Motion: Speed & Acceleration Welcome to Physics

Main Principal #1: Physics is the study of moving objects! We want to be able to predict how objects will move in all situations

But here’s the problem: Q: How do you know if an object is actually “moving”? A: When it’s “changing position” (travelling a certain distance)? If a person is “standing still”, we assume he/she is “not moving” But what about the planet that person is standing on? It is moving around a star!!! Is the person therefore “moving” anyway? Oh boy; welcome to physics!!! So how do we tell if something really is “moving”?

We use a principal called: “Point of Reference” “PoR” means: what appears to be moving to you? It can be tricky to really know what is moving and what isn’t! Point out the difference of distance The hands on a clock don’t seem to move; but they are constantly moving very slowly! Are the passengers in a boat moving compared to a person standing on the shore? Or is the person standing on the shore moving compared to the boat in the water? Fwew! It can be tricky!

Main Principal #2 & 3: #2: Basically, (if you really think about it) all objects are moving some amount, all the time. #3: We compare the motion of the object that appears to move to a “stationary” object (one that doesn’t appear to move). An object is moving “slowly” if it moves a short distance in a long time. An object is moving “quickly” if it moves a long distance in a short time. There are several ways we use to describe the motion of an object:

“Velocity” and “Speed” “Speed” means distance travelled in a certain amount of time “Velocity” is the speed in a certain direction Think: What are two ways to increase speed (go faster)? Increase distance traveled in the same amount of time Decrease the time it took to go the original distance ***Since they are almost the exact same thing, “Speed” and “Velocity” are used for the same motion Formula for Speed: Speed =

Graphing Velocity What you want to be able to do is Graphs can help us Interpret what has actually happened! The shape of the line and steepness of the “slope” can tell us what has happened and how fast it occurred! That’s right! We are NOT done with graphs!!! We are only just beginning!!! What you want to be able to do is know what the graph is telling you by looking at the line… This is NOT the graph for your notes...

Interpreting Velocity on a Graph Draw this graph in your notes… Interpreting Velocity on a Graph You can use the slope of a line to find Velocity 15 m More Distance In Less Time (Faster) Distance 10 m 5 m Less Distance In More Time (Slower) 5 s 10 s 15 s Time Think of the Velocity graph this way: Which one of these balls will fall faster? The graph with more steepness is the faster velocity. The less steep, the slower.

Interpreting Speed on a Graph: Q: What should you learn from this? A: Steep lines on motion graphs, mean an object is moving quickly! Non-steep lines mean the object is moving slowly! As a group, describe how fast the object is probably moving as you travel along the line (left to right) (After you try it together, click to see the real motion) Interpreting Speed on a Graph: Q: HOW CAN YOU TELL?!?! It can help to think about the speed of a rolling ball on the slope 15 m Now it’s moving fast! Distance 10 m This ball would roll fast! This ball would roll slowly… 5 m Started slowly… Stopped moving! This ball won’t roll at all! 5 s 10 s 15 s Draw this graph in your notes… Time

What should we learn? WATCH YOUR VARIABLES CAREFULLY! The two lines look different, but still describe the same objects motion! Graphing Speed: Let’s dim that last line but keep it here to compare to the new line… This is basically the line of the data you see in the table… It all can change if you pick different variables to graph! But this graph is not the whole story!!! Time (min) Dist. (meters) 1 2 3 4 5 Speed 100 m/min 100 220 110 m/min Speed (m/min) Distance (m) 300 100 m/min 430 107 m/min 500 100 m/min Now we graph the line of this data… Time (min)

Houston; um…Where are we going? Acceleration! Houston; um…Where are we going?

Think: How would you describe the motion of floating balloons? The movement of a balloon is a great example of an object Accelerating! Balloons just kinda move all over the place. Sometimes they go straight up, sometimes they fly more sideways. CRAZY! Think: How would you describe the motion of floating balloons? Do they move in a “constant” speed or direction? (“constant” means: “staying the same”)

What is “Acceleration”? You probably think it simply means to “speed up.” Well yes, and a bit more: A: The rate at which velocity changes! In other words: How fast something speeds up, slows down, or even changes direction. Speeding up: Slowing down: Changing direction:

Acceleration = A change in Velocity You already learned the Velocity equation: Here is the equation for Acceleration: Like this: = Final Velocity – Start Velocity Final Time – Start Time ____________________ So how do you do that? Means “change”

Interpreting Acceleration on a Graph Compare it to the line of an object with a changing Velocity = Acceleration! Notice the downward curve 15 m/s It is a nice straight line! Distance Velocity 10 m/s This is the line of an object with an “constant velocity” Curved lines mean a change has occurred = Acceleration! 5 m/s Straight lines on a graph mean “constant” It is also possible to have an object with a negative acceleration We can call this “Deceleration” 5 s 10 s 15 s Time Notice also, the different Y-axis variable!

Acceleration Graphing Practice Time Dist. Acc. 1 2 3 4 5 Think: Why is the acceleration staying the same if the car is speeding up more and more? 2 m 2 m/s2 8 m 2 m/s2 18 m 2 m/s2 32 m 2 m/s2 50 m 2 m/s2 Now try and graph both the Speed line AND the Acceleration line. Compare them! Do this on the very back page of your packet!