Overview of Newton's Second Law of Motion Video

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  • 00:00 Trucks and Sports Cars
  • 00:58 Acceleration and Mass
  • 1:58 Force
  • 2:47 Manipulating the Formula
  • 3:51 Lesson Summary
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Lesson Transcript
Instructor: Kevin Newton

Kevin has edited encyclopedias, taught middle and high school history, and has a master's degree in Islamic law.

Have you ever had to push a broken-down car? Hopefully, the answer is no but if you have, you definitely know that the lighter the car, the easier the job. In this lesson, we learn why that is the case.

Trucks and Sports Cars

Imagine for a moment that you had your dream car. For many people, chances are that your dream car is either a little sporty coupe from a German or Italian manufacturer, or a much more solid vehicle, like a truck or an SUV.

Now, imagine that your dream car has broken down on the side of the road. There is a service station 300 yards away, and you and your friends feel foolish calling a tow truck for such a short distance. Plus, the towing company said it would be $300, which no one wants to pay. Reluctantly, you and your friends get out and start to push. Which car do you want now? The tiny little sports car or the heavy, giant truck?

Most people would choose the smaller car at this moment, as it is easier to push. Why is that? The answer is relatively simple, according to Isaac Newton. In fact, it's his Second Law of Motion: Force = Mass * Acceleration. What does that mean? Let's find out.

Acceleration and Mass

First, let's examine what is meant by two of those words: mass and acceleration. Mass is probably the easier of the two to define, as it means the total amount of material present. Mass is different from weight, in that weight is the effect of gravity on an object. As such, due to differences in gravity, a 220 pound man on earth would only weigh about 37 pounds on the moon, but his mass of 100 kilograms would be identical anywhere. In our example, the sports car had a much lower mass than the SUV.

So what about acceleration? Acceleration is the term given to the measure of the change of velocity over time. This is why you'll often hear scientists call it 'meters per second per second,' or 'meters per second squared.' We are tracking how the force increases. Someone who goes the speed limit consistently on the highway will have a much lower acceleration than someone who speeds up, slows down, and slams on the brakes.

Force

Multiplying acceleration and mass gives us the force of an object. Force is measured in a unit known as a newton, which is a kilogram meter per meter squared. In other words, it's simply the units of acceleration and mass pushed into one unit.

Think about it like this. A baseball on a shelf is unable to exert any force, but when thrown it will exert a force due to the fact that it has gone from not moving to moving at more than 100 kilometers an hour!

So, back to your broken down car. What we are trying to find is the force needed to cause your car to change its velocity; in other words, to accelerate. If you put the mass of the sports car at 500 kg and the mass of the SUV at 2,000 kg, you will need four times the acceleration to get the same amount of force.

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