DYNAMICS

Unit Introduction

This unit covers aspects of dynamics as it relates to motion under the actions of forces.

What are Dynamics?

The concept of dynamics can be thought of as any motion and how forces affect that motion. In order to comprehend dynamics, you must first have an understanding of balanced and unbalanced forces and their effects on motion.

Balanced and Unbalanced Forces

By definition, balanced forces produce no change in an object’s motion. If you draw a force diagram for all forces acting on an object and the result is 0N, the forces are balanced. If the object is moving, the velocity will remain constant and there will be no acceleration. If the object is at rest, it will stay at rest.

Example: Imagine a box is sitting on the floor. The box has a weight (or gravitational force) of 3N. The ground exerts an upward normal force of 3N. You try to push the box to the right with a force of 5N, but the surfaces in contact also produce friction with a force to the left of 5N. The forces acting on the box are balanced, so there is no change in the box’s motion.

Unbalanced forces produce a change in an object’s motion because the net force acting on the object is greater than 0N. If the object is moving, unbalanced forces cause the object to accelerate (can be a positive or negative change). If the object is at rest, unbalanced forces cause the object to start moving.

Example: Imagine the same box from the previous example is still sitting on the floor. All forces are the same, except this time you use all your strength to apply a force of 10N to the right. The net force is now 5N to the right, which is an unbalanced force. This produces a positive acceleration to the box’s motion.

Watch the following video clip to learn more about balanced and unbalanced forces. Pay attention to the force diagrams given in the examples and predict whether the net forces are balanced or unbalanced. Continue watching to the end to see if your predictions were correct.

How do unbalanced forces affect motion? The next video clip demands critical thinking and application for you to decide what is true about net forces and how they influence objects’ motion. Complete the video notes and submit your work as question #11 in the assessment portion of the unit.

Printable: KHAN VIDEO NOTES

Newton’s Laws of Motion

Perhaps one of the greatest minds to investigate dynamics in forces and motion was Sir Isaac Newton. Watch this interesting clip which highlights some of Newton’s personal and professional life.

Life of Sir Isaac Newton (06:57)

Through his research, Newton developed three now-famous laws of motion.

Newton’s First Law of Motion

Newton’s first law is commonly known as the law of inertia. The law states “an object at rest will stay at rest and an object in motion will stay in motion until acted upon by an outside force.”

For example, a soccer ball will remain at rest on the field until a soccer player kicks it and starts its motion with the force of the kick.

Newton’s Second Law of Motion

Newton’s second law states “The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.” In simpler terms, force is equal to mass times acceleration, or F = ma.

For example, it may take a small force for you to push an empty box across the floor, but it will take a greater force for you to push the box if it is full of books.

Newton’s Third Law of Motion

Newton’s third law states “For every action, there is an equal and opposite reaction.”

For example, a spaceship expels gas downward and it boosts the spaceship upward.

Read the following articles for an expanded look at Newton’s laws of motion. Pay close attention to the examples in each article.

First Law:

Second Law:

Third Law:

Interactive Media: Newton’s Laws of Motion

This website has links to several interactive activities to allow you to see Newton’s laws of motion in action.

http://www.physicsclassroom.com/Physics-Interactives/Newtons-Laws

Force Pairs

A force is a type of interaction between two objects. The objects experience an equal amount of force in opposite directions, as Newton’s third law states. These interacting force pairs are not the same as balanced forces, and they do not cancel each other out since they act on different objects. To identify objects involved in an interacting force pair, look for this format: “A acts on B, so B acts on A.”

An example of this is a baseball bat swinging leftward into a baseball, which pushes rightward into the baseball bat.