What is MOTION?

Motion occurs when an OBJECT CHANGES its POSITION or in other words...

travels a DISTANCE between two points.

 

Proving that something is moving requires something to be still.

Have you ever felt like you were moving when in reality it was something else?

The moving sidewalk and escalator act like a treadmill when you walk on it the wrong way. When you match its speed, YOU feel like you are moving but you have not changed your position.

Notice the Mythbusters matching the forward speed with the same backward speed. The result is what seems like no motion from a certain point of view.

 

 
 
Your body may seem like it has gone a distance but if you don't change position, you have not DISPLACED.
 
Motion is relative.
We must always compare our starting position with something else. This helps us "get our bearings". This object is called the REFERENCE POINT. Without a reference point we can never determine what is moving, let alone how far it has moved. If two things are moving at the same time, motion must be defined from both points of view.
     
Consider all of the ways your are moving even while you are "sitting still".

While "sitting" YOU are rotating around Earth's axis at 1600 km/hr.

You AND the Earth are orbiting the Sun at a rate of 30 km/s to make a Solar System.

You, the Earth, the Sun, AND the Solar System are orbiting the galaxy at 4281 km per second !!!

You, the Earth, the Sun, the Solar System, AND the galaxy are all moving but so are the other galaxies so there is no clear reference point.

 

There are FOUR types of Motion

Linear Motion (straight line)

an object moves in one direction only

Projectile Motion

an object near the Earth's surface moves in a curved path (moving forward WHILE falling)

Circular (rotational) Motion

an object moves forward WHILE being pulled to the center of a circle

Vibrational Motion

an object has a very quick back and forth motion

 

Speed and Velocity

Speed = a change in position over a period of time

Velocity = speed in a specific direction

 

Speed is calculated by dividing the distance traveled by the time it took to do it.
Try to get the alien back to his home! An Interactive Distance /Time graph

 

Acceleration

Any change in velocity. There are three ways to accelerate. Do you remember?

1. speed up
2. slow down
3. change direction
These can ONLY occur when an unbalanced force acts on an object.

 

Newton's 1st Law of Motion

An object at rest remains at rest.

An object in motion remains in motion at constant velocity until an unbalanced force acts on it.

An object at rest remains at rest.

These balls are both at rest but the one on the right "wants" to be at rest more. Being a hunk of concrete, it has more mass than the kickball. Therefore, the concrete ball has more INERTIA. Inertia is an object's resistance to a change in motion. While it would be fun to launch the kickball across the field with your foot. I wouldn't recommend kicking the concrete ball in front of Target. The more mass something has, the more difficult it is to get moving...

 

An object in motion remains in motion...

...but if you DID get it moving, it would be a very difficult thing to stop. Having inertia means resisting ANY change in motion. Sometimes we talk about how difficult it is to stop an object. We say the mass had a great deal of MOMENTUM. You can think of momentum as inertia in motion.

...at a CONSTANT VELOCITY...

An object in motion will not want to change its speed or direction. If we open the cargo bay doors (with our space suits on) and drive a golf ball into space, it will travel at the same speed in the same direction FOREVER. We call this CONSTANT VELOCITY! It is very difficult to experience this on Earth but if you have ever slid all the way across a patch of ice you'll start to get the idea.

...until an UNBALANCED force acts on it.

 

Newton's 2nd Law of Motion

An unbalanced force causes a mass to accelerate in the same direction as the force.

An unbalanced force

causes a mass to

ACCELERATE...

 

...in the same direction

as the force.

 

 

Newton's 3rd Law of Motion

Every action force has an equal and opposite reaction force.

Every action force

has an EQUAL and OPPOSITE

reaction force.

 

 

 

 

HONORS: How is motion graphed?

What would a position/time graph look like?

What would a velocity/time graph look like?

Watch these animations from "the physics classroom" website very closely. You will need to be able to interpret graphs like this. I have organized their work in one place for easier study.

Constant Positive Velocity

Observe that the object (to the left) moves with a constant velocity in the positive direction. The dot diagram shows that each consecutive dot is the same distance apart (i.e., a constant velocity). The position-time graph shows that the slope is both constant (meaning a constant velocity) and positive (meaning a positive velocity). The velocity-time graph shows a horizontal line with zero slope (meaning that there is zero acceleration); the line is located in the positive region of the graph (corresponding to a positive velocity). The acceleration-time graph shows a horizontal line at the zero mark (meaning zero acceleration).

Constant Negative Velocity


Observe that the object (to the left) moves with a constant velocity in the negative direction. The dot diagram shows that each consecutive dot is the same distance apart (i.e., a constant velocity). The position-time graph shows that the slope is both constant (meaning a constant velocity) and negative (meaning a negative velocity). The velocity-time graph shows a horizontal line with zero slope (meaning that there is zero acceleration); the line is located in the negative region of the graph (corresponding to a negative velocity). The acceleration-time graph shows a horizontal line at the zero mark (meaning zero acceleration).

Positive Velocity and Positive Acceleration

Observe that the object (to the left) moves in the positive direction with a changing velocity. An object which moves in the positive direction has a positive velocity. If the object is speeding up, then its acceleration vector is directed in the same direction as its motion (in this case, a positive acceleration). The dot diagram shows that each consecutive dot is not the same distance apart (i.e., a changing velocity). The position-time graph shows that the slope is changing (meaning a changing velocity) and positive (meaning a positive velocity). The velocity-time graph shows a line with a positive (upward) slope (meaning that there is a positive acceleration); the line is located in the positive region of the graph (corresponding to a positive velocity). The acceleration-time graph shows a horizontal line in the positive region of the graph (meaning a positive acceleration).

Negative Velocity and Negative Acceleration

Observe that the object (to the left) moves in the negative direction with a changing velocity. An object which moves in the negative direction has a negative velocity. If the object is speeding up then its acceleration vector is directed in the same direction as its motion (in this case, a negative acceleration). The dot diagram shows that each consecutive dot is not the same distance apart (i.e., a changing velocity). The position-time graph shows that the slope is changing (meaning a changing velocity) and negative (meaning a negative velocity). The velocity-time graph shows a line with a negative (downward) slope (meaning that there is a negative acceleration); the line is located in the negative region of the graph (corresponding to a negative velocity). The acceleration-time graph shows a horizontal line in the negative region of the graph (meaning a negative acceleration).

Positive Velocity and Negative Acceleration

Observe that the object (to the left) moves in the positive direction with a changing velocity. An object which moves in the positive direction has a positive velocity. If the object is slowing down then its acceleration vector is directed in the opposite direction as its motion (in this case, a negative acceleration). The dot diagram shows that each consecutive dot is not the same distance apart (i.e., a changing velocity). The position-time graph shows that the slope is changing (meaning a changing velocity) and positive (meaning a positive velocity). The velocity-time graph shows a line with a negative (downward) slope (meaning that there is a negative acceleration); the line is located in the positive region of the graph (corresponding to a positive velocity). The acceleration-time graph shows a horizontal line in the negative region of the graph (meaning a negative acceleration).

Negative Velocity and Positive Acceleration

Observe that the object (to the left) moves in the negative direction with a changing velocity. An object which moves in the negative direction has a negative velocity. If the object is slowing down then its acceleration vector is directed in the opposite direction as its motion (in this case, a positive acceleration). The dot diagram shows that each consecutive dot is not the same distance apart (i.e., a changing velocity). The position-time graph shows that the slope is changing (meaning a changing velocity) and negative (meaning a negative velocity). The velocity-time graph shows a line with a positive (upward) slope (meaning that there is a positive acceleration); the line is located in the negative region of the graph (corresponding to a negative velocity). The acceleration-time graph shows a horizontal line in the positive region of the graph (meaning a positive acceleration).