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Lecture 3

PHY 101 Lecture 3: PHY 101 Chapter 3

Course Code
PHY 101

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Chapter 3 Notes
Motion is Relative
o Everything in the universe is moving
o When we talk about the speed of something, we are talking about it relative to the
surface of the Earth.
o Speed = Distance / Time
o A cyclist who covers 16 meters in 2 seconds has a speed of 8 meters per second
o Common units
Meters per second
Miles per hour
Km per hour
Meters per minute
o Instantaneous Speed
The speed at any instant in time
A speedometer in a car shows the instantaneous speed of the car
o Average Speed
Average speed = Total distance covered / Total time interval
We travel 80 km in 1 hour our average speed is 80km/hr. If we go 320km in 4
hours, the average speed is still 80km/h
Total Distance Covered
o Total distance covered = average speed X time interval
o When we know both the speed and the direction of motion of an object
o A car traveling at 60km/h is its speed
o A car traveling at 60km/h north is its velocity
o Velocity is a vector quantity since it has a magnitude and a direction
o Constant velocity
Something with a constant velocity is something that is not speeding up or
slowing down, and it also does not change direction
o Changing velocity
Change the speed
Change the direction
o How quickly and in what direction velocity changes
o Acceleration = change of velocity / time interval
o When an object is slowing down, it is decelerating
o When an object is speeding up it is accelerating
o When an object is reversing direction, the acceleration becomes negative
o When an object is moving at a constant velocity, or at rest it has an acceleration of 0
Acceleration on Galileo's inclined planes
o Galileo measured speed and acceleration using inclined planes and a ball
o The steeper the inclined plane was, the faster the ball would accelerate as it went
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