Pre-Test
Q&A	Measurement

Instruction 1-1

Solving Constant Speed and Average Speed Problems | Balanced Forces | Newton's First Law | One Dimensional Motion Problems (Newton's Second Law) | Universal Law of Gravitation | Applying Forces to an Object (Newton's 3rd Law) | Summary

Speed is a measure of how fast something is moving. For example a car may have a speed of 40mph or 60m/s. The unit’s mph, miles per hour, and m/s, meters per second, hint toward the answer to our question. The units show that to obtain a speed, you must divide the distance traveled by the time it took to travel this distance. Simple as that!

Try the experiment at the link below to calculate your own speed.
http://www.middleschoolscience.com/walk.html

Before we move on, let's make sure you know the difference between speed and velocity. Speed and velocity are both ways of describing how fast an object moves. Speed is a scalar quantity meaning it does not depend on direction. Velocity is a vector quantity meaning that it does require a direction.
http://www.physics.uoguelph.ca/tutorials/vectors/vectors.html

Speed examples: 20mph, 60ft/s, 76m/s
Velocity examples: +20mph, 60ft/s north, -76m/s

* If a velocity is shown with no direction, assume it is positive. .

If an object remains at the same speed the entire time it is said to have constant speed. A space ship at a constant speed does not move faster or more slowly. An object at constant speed will travel the same distance every time interval. (The time interval is just the difference between two times; seconds, minutes, hours...etc.). For example, if your car travels at a constant speed of 10m/s, the distance traveled every second is 10m. The time interval would be one second.

Constant speed

http://www.physicsclassroom.com/Class/1DKin/U1L3a.html

At most times, a car will change its speed throughout a trip. It may stop at a traffic light, slow down in traffic, or speed up to pass a slowly moving car. It is not maintaining constant speed. In this case, it may be useful to describe the car's average speed. Average speed is just the total distance covered divided by how long the trip took. So, what would your average speed have been if you traveled by car to your friend's house 100 miles away and it took 2 hours?

It took 2 hours to travel 100 miles at an average speed of 50 miles per hour. On average, you were traveling at 50mph. Although, at times, you may have stopped or traveled over or under 50 mph, your average speed was still 50mph.

There's one more kind of speed we should talk about. If you take a look at the speedometer in the car as you travel it tells you the instantaneous speed. That is the speed that you're traveling at that moment or instant.

http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/kinema/trip.html

A graph of an object's position and time can also be useful. These are called position-time graphs. The time data is plotted on the x, or horizontal axis and the position data is plotted on the y, or vertical axis.
Take a look at the data table and graph below.

Let's say this is the graph of a jogger. The line represents the most likely positions of the jogger at the times between the recorded data points. You can use the graph to estimate his position at any time during the run. Try it. Where was the jogger when 2 seconds had passed? Find the point on the line at 2 seconds. Follow it over to the y-axis to determine his position. I hope you got 20 meters from his starting point.
http://dev.physicslab.org/Document.aspx?doctype=3&filename=Kinematics_PositionTimeGraphs.xml

There are other things that you can determine from this graph. Take a look at the line. Think about what would happen if that line was lowered or raised like a ladder on a wall. A steeper line (ladder raised) indicates a greater change in position during the same time interval. That means he must be running faster!

Take a look at the graphs below. Which graph depicts slower motion?

http://www.physicsclassroom.com/Class/1DKin/U1L3a.html

I hope you picked the one on the right! That jogger would be covering less distance per time interval. Therefore, he must be moving slower.

Do you know how to find the value of the slope of a line? Rise over Run. If not, check out this website to learn.
http://www.wtamu.edu/academic/anns/mps/math/mathlab/int_algebra/int_alg_tut15_slope.htm

The value of the slope, (M), of a position-time graph can tell you the average speed or velocity of the object.
The sign of the slope tells you which direction the object is moving. The way that the slope of the line is calculated shows that the slope is the change in position divided by the time interval. The slope is the object's average velocity.

You can also determine how far an object travels, d_f, at a constant average speed. An object's initial position, d_i, its constant average speed or velocity, v_i, and the time, t, are related by a simple equation.

http://www.glenbrook.k12.il.us/gbssci/phys/Class/1DKin/U1L1d.html

http://www.hawaii.edu/suremath/k4_12dir/k4_12menu.html