Lesson 1: Newton's First Law of Motion

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Balanced and Unbalanced Forces

Newton's first law of motion has frequently been stated throughout this lesson.

An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

But what exactly is meant by the phrase "unbalanced force?" What is an unbalanced force? In pursuit of an answer, we will first consider a physics book at rest on a table top. There are two forces acting upon the book. One force - the Earth's gravitational pull - exerts a downward force. The other force - the push of the table on the book (sometime's referred to as a normal force) - pushes upward on the book.

Since these two forces are of equal magnitude and in opposite directions, they balance each other. There is no unbalanced force acting upon the book and thus the book maintains its state of motion.

Consider another example of a balanced force - a person standing upon the ground. There are two forces acting upon the person. The force of gravity exerts a downward force. The floor of the floor exerts an upward force.

Since these two forces are of equal magnitude and in opposite directions, they balance each other. There is no unbalanced force acting upon the person and thus the person maintains its state of motion.

Now consider a book sliding from left to right across a table top. Sometime in the prior history of the book, it may have been given a shove and set in motion from a rest position. Or perhaps it aquired its motion by sliding down an incline from an elevated position. Whatever the case, our focus is not upon the history of the book but rather upon the current situation of a book sliding across a table top. The book is in motion and at the moment there is no one pushing it to the right. (Remember: a force is not needed to keep a moving object moving to the right.) The forces acting upon the book are shown below.

The force of gravity pulling downward and the force of the table pushing upwards on the book are of equal magnitude and opposite directions. These two forces balance each other. Yet there is no force present to balance the force of friction. As the book moves to the right, friction acts to the left to slow the book down. There is an unbalanced force; and as such, the book changes its state of motion. Unbalanced forces cause accelerations. In this case, since the unbalanced force is directed opposite the object's motion, it will cause a deceleration (a slowing down of the object).

To determine if the forces acting upon an object is balanced or unbalanced, an analysis must first be conducted to determine what forces are acting upon the object and in what direction. If two inidividual forces are of equal magnitude and opposite direction, then the forces are said to be balanced. An object is said to be "acted upon by an unbalanced force" only when there is an individual force which is not being balanced by a force of equal magnitude and in the opposite direction .

 

 

Check Your Understanding

James drops a 5.0 kg box of shingles (weight = 50.0 N) off the barnhouse roof into a haystack below. Upon encountering the haystack, the box of shingles encounters a 50.0 N upward restraining force. Use this description to answer the following questions. Depress the mouse on the "pop-up menus" to view the correct answers.

1. Which one of the velocity-time graphs best describes the motion of the shingles? Support your answer with sound reasoning.

   

   Depress mouse to view answer. Graph B is correct. The shingles first accelerate with a negative (downward) acceleration until they hit the haystack; upon hitting the haystack, the shingles experience a balance of forces (50 N downwards due to gravity and 50 N upwards due to the haystack). Thus, the shingles will finish their motion moving with a constant velocity. Graph B depicts both the initial negative acceleration and the final constant velocity.

    

    

2. Which one of the following ticker tapes best describes the motion of the falling shingles from the time that they are dropped to the time that they hit the ground? The arrows on the diagram represent the point at which the shingles hit the haystack. Support your answer with sound reasoning.

   

   Depress mouse to view answer. Tape A is correct. The shingles first accelerate with a negative (downward) acceleration until they hit the haystack; upon hitting the haystack, the shingles experience a balance of forces (50 N downwards due to gravity and 50 N upwards due to the haystack). Thus, the shingles will finish their motion moving with a constant velocity. Tape A depicts both the initial downward acceleration and the final constant velocity.

    

    

3. Several of James' friends were watching the motion of the falling shingles. Being "physics types", they began discussing the motion and made the following comments. Indicate whether each of the comments are correct or incorrect?
   1. Once the shingles hit the haystack, the forces are balanced and the shingles will stop.

      True or False? Depress mouse to see. False. Once the shingles hit the haystack, the forces are balanced (50 N down and 50 N up). However, an object in motion (such as the shingles) will continue in motion at the same speed and in the same direction. When the shingles strike the haystack, they stop accelerating; yet they do not stop moving.

       

       

   2. Upon hitting the haystack, the shingles will accelerate upwards because the haystack applies an upward force.

      True or False? Depress mouse to see. False. Once the shingles hit the haystack, the forces are balanced (50 N down and 50 N up). The upward force of the hay is balanced by the downward pull of gravity. The shingles will continue in motion at constant speed.

       

   3. Upon hitting the haystack, the shingles will bounce upwards due to the upwards force.

      True or False? Depress mouse to see. False. Once the shingles hit the haystack, the forces are balanced (50 N down and 50 N up). The shingles would only bounce upwards if the haystack applied an upward force greater than 50 N. As stated in the problem, the haystack applies only 50 N of upward force. Furthermore, the upward force would first contribute to slowing the shingles down (an upward acceleration) before it could begin to actually move it upward.

       

 

 

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Lesson 1: Newton's First Law of Motion

• Newton's First Law
• Inertia and Mass
• State of Motion

Go to Lesson 2