Identifying Variables

Identifying Variables

If you ask me how much money I have, and I answer 5, you still don't know how much money I have. Do I have 5 dollars, or 5 cents. That word after the "5" is the unit. Units tell us how we measured the variable.

The chart to the right tells you the units we use in physics for each variable. You need to make certain that the unit for each variable is written down on your formula sheet. Do that now.

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​Variables and Units

Match

Match the following

• velocity

• distance

• acceleration

• time

m/s

m

m/s/s

sec

• m/s

• m

• m/s/s

• sec

​In the previous matching problem,

You should have matched distance with meters (m), time with seconds (sec), velocity with meters per second (m/s), and acceleration with meters per second per second (m/s/s). If you had trouble with this, make certain that you write down the units for each variable on your formula sheet. If you are still having trouble, please call your teacher to come explain!

Why Units are Important

​Units are the secret to making this easier

When you read a word problem, look for all of the numbers. After each number, you'll see a unit, usually abbreviated. That unit tells you which variable is associated with that number. If you see 5 m, then you know that the distance is 5 meters. If you see 5 sec, then you know that the time is 5 sec.

Multiple Choice

A 50 pound dog walked 200 m in 300 sec. In this problem, the distance is:

• 50 pounds

• 200 meters

• 300 seconds

• unknown

​In the dog problem

You should have chosen 200 meters (m), because meters is the unit for distance.

50 pounds is a weight, and 300 seconds is a time.

Multiple Choice

A plane with 300 passengers accelerated to 250 m/s over the course of 1200 sec. What is the time in this problem?

• 250 m/s

• 300 passengers

• 1200 sec

• unknown

​In the plane problem,

You should have chosen 1200 seconds (sec) because seconds is the unit for time. 250 meters per second (m/s) is a velocity, and 300 is the number of passengers.

Dropdown

After traveling for 500 sec, a 2000 kg truck had an average velocity of 15 m/s. How far did the car travel? The velocity was ​

Select answer

, the time was ​

Select answer

and the distance was ​

Select answer

​In the truck problem,

You should have chosen 15 meters per second (m/s) for the velocity, and 500 seconds (sec) as the time. The distance was unknown - that is the variable that you would have to solve for. The mass of the truck doesn't match with any of the variables, so you can ignore that for now.

Acceleration happens any time something speeds up, slows down, or changes directions. In this class, you won't have to do any calculations on something changing directions, so we're going to concentrate on things that are speeding up and slowing down.

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​Acceleration

Initial Velocity and Final Velocity

Initial velocity just means. "the velocity at the beginning of the problem". Final velocity means, "the velocity at the end of the problem". If the object is speeding up, the final velocity will be higher. If the object is slowing down, the final velocity will be slower.

Multiple Choice

A soccer ball is kicked. It accelerates from 2 m/s to 35 m/s in 0.5 sec. The initial velocity of the ball is:

• 2 m/s

• 35 m/s

• 0.5 s

• 9.8 m/s/s

​In the first soccer ball problem,

The initial velocity is 2 m/s, because that is the velocity of the ball at the beginning of the problem.

Multiple Choice

A soccer ball is kicked. It accelerates from 2 m/s to 35 m/s in 0.5 sec. The final velocity of the ball is:

• 2 m/s

• 35 m/s

• 0.5 s

• 9.8 m/s/s

​In the second soccer ball problem,

The final velocity is 35 m/s, because that is the velocity of the ball at the end of the problem.

​A special note when the velocity is 0 m/s

Sometimes in a problem, an object isn't moving. You might see the words "not moving" or "stopped" or "at rest". The problem might even assume that you know that the object is stopped, for example if a car is at a red traffic light. If you see those words, the velocity at that time is 0 m/s. It's easy to accidentally find another number in the problem and use that instead, so be careful!

Dropdown

When a traffic light turns from red to green, a 400 pound motorcycle accelerates to 20 m/s in 10 sec. The time is ​

Select answer

, the final velocity is ​

Select answer

and the initial velocity is ​

Select answer

​In the motorcycle problem

You should have chosen "0 m/s" as the initial velocity, because the motorcycle was at a red traffic light. That means it was stopped.

Dropdown

At the end of a 10 km race, a runner traveling at 5 m/s comes to a stop in 3 sec. The time is ​

Select answer

, the initial velocity is ​

Select answer

, and the final velocity is ​

Select answer

​In the problem with the runner

You should have chosen "0 m/s" as the final velocity, because the runner is coming to a stop at the end of the race.