Lesson 1: Newton's First Law of Motion

Balanced and Unbalanced Forces

Newton's first law of motion has been stated frequently 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 does the phrase "unbalanced force" mean? What is an unbalanced force? In pursuit of an answer, 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 second force – the push of the table on the book (sometimes 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. The book is said to be at equilibrium. There is no unbalanced force acting upon the book and thus the book maintains its state of motion. When all the forces acting upon an object balance each other, the object will be at equilibrium; it will not accelerate. (Note: diagrams such as the one above are known as free-body diagrams and will be discussed in detail in Lesson 2.)

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 push of the floor exerts an upward force.

Since these two forces are of equal magnitude and in opposite directions, they balance each other. The person is at equilibrium. There is no unbalanced force acting upon the person and thus the person maintains his/her state of motion. (Note: diagrams such as the one above are known as free-body diagrams and will be discussed in detail in Lesson 2.)

Now consider a book sliding from left to right across a table top. Sometime in the history of the book, it may have been given a shove and set in motion from its rest position. Or perhaps it acquired its motion by sliding down an incline from an elevated position. Whatever the case, the focus is not upon the history of the book but rather upon the current situation of the 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.) The forces acting upon the book are shown below.

The force of gravity pulling downwards and the force of the table pushing upwards on the book are of equal magnitude and in opposite directions. These two forces balance each other. However, 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. This is an unbalanced force; and as such, the book changes its state of motion. The book is not at equilibrium and it subsequently accelerates. Unbalanced forces cause accelerations. In this case, since the unbalanced force is directed opposite to the object's motion, it will cause a deceleration (a slowing down) of the object.

To determine if the forces acting upon an object are balanced or unbalanced, an analysis must first be conducted to determine which forces are acting upon the object and in what direction. If two individual forces acting on an object are of equal magnitude and opposite direction, then these 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 acting on the object which is not balanced by another force of equal magnitude and in the opposite direction . Such analyses are discussed in Lesson 2 of this unit and applied in Lesson 3.

Check Your Understanding

Luke Autbeloe drops a 5.0 kg box of shingles (weight approximately 50.0 N) off the barn house roof into a haystack below. Upon hitting the haystack, the box of shingles encounters an upward restraining force of 50.0 N . 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 following 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 they are dropped to the time 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 Luke's 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 is correct or incorrect. Support your answers.

A. Once the shingles hit the haystack, the forces are balanced and the shingles will stop.

True or False? Depress mouse to view answer. 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 if there is no unbalanced force. When the shingles strike the haystack, they stop accelerating; but they do not stop moving.

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

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

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

True or False? Depress mouse to view answer. 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 them upward.

4. If the forces acting upon an object are balanced, then the object

A. must not be moving.

B. must be moving with a constant velocity.

C. must not be accelerating.

D. none of the above.

Depress mouse to view answer. Answer: C The answer could be A (but does not have to be A) and it could be B (but does not have to be B). An object having balanced forces definitely cannot be accelerating. This means that it could be at rest and staying at rest (one option) or could be in motion at constant velocity (a second option). Either way, it definitely is not accelerating - choice C of your four choices.

Go to Lesson 2