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Newton’s Third Law

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Submitted by DaveBakker on Tue, 04/23/2019 - 18:14

Engineering Crash Cushions to Learn Newton's Third Law

Newton's third law states that for every action, there is an equal and opposite reaction. By crashing a physics cart into a wall, various crash cushions can be used to reduce the forces experience by the cart.

Newton's Third Law Example

Car crashes are an example of Newton's Third Law. The car exerts a large force on the wall and the wall then exerts a large force back onto the car. Civil engineers are always trying to think of new ways to make highways safer. Building crash cushions along highways that reduce the impact force of the collision will, according to Newton's Third Law, also reduce force experienced by the passengers of the car. This can save lives. But how should these crash cushions be designed? 

Your group is a team of civil engineers that work for the Department of Highway Safety to make roadways safer. Using the available supplies, and the principals of Newton's Third Law, your group must design two model crash cushions and test which model would be better to build for the highway. You will test each model crash cushion design by collecting acceleration data with PocketLab Voyager. Using the data collected from your crash cushion models, you will then draw a conclusion about how to design the best real-world crash cushion to make car crashes safer. 

NGSS Alignment

This investigation is aligned with NGSS MS-PS2-1. For a more detailed description of the alignment, along with example data and results., see our lesson on how to teach NGSS Newton's Third Law. 

Newton's Third Law Investigation Questions

How should crash cushions be designed to help make car crashes on highways safer? How does the design relate to Newton's Third Law of Motion.  

Objective

Students will be able to: 
-Design two solutions for a crash cushion that will reduce the impact force experienced during a car crash. 
-Use scientific reasoning and evidence collected during the investigation to determine which solution is better at making car crashes safer. 

Materials

-PocketLab Voyager
-PocketLab HotRod or other dynamics cart
-Cardboard box to represent wall
-Ramp
-Materials for crash cushion design: construction paper, scissors, masking tape, staples

Discussion

Part 1
Newton’s Third Law of Motion states: For every action, there is an equal and opposite reaction. 

In the cart crash, the cart crashes into the wall and exerts a force on the wall. How does Newton’s Third Law of Motion relate to the impact force experienced by the passengers in the car? Discuss your answer with your group and then write a response that says how the diagram final vector arrow (Fwall on cart) on the diagram should be drawn.

Newton's Third Law Diagram

Part 2  

Newton’s Second Law of Motion:
Newton’s Second Law tells us that acceleration and force are proportional. For the purposes of this lab, we will use PocketLab to measure acceleration at impact to determine the proportional force experienced in the crash. 

Momentum: 
Momentum is the product of an object’s mass and velocity. We can think of momentum as inertia in motion. 

Equation for momentum: p = mv where p = momentum, m = mass, and v = velocity

In this experiment, you will control the cart’s mass and velocity and therefore its momentum going into the crash. 

Impulse: 
Impulse is the change in momentum of an object. Whether you are using a crash cushion or not, the change in momentum of the cart will always be the same. If your experiment is properly controlled, the cart will crash with the same momentum and then, after the crash, come to a stop with no momentum. Therefore, if the cart is crashing directly into the wall or the into any of your crash cushions, the impulse for every crash will always be the same. 

Equation for impulse: J=F· ∆t  where J=impulse F=force ∆t=change in time

The cushion cannot change the velocity or momentum of the cart before it crashes, so it can’t change the impulse. However, an effective crash cushion will still find a way to change the force (F) in the equation above, even when the impulse cannot be changed.

Hypothesis

Write a prediction to answer the following question:
The objective of your crash cushion is to reduce the force experienced by the cart. Thinking about the information from the discussion section above, identify how this is possible. What will your crash cushion need to do during the crash in order to reduce the force experienced by the cart?

Set-up Experiment

Set-up your materials as shown in the diagram. 

Newton's Third Law Crash Cushion Diagram


Control Variables

For every new run, you will change the independent variable but keep a set of control variables the same. This is to make sure that the experiment is accurate. Respond below to record your control variables. 

1) For every run, what will be the angle of the ramp? 

2) For every run, where will the cart be placed on the ramp? 

3) For every run, what is the distance the cart will travel between the bottom of the ramp and where the crash begins? 

 

Independent and Dependent Variables

For every run, you will be testing a different value of the independent variable. You will see how these different independent variable values affect your dependent variable, measured in your data and seen in your observations. 

What is the Independent Variable in this experiment? 

What is the Dependent Variable in this experiment?  
 

Runs

Control Run
Run 1 is a control run. You will crash the cart into a wall with no cushion. Perform three trials. Determine what in your collected data should be your Trial Result. Enter that into the Trial Results for every trial. You can also add additional notes to the Trial Notes. 

Steps for Control Run:

  • Set up the cart at the top of the ramp according to your control variables. 
  • Begin recording data for Trial 1. 
  • Roll the cart down the ramp so it crashes into the wall.
  • Stop Recording data. Save your data. 
  • Find and record the Trial Result.
  • Repeat steps for Trials 2 and 3.
  • Record any overall observations, including what the crash looked like.


Crash Cushion Design 1
Your next Run will be for Crash Cushion Design 1. You will crash the cart into your first crash cushion design. If you can, include a picture or video of your design here. Include an explanation of your design and the reasoning behind it.

Steps for testing Crash Cushion Design 1: 

  • Set up the cart at the top of the ramp according to your control variables. 
  • Begin recording data for Trial 1. 
  • Roll the cart down the ramp so it crashes into your crash cushion.
  • Stop Recording data. Save your data. 
  • Find and record the Trial Result.
  • Repeat steps for Trials 2 and 3.
  • Record any overall observations, including what the crash looked like. 


Crash Cushion Design 2
Your next Run will be for Crash Cushion Design 2. You will crash the cart into your second crash cushion design. If you can, include a picture or video of your design here. Write an explanation of your design and the reasoning behind it. Include an explanation of what you did to improve upon Design 1. 

Steps for testing Crash Cushion Design 2: 

  • Set up the cart at the top of the ramp according to your control variables. 
  • Begin recording data for Trial 1. 
  • Roll the cart down the ramp so it crashes into your crash cushion.
  • Stop Recording data. Save your data. 
  • Find and record the Trial Result.
  • Repeat steps for Trials 2 and 3.
  • Record any overall observations, including what the crash looked like.

Data Analysis

  1. What data did you determine was the Trial Result for each trial? Why? 
  2. What did the collected data tell you about your Investigation Question? How did your data analysis help you understand the results better?

Conclusions about Newton's Third Law

  1. Which of your crash cushion designs was most effective at making car crashes safer? Support your conclusion with evidence that you gathered from the lab. What scientific concepts or principles can explain the evidence? 
  2. Was your hypothesis valid or invalid? Explain.  
  3. How did you see evidence of Newton’s Third Law in your experiment? 


Question for Further Discussion

If you wanted to make cars safer during head-on collisions with other cars, what would you design the front of the car to do at impact? Relate your answer to your conclusions about crash cushion?