Chapter 2 Linear Motion What is linear motion

Chapter 2 Linear Motion

What is linear motion? u Motion- change in position over time u linear motion- motion in a straight line u or motion in 1 dimension (1 -D)

u Is How do you determine if something is moving? a person sitting still in a car going 35 m/s moving? u Are you moving now? u Earth orbits at 30 km/sec, and revolves at 266 mph u What about a character on a video game that is apparently running across the TV? u In order to determine if something is moving you have to compare it to something that is considered to be not moving or stationary.

Frame of reference u Whatever you compare it to is called your frame of reference. u The most common frame of reference is the Earth. u *there is no such thing as a perfect frame of reference, everything is moving.

Motion is always relative to something. u Using the person in a car going 30 m/s. Describe the motion from each frame of reference. – The Earth the person is moving fast (30 m/s) – The car’s interior the person is not moving – To another car going 31 m/s the person is slowly moving backwards (1 m/s)

Vector vs. Scalar Units Scalar unit- is any unit that doesn’t have a direction- (total distance you walked to this class, walking up stairs, speed on speedometer, directions from mapquest) u Vector Unit- Any unit that has a constant direction associated with it (how high is the 2 nd floor, straight line on a map) u Scalars Vectors Units u Distance Displacement meters (m) u Speed Velocity m/s u

We will use all metric units u It is significantly easier if we do. u A meter is a little more than a yard. u For speed, m/s is just under half of miles per hour. u 68 mph = 30 m/s

What is speed? Speed- how fast something is moving u or how much distance is covered in a certain amount of time u There are two types of speed we will discuss. u Instantaneous speedu An object’s speed at any given instant. u Average speedu An average of all instantaneous speeds u

To calculate average speed u average us speed = total distance/time = d/t u If you cover 75 m in 3 sec, how fast are you going? u 25 m/s u If you travel 20 m/s for 4 m, how long did it take you? u. 2 s

Velocity u Velocity is the same as speed, but it has a direction associated. u Can something have a constant speed but a changing velocity? u Yes, all it has to do is turn.

Negative velocity and speed u Is it possible for something to have a negative velocity? u Yes, it means it is going backwards. u -5 m/s N is the same as 5 m/s S. u Is it possible for something to have a negative speed? u No, there is no specific direction u Backwards depends on your frame of reference

Combining vectors in 1 -D u simply add them together. u If you walk 5 m N, 2 m S and 3 m N, what distance have you walked and what is your displacement. u distance- 10 m u displacement- 6 m N u If you are walking towards the back at 1 m/s in a bus that is moving 10 m/s… u you are moving 9 m/s (forward) u *note* you cannot combine speeds

Acceleration u Acceleration- change in velocity over time. u to calculate acceleration: a= vf – vi (f and i mean final t and initial) The units of acceleration are m/s/s or m/s 2

Negative Acceleration u What does a negative acceleration mean? u The object is slowing down. This is also sometimes called deceleration. u It does NOT necessarily mean the object is going backwards (yet)