Newton's First Law and Balanced Forces

The Ancient View of Motion

The belief in ancient Greece, in the time of Aristotle, was that objects had a natural position at rest on the surface of the earth. If you gave the object a push, you gave it some impetus, which was a property that caused it to move. The object would continue to move in a straight line until it had used up its impetus, at which point it would return to its natural position at rest on the surface of the earth.

The reason it would move in a straight line is that straight line motion was crude and earthly, while curved motion was beautiful and reserved for the gods and heavenly objects.

This belief conformed to everyday experience. Just try it. Pick up a baseball and give it a throw. Chances are, after it has gone for a while, it will end up back at rest on the ground, just like Aristotle believed. Since it was so obvious that this was the case, it should be no surprise that this view persisted until the time that Galileo and several others began to correct it and finally Isaac Newton put the belief to rest.

Newton's First Law of Motion

In this presentation, you will take a look at Newton’s First Law of motion, and see how objects behave when forces on them are balanced.  You will see how to apply Newton’s First Law to situations involving pulleys, which illuminates the usefulness of a First Law analysis. Click the player to begin.

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Friction

It is now time to take a closer look at the force of friction and see what factors contribute to the amount of frictional force. Friction is important in many sports.  For example, when drag racers leave the line, they want to accelerate as fast as they can, but if they let the engine apply too much force, then the tires will begin to slip instead of sticking to the surface of the track. If the they begin to slip, a burst of smoke is produced and the driver will usually immediately let off the gas, realizing that they've lost the race, because their ability to continue to accelerate is greatly diminished.  This is called “smoking the tires”. Anti-lock brakes also work on this principle.  When the car senses that the brakes have been forcefully applied, the onboard computer will cycle the brakes on and off, allowing the tires to alternate between slipping and turning.  Each time the tires can turn, the surface of the tire is at rest with respect to the road, and can take advantage of the greater static friction to slow down and turn with more control. At the end of the presentation, you will work through the simulation below. Click the player to begin.

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Forces in 1 Dimension Simulation

View these instructions for "Forces in 1 Dimension". Open the Forces in 1 Dimension Simulation from PhET by clicking on the image to the right. Please be aware that the site will open in a new window and you will need to allow the program to open. If you need help opening the program please visit the Developmental Module of this course. Follow the directions you just viewed in the instructions as you work through the simulation and try to answer these questions as you play with the simulation. After you work through the simulation complete the practice problems below.

 

Balanced Forces Practice

Now, see if you can apply your knowledge of Newton’s First Law to answer these questions involving balanced forces, forces on pulleys, and calculating the force of friction in various situations. Work through each problem on your own prior to checking your answers. Click the player to get started.