Chapter 2: Motion Section 1: Describing Motion

What is Motion? What is motion? How can we tell if something has moved or is moving?

Motion Motion occurs when an object changes position relative to a certain point Whenever an object moves, we compare that object to something in the background that is assumed to be not moving. The background is your frame of reference. Frame of Reference Animation Photo courtesy of: http://www.physicsclassroom.com/class/vectors/u3l2a.cfm

Describing Motion Motion can be described using the following: Distance Displacement Speed Velocity

Distance vs. Displacement Distance –how far an object has moved Measured in base units of meters Other Metric units: kilometers, centimeters, millimeters, etc. Other English units: feet, miles, inches, etc. Displacement – how far an object has moved from the starting point and the direction of the movement relative to the starting point Values should have units of length and a direction What is the runner’s distance travelled? What is the runner’s displacement?

SPEED vs. Velocity Speed–the distance an object travels in a given amount of time Objects usually do not travel at a constant speed so we use average speed to describe the motion of the object. Instantaneous speed is the speed of an object at a given point in time. (Speedometers measure instantaneous speed!) Values should include a unit of length per unit of time i.e. 20 meters/second Velocity– the displacement of an object in a given amount of time Values should include a unit of length per unit of time and a direction i.e. 20 meters/second north

Calculating Average Speed Speed is the rate of an object’s change in position A rate is any change over time To calculate average speed (s), you divide the total distance travelled (d) by the total time (t) it took to travel that distance avg. speed = 𝐝𝐢𝐬𝐭𝐚𝐧𝐜𝐞 𝐭𝐢𝐦𝐞 Because speed is a rate, the units should always have two parts: units of length over units of time i.e. meters per second (m/s), miles per hour (mi/hr)

Speed Problems For each problem: List the givens List the formula you will use Write the givens in your formula, including units Show all math calculations List appropriate units on your final answer If a plane traveled 1760m in 8 seconds, what was its average speed?

Speed Problems For each problem: List the givens List the formula you will use Write the givens in your formula, including units Show all math calculations List appropriate units on your final answer 2. If a car takes 25 seconds to travel 750 meters, what was its average speed?

Speed Problems For each problem: List the givens List the formula you will use Write the givens in your formula, including units Show all math calculations List appropriate units on your final answer 3. It takes you 15 minutes to walk to the grocery store; If you walk at an average speed of 4 miles/hour, how far away is it?

Speed Problems For each problem: List the givens List the formula you will use Write the givens in your formula, including units Show all math calculations List appropriate units on your final answer 4. A cruise ship is traveling at an average rate of 50km/hr. How long does it take to go 700 km?

Distance- Time Graphs Distance-time graphs show the motion of an object over a certain period of time X-axis: Time Y-axis: Distance We can use the slope (rise over run) of the line to determine the speed of the object A straight line indicates constant speed A horizontal line indicates that the object is not moving A curved line indicates changing speed

Distance- Time Graphs Practice 1. At which section(s) on the graph does the object have a constant speed? 2. At which section(s) on the graph does the object have a changing speed? 3. At which section(s) on the graph is the object stopped? 4. What is the average speed of the object for section “a”? 5. What is the total distance the object travelled?