Science With Mr. Milstid

Speed, velocity and acceleration all deal with motion.
Motion is defined as: a change in position of an object, relative to a reference point.
All motion is relative - without using stationary or other comparative objects to guage motion, we would not be able to determine if and when objects are moving.

For Instance:

When standing on a subway platform, the platform is your reference point, so the train appears to be moving. BUT! when standing inside the train, it is your reference point, so everything outside the train appears to be in motion.

When watching a dog run across a field, the surface of the earth, trees, and anything in motion slower (or faster) than the dog is a reference point.

This interactive from the American Museum of Natural History provides a great example of how frames of reference work in regards to motion.

With all motion comes speed.
Speed is defined as: the distance traveled by an object, divided by the time it took to travel that distance, or: s=d/t. Speed is, therefore, evalauted in units of distance over time, using meters/seconds.

For Instance:

If a hot air balloon floats 50 meters through the air in 10 seconds, its speed is 5m/s (50m / 10s = 5 m/s)

Speed is a scalar measurement: it has magnitude only.

We will discuss 2 types of speed: Instantaneous Speed (speed at any exact moment) and Average Speed.
Most of the time, objects do not travel at a constant speed.

So, we will most often talk about speed as average speed: average speed equals the total distance traveled by an object divided traveled by the total time to do so. (Total Distance / Total Time)

Often, people confuse speed and velocity…typically, we think of them as one and the same.
Velocity is defined as: the measurement of speed and direction of travel of an object. It is a vector quanitity - it has magnitude and direction. This is a very important distinction: 50 m/s North is a velocity, while 50 m/s is not…it is simply speed.

Because velocity measures speed & direction, it changes a lot.
Any time speed, direction or both change, velocity changes.

For Instance:

If a bus is travelling at 15m/s and speeds up, its velocity has changed. If a bus is traveling at 15m/s and turns left, while remaining at 15 km/h, it velocity has still changed.

The only time velocity remains constant is when speed and direction are constant (which almost never occurs with objects in motion).

Because velocity is relative, and relies on direction, multiple velocities can be combined. This is called resultant velocity: the addition of two velocities.

For Example:

If you are moving on a Septa bus that is going 30 m/s, and get up to walk to the front of the bus at 5 m/s, your resultant velocity is 35 m/s (Because you are on the bus, you are already moving 30 m/s. By moving forward in the bus, you’ve increased your velocity an additional 5m/s.)
If you are moving on a Septa bus that is going 30 m/s, and walk backwards, from the front of the bus to the rear at 5 m/s, your resultant velocity is 25 m/s.

Another insteresting effect of velocity’s reliance on direction is the difference between average speed of motion and the average velocity of motion.
Unlike when calculating average speed, where we divide the total distance traveled by the time of motion, to calculate average velocity of motion, we divide the total displacement of an object by the time of motion (Total Displacement/Time).

The animation below provides a good refresher on the difference between distance and displacement.

Acceleration is another measurement of motion, and is a whole different ball of wax. We usually consider acceleration to be “speeding up.”
This is not true! Acceleration is actually equal to how quickly velocity changes, and can be positive (speeding up), or negative (slowing down)!
Anytime velocity changes, acceleration is affected.

A neat result of this is:

in our car, the gas pedal is often called the accelerator, however, it only speeds us up. Any other instrument that changes velocity in our car is also an accelerator. This means that the brake, and also the steering wheel fit in this category as well.

Just as we discuss average speed and average velocity, we will often discuss average acceleration.
Average acceleration is calculated using the following formula:
Average Acceleration = (final velocity – starting velocity)/time to change velocity.
It is expressed in m/s/s, or m/s².

Centripetal Motion
Objects in circular motion are constantly changing direction.
As such, they are constantly changing velocity, and, therefore, constantly accelerating.

Practice recognizing speed, velocity and acceleration on a graph: Play the game linked below. Take notice of how velocity graphs change over time with changes in velocity.

	Click the image to the left to download Matter in Motion classwork/sample items.
	Click the image to the left to download the Matter In Motion comprehensive practice set 1

Tags: Acceleration, Forces, Motion, Physics & Physical Science, Speed, Velocity
Posted in Forces & Motion, Physics & Physical Science