# per3 - Chapter 3 Motion and Forces Goals of Period 3...

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33 Chapter 3: Motion and Forces Goals of Period 3 Section 3.1: To define speed and velocity as rates Section 3.2: To explain the cause of changing velocity – forces Section 3.3: To define changes in velocity - acceleration Section 3.4: To examine the acceleration of falling objects Section 3.5: To calculate the force required to accelerate an object Our Period 2 discussion of energy forms and conversions illustrated some of the many ways in which energy is used. Forces are responsible for energy. Period 3 introduces the concept of forces acting on objects to make them move. Since motion can involve either a fixed velocity or a change in velocity (acceleration), our explanation of forces includes a discussion of speed, velocity, and acceleration. 3.1 More about Rates: Speed and Velocity As explained in Period 1 , per means for each and designates a ratio. For example, 55 miles per hour (55 MPH) can be written as the fraction 55 miles/hour, which tells us how many miles (55) are traveled in one hour. This ratio is an example of a rate , a ratio that tells how fast or how often something occurs. For example, 25 miles per gallon (25 MPG) is a rate that tells how many miles we can travel on one gallon of gas. We begin the discussion of rates by carefully defining the quantities used. In physics words are used in very specific ways. Such words include distance, time, speed, velocity, and acceleration. Distance: We use the symbol D to represent the distance between two points. Distances are measured by comparing the length of the path between two points to standard lengths such as meters or feet. We assume that the length of a meter stick held in our hands is constant, so that a meter stick is one meter long no matter where we are in the Universe. Time: We may wish to know how much time has passed between events. To measure time, we need an event that repeats, such as the swing of a pendulum clock, where each swing marks a unit of time. Some clocks are more accurate than others. Atomic clocks, which use cesium atoms' radiation to mark units of time, are accurate to billionths of a second. We often use seconds as our standard unit of time. The symbol t represents the time elapsed between two events. Speed: Speed is a rate that measures the distance an object travels in a given time and can be written as: Speed = Distance traveled Time elapsed

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34 Using symbols to represent the quantities, t D s = where s = speed (in meters/second or miles/hour) D = distance traveled (in meters or miles) t = time elapsed (in seconds or hours) (Example 3.1) If you drive for 2 hours at a constant speed and travel 120 miles, what is your speed during the trip? t D s = Example 3.1 involved motion at a constant speed. In reality, few objects travel at a constant speed for long. In most real life examples, we use the average speed of an object as it travels between two points, as shown in example 3.2. (Example 3.2
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per3 - Chapter 3 Motion and Forces Goals of Period 3...

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