rest to 56 m/s in just 2.8 s. If the fighter jet has a mass of 13,000 kg, what is the force required?



  • Tried this out yesterday for one of my At-Bats. It unfortunately did not go as expected. Almost every step took way more time than I thought it would, starting with the Do Now. It was clear to me that students still had a lot of remaining questions about Newton’s First Law; in trying to stick with the plan, I wrapped it up too quickly (even though it had already taken 10 minutes longer than I had planned).

    First critical mistake: What would have served students best during the Do Now? I think I should have thrown the plan out the window and focused on making sure that they had the prerequisite understanding of net force before moving to acceleration (which is a consequence of net force).

    Second critical mistake: What is the main learning objective of the lesson? In between performing the force/acceleration simulation on the dog and performing it on the textbook, I had a choice to make in the moment about whether I should forget about the textbook and go to graphing with the data we had, or keep going forward with the plan. Again, I think I chose poorly to stay the course. Given that we were already behind, allowing more time later for the real meat of the lesson, which is having students discover F = m a from empirical data. The high-cognitive-demand portion of the lesson that I wanted students to really be grappling with was shortchanged due to the time constraints, and we didn’t even have time for the exit ticket. Argh.

    Ah well. I think it’s a good lesson, and I want to keep improving it. I need to get better about focusing on what’s really important and practicing good time management in the moment. I don’t think I like the simulation either; much better would be a good, simple physical experiment. Will have to think more about this.

    Comments and suggestions welcome.


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