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Physics Ch. 3
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Gravity
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As you read this in your chair, how fast are you moving relative to the chair? Relative to the Sun?
You are not moving relative to the chair, but you are moving over 100,000 km/h relative to the Sun.
What two units of measurement are necessary for describing speed?
Distance and time
What kind of speed is registered by an automobile speedometer?
Instantaneous speed
What is the average speed in kilometers per hour of a horse that gallops a distance of 15 km in a time of 30 min?
30 km/hr
How far does a horse travel if it gallops at an average speed of 25 km/h for 30 min?
12.5 km
What is the main difference between speed and velocity?
Velocity includes a direction, but speed does not.
If a car moves with a constant velocity, does it also move with a constant speed?
Yes, because constant velocity requires constant speed in the same direction.
If a car is moving at 90 km/h and it rounds a corner, also at 90 km/h, does it maintain a constant speed? A constant velocity?
It maintains a constant speed, but does not maintain a constant velocity.
What is the acceleration of a car moving along a straight road that increases its speed from 0 to 100 km/h in 10 s?
10 km/h·s
What is the acceleration of a car that maintains a constant velocity of 100 km/h for 10 s?
0 km/h·s
When are you most aware of your motion in a moving vehicle: when it is moving steadily in a straight line or when it is accelerating? If you were in a car that moved with absolutely constant velocity (no bumps at all), would you be aware of the motion?
Accelerating. You would not be aware of the motion if you did not look outside the car.
Acceleration is generally defined as the time rate of change of velocity. When can it be defined as the time rate of change of speed?
When moving in a straight line, the time rate of change of speed is acceleration.
What did Galileo discover about the amount of speed a ball gained each second when rolling down an inclined plane? What did this say about the ball's acceleration?
The ball had constant acceleration, so it gained the same speed every second.
What relationship did Galileo discover about a ball's acceleration and the steepness of an incline? What acceleration occurs when the plane is vertical?
The steeper the incline, the greater the acceleration. On a vertical incline, the ball is in free fall.
What exactly is meant by a "freely falling" object?
An object in free fall falls under gravity alone with no friction.
What is the gain in speed per second for a freely falling object?
It gains approximately 10 m/s.
What is the speed acquired by a freely falling object 5 s after being dropped from a rest position? What is the speed 6 s after?
The speed is 50 m/s after 5 s and 60 m/s after 6 s.
The acceleration of free fall is about 10 m/s2. Why does the seconds unit appear twice?
The second appears once in the denominator of the unit for speed and once for the interval of time.
When an object is thrown upward, how much speed does it lose each second (ignoring air resistance)?
It loses 10 m/s of speed each second until it reaches the high point, then it gains 10 m/s each second.
What relationship between distance traveled and time did Galileo discover for freely falling objects released from rest?
Galileo discovered that distance increased as the time squared.
What is the distance fallen for a freely falling object 1 s after being dropped from a rest position? What is the distance for a 4-s drop?
It falls 5 m in 1 s and 80 m in 4 s.
What is the effect of air resistance on the acceleration of falling objects?
Air resistance decreases the acceleration of falling objects.
Given the measurements 10 m, 10 m/s, and 10 m/s2, which is a measure of speed, which is a measure of distance, and which is a measure of acceleration?
10 m is distance, 10 m/s is speed, and 10 m/s2 is acceleration.
What is the speed over the ground of an airplane flying at 100 km/h relative to the air caught in a 100-km/h right-angle crosswind?
141 km/h
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