What Is Motion?
Motion is defined as the movement of an object. Inertia is the tendency of an object to resist changes in its state of motion. As discussed, all objects are moving, even those that appear to be completely still. Furthermore, all objects have inertia and thus they tend to resist changes in their motion. The state of motion of an object is defined by its velocity. Velocity is the speed at which an object is moving with regard to direction. Thus, inertia can be phrased as the tendency of an object to resist a change in its velocity. For example, a car may be traveling with a velocity of 65 MPH to the west. In other words, the car's state of motion is 65 MPH to the west. As the car has inertia, it will tend to maintain its velocity of 65 MPH to the west.
A car traveling at 65 MPH to the west will tend to maintain this velocity due to inertia.
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At Rest?
An object at rest is said to have zero velocity. Therefore, its state of motion is also zero velocity. I just told you that all objects are moving. So, what does it mean to say an object is at rest has zero velocity? An object is said to be at rest if it is not moving relative to some reference point. Let's look at an example: my computer is at rest on my table. It is not moving relative to the table. The table is not moving relative to the floor that it is sitting on within my house, and my house is not moving relative to the earth. The earth, as we know however, is moving.
Acceleration
If an object changes its velocity it has what we call acceleration. If an object increases its velocity, we call that positive acceleration. If an object decreases its velocity, it has negative acceleration. Any time an object accelerates, it changes its state of motion.
Now that we've established that everything has motion, let's consider what is responsible for causing that motion. The answer to this question is force. A force is any influence that causes an object to change its shape or motion. There are usually multiple forces acting on an object at any particular time. For example, a ball at rest on the ground is pulled down by the force of gravity and it's pushed up by the force of the earth's surface. Additionally, the air molecules are constantly bombarding the ball on the outside as well as the inside. All these forces have magnitude; that is, they have size. If the forces are acting in the same direction, then they are added together and cause motion to occur in the same direction. If the forces are acting in opposite directions, those forces are subtracted from each other and they cause motion in the opposite direction. The units of force are usually expressed as Newtons in honor of Sir Isaac Newton, who did much to advance our understanding of mechanics.
An object usually has multiple forces acting upon it at any given time.
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Simple and Complex Movement
The study of motion can be simple and not so simple. The simplest movement would be an object at rest, moving in a straight line with a constant velocity. These objects are in a state of equilibrium in which all forces are balanced. Let's consider the computer sitting on the table in front of you. It is at rest as long as the forces acting on it are balanced. The force of gravity pulling down is balanced by the force of the table pushing up on the computer. Should your computer experience an unbalanced force, it will change its velocity. Likewise, a car traveling at 65 MPH west has a constant velocity, as all the forces acting on it are balanced. It's in equilibrium. The force of the car pushing it forward is balanced by the force of friction pushing it backwards. As these forces are balanced, the car's velocity, that is, its state of motion, remains unchanged. The car's velocity will change if the brakes are applied or if you push down on the accelerator.
More complex movements occur when an object changes its direction. This may involve a curved path of motion such as a football thrown into the air to a receiver. The football travels up, forward, and down as influenced by different forces. That's complex!
The curved path a football takes when thrown is an example of a complex movement.
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Lesson Summary
To summarize, mechanics is a field of science designated for the study of moving objects. Motion is defined as the movement of an object. Inertia is the tendency of an object to resist changes in its state of motion. The state of motion of an object is defined by its velocity. Velocity is the speed at which an object moves with respect to its direction. Thus, inertia can be phrased as the tendency of an object to resist a change in velocity. If an object changes its velocity it has acceleration. Objects change their state of motion when forces act upon them. A force is any influence that causes an object to change its shape or motion. These objects are in a state of equilibrium in which all forces are balanced. The units of force are usually expressed as Newtons.
Learning Outcomes
After finishing this lesson, you should be able to:
- Define mechanics, motion, inertia, and velocity
- Explain the difference between an object that is at rest, accelerating, and in equilibrium
- Understand what causes an object's motion
- Describe simple and complex movements
