Motion :
Motion Slide 2:
Some Motion Terms Distance & Displacement
Velocity & Speed
Acceleration
Uniform motion
Scalar .vs. vector Scalar versus Vector :
Scalar versus Vector Scalar - magnitude only (e.g. volume, mass, time)
Vector - magnitude & direction (e.g. weight, velocity, acceleration) Pictorial Representation :
Pictorial Representation An arrow represents a vector
Length = magnitude of vector
Direction = direction of vector Pictorial Representation :
Pictorial Representation This arrow could represent a vector of magnitude 10 point to the “right”
This arrow could represent a vector of magnitude 5 point to the “left” Distance & Displacement :
Distance & Displacement Distance is the actual distance traveled.
Displacement depends only on Start & Finish line
Displacement is the distance traveled , in a certain direction. Displacement Isn’t Distance :
Displacement Isn’t Distance The displacement of an object is not the same as the distance it travels
Example: Throw a ball straight up and then catch it at the same point you released it
The distance is twice the height
The displacement is zero Distance & Displacement :
Distance & Displacement Distance & Displacement :
Distance & Displacement B A C 5 m 4 m 3 m You walk from A to B to C.
Your distance traveled is 7m
Your displacement form A is 5 m Velocity & Speed :
Velocity & Speed Velocity is the displacement traveled in a certain time.
Speed is the distance traveled in a certain time.
Velocity is speed in a given direction. Slide 11:
Instantaneous Speed is the speed at any specific instance
Average Speed is the total distance covered divided by total time Types of Speed Speed :
Speed The average speed of an object is defined as the total distance traveled divided by the total time elapsed
Speed is a scalar quantity Velocity :
Velocity The average velocity of an object is defined as the total displacement traveled divided by the total time elapsed
Velocity is a vector quantity Speed, cont :
Speed, cont Average speed totally ignores any variations in the object’s actual motion during the trip
The total distance and the total time are all that is important
SI units are m/s Velocity :
Velocity It takes time for an object to undergo a displacement
The average velocity is rate at which the displacement occurs
generally use a time interval, so let ti = 0 Velocity continued :
Velocity continued Direction will be the same as the direction of the displacement (time interval is always positive)
+ or - is sufficient
Units of velocity are m/s (SI), cm/s (cgs) or ft/s (US Cust.)
Other units may be given in a problem, but generally will need to be converted to these Speed vs. Velocity :
Speed vs. Velocity Cars on both paths have the same average velocity since they had the same displacement in the same time interval
The car on the blue path will have a greater average speed since the distance it traveled is larger Speed vs. Velocity :
Speed vs. Velocity You drive from Yakima to Seattle (140 miles away)
You stop in Ellensburg for a 2 hr lunch with a friend.
Your total driving time is 2 hours Uniform Velocity :
Uniform Velocity Uniform velocity is constant velocity
The instantaneous velocities are always the same
All the instantaneous velocities will also equal the average velocity Velocity Example :
Velocity Example How fast is the plane moving in respect to the ground? :
How fast is the plane moving in respect to the ground? Velocity again How fast is the plane moving in respect to the ground? :
How fast is the plane moving in respect to the ground? Velocity, yet again How fast is the plane moving in respect to the ground? :
How fast is the plane moving in respect to the ground? Velocity (finally) How fast is the plane moving in respect to the ground? :
How fast is the plane moving in respect to the ground? Velocity again (??) How fast is the plane moving in respect to the ground? :
How fast is the plane moving in respect to the ground? Velocity - the last time How fast is the plane moving in respect to the ground? :
How fast is the plane moving in respect to the ground? (Last) Velocity… Acceleration :
Acceleration Change in velocity divided by the change in time Acceleration :
Acceleration Changing velocity (non-uniform) means an acceleration is present
Acceleration is the rate of change of the velocity
Units are m/s2 (SI), cm/s2 (cgs), and ft/s2 (US Cust) Average Acceleration :
Average Acceleration Vector quantity
When the sign of the velocity and the acceleration are the same (either positive or negative), then the speed is increasing
When the sign of the velocity and the acceleration are in the opposite directions, the speed is decreasing Instantaneous & Uniform Acceleration :
Instantaneous & Uniform Acceleration The limit of the average acceleration as the time interval goes to zero
When the instantaneous accelerations are always the same, the acceleration will be uniform
The instantaneous accelerations will all be equal to the average acceleration Relationship Between Acceleration & Velocity :
Relationship Between Acceleration & Velocity Uniform velocity (shown by red arrows maintaining the same size)
Acceleration equals zero Relationship Between Velocity & Acceleration :
Relationship Between Velocity & Acceleration Velocity and acceleration are in the same direction
Acceleration is uniform (blue arrows maintain the same length)
Velocity is increasing (red arrows are getting longer)
Positive velocity and positive acceleration Relationship Between Velocity & Acceleration :
Relationship Between Velocity & Acceleration Acceleration and velocity are in opposite directions
Acceleration is uniform (blue arrows maintain the same length)
Velocity is decreasing (red arrows are getting shorter)
Velocity is positive and acceleration is negative Kinematic Equations :
Kinematic Equations Used in situations with uniform acceleration Kinematic Equations - Ex #1 :
Kinematic Equations - Ex #1 A car traveling with an initial velocity of 6 m/s, accelerates at 2 m/s2, for 6 seconds. What is the car’s final velocity? Kinematic Equations - Ex #1 - Ans :
Kinematic Equations - Ex #1 - Ans A car traveling with an initial velocity of 6 m/s, accelerates at 2 m/s2, for 6 seconds. What is the car’s final velocity? Galileo Galilei :
Galileo Galilei 1564 - 1642
Galileo formulated the laws that govern the motion of objects in free fall
Also looked at:
Inclined planes
Relative motion
Thermometers
Pendulum Free Fall :
Free Fall All objects moving under the influence of gravity only are said to be in free fall
Free fall does not depend on the object’s original motion
All objects falling near the earth’s surface fall with a constant acceleration
The acceleration is called the acceleration due to gravity, and indicated by g Acceleration due to Gravity :
Acceleration due to Gravity Symbolized by g
g = 9.81 m/s2
g is always directed downward
toward the center of the earth
Ignoring air resistance and assuming g doesn’t vary with altitude over short vertical distances, free fall is constantly accelerated motion Free Fall – an object dropped :
Free Fall – an object dropped Initial velocity is zero
Let up be positive
Use the kinematic equations
Generally use y instead of x since vertical
Acceleration is g = -9.81 m/s2 vo= 0
a = g Free Fall – an object thrown downward :
Free Fall – an object thrown downward a = g = -9.81 m/s2
Initial velocity ≠ 0
With upward being positive, initial velocity will be negative vo 0
a = g Free Fall - example :
Free Fall - example If a rock is dropped from a building, and it takes 18 seconds to reach the ground, how tall is the building? Free Fall - answer :
Free Fall - answer What do we know? Free Fall - answer :
Free Fall - answer Motion :
Motion The End