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After watching this lesson and completing the lab, you will be able to explain what Newton’s 3rd Law is, and use an equation to show why two forces can be equal and opposite, and still lead to two objects moving in different ways. A short quiz will follow.

Newton’s 3rd Law

Newton’s 3rd Law tells us that if body A exerts a force on body B, body B exerts an equal but opposite force on body A. Or in other words, every action has an equal and opposite reaction.

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If you push down on your desk, you are applying a force to the desk, and the desk is applying an equal but opposite force back to you. But if that’s true, why does anything move at all? Let’s investigate!

Physics Lab Steps

Note that it’s important to take care when completing this physics lab. We will be creating a chemical reaction and sending a cork flying through the air. You must make sure that the cork isn’t allowed to point in the direction of a human being (including yourself!).

For this physics lab, you will need:

  • A wine bottle
  • A reasonably intact cork
  • Baking soda
  • Vinegar
  • A paper towel
  • A LARGE toy car
  • Duct tape
  • And, a flat, long surface, like a driveway

Step 1: Strap the wine bottle to the top of the toy car using the duct tape. The neck of the bottle should point towards the back of the car. Measure the mass of your contraption and note it down. Also measure the mass of the cork.Step 2: Measure out enough vinegar to fill a few centimeters of the bottle, and then note down the mass of the vinegar. Then pour those few centimeters of vinegar into the bottle.

Step 3: Stuff some paper towels in the neck of the bottle. They should hold in place, but not too tightly.Step 4: Fill the rest of the neck of the bottle with baking soda, and close the bottle with a cork.Step 5: Tap the bottle on the ground hard enough to make the wad of paper fall into the bottle, and shake to make the vinegar and baking soda mix.Step 6: Put it down on the driveway, but be SUPER CAREFUL and make sure the cork isn’t pointing towards anyone.

Step 7: Sit down and watch.At this point, if you haven’t already, you should pause the video to complete the experiment and note down your observations. We will then discuss the expected results.

Observations and Conclusions

Okay, now that we’ve completed the lab, let’s talk about the results. What did you observe in the experiment?You should have seen the cork forced out, pushed out by the gas created in the reaction between the vinegar and baking soda. The cork will have shot one way and caused the car to shoot the other way.

When done well, the car may have moved as much as five feet and the cork around 10-20 feet.This happens because of Newton’s 3rd Law. Forces internal to the car applied a force to the cork, so the cork must also apply an equal but opposite force back onto the car. But if the forces are equal and opposite, why does the cork go so far and the car moved so little?Take a look at your mass measurements. The cork has a much smaller mass than the car. If we take Newton’s 2nd Law in equation form, F = ma, we see that a force is equal to a mass multiplied by an acceleration. The mass of the car was LARGE, and based on your observations of its motion, the acceleration must have been SMALL.

But the mass of the cork was really SMALL, and its acceleration was LARGE. The two forces, F, are equal to each other. A small m times a large a equals a large m times a small a.

That’s how the forces are equal.

Lesson Summary

Newton’s 3rd Law tells us that if body A exerts a force on body B, body B exerts an equal but opposite force back on body A. Or in other words, every action has an equal but opposite reaction.

This can be a bit confusing, because when you push on a shopping cart, the cart moves. Yet if the forces are equal and opposite, it’s easy to imagine that nothing would ever move. The shopping cart would push back on you with the same force.To investigate this, we completed a physics lab that involved strapping a wine bottle to the back of a large toy car.

A reaction was created inside the wine bottle, causing the cork to pop out. The cork moved one way because a force was applied to it, but an equal but opposite force applied to the car, making it move the other way. If it wasn’t for friction, the car would have moved even further than it did. But even with no friction, the cork would always move further than the car.

This is because the car has a much larger mass. We know from Newton’s 2nd Law that F = ma. So a large mass undergoing a small acceleration and a small mass undergoing a large acceleration can be created by exactly the same amount of force. And that’s why the cork moved further than the car.

Learning Outcomes

Following this lesson, you’ll be able to:

  • Recall Newton’s 3rd Law
  • Describe the relationship between Newton’s 2nd Law and 3rd Law
  • Explain an experiment that shows the relationship between these two principles

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