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Motion & Forces Describing Motion Motion Speed & Velocity Newton’s First Law Newton’s First Law of Motion An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net force. constant velocity net force motion Motion Problem: Is your desk moving? We need a reference point... nonmoving point from which motion is measured Motion Motion Change in position in relation to a reference point. Reference point Motion Relative Motion If you are sitting in a chair reading this sentence, you are moving. You are not moving relative to your desk or your school building, but you are moving relative to the other planets in the solar system and the Sun. Motion • Distance • Distance describes how far an object has moved. • The SI unit of length or distance is the meter (m). Longer distances are measured in kilometers (km). • Shorter distances are measured in centimeters (cm). Motion • Displacement is the distance and direction of an object's change in position from a reference point. • Suppose a runner jogs to the 50-m mark and then turns around and runs back to the 20-m mark. • The runner travels 50 m in the original direction (north) plus 30 m in the opposite direction (south), so the total distance she ran is 80 m. Speed & Velocity Speed rate of motion distance traveled per unit time d s t distance speed time Calculating Speed The SI unit for distance is the meter and the SI unit of time is the second (s), so in SI, units of speed Sometimes it is more convenient to express speed in other units, such as kilometers per hour (km/h). Speed & Velocity Instantaneous Speed speed at a given instant Speed odometer reading Average Speed the total distance traveled divided by the total time of travel total distance avg. speed total time Changing Instantaneous Speed When something is speeding up or slowing down, its instantaneous speed is changing. If an object is moving with constant speed, the instantaneous speed doesn't change. To determine direction you need to know the velocity Speed & Velocity Problem: A storm is 10 km away and is moving at a speed of 60 km/h. Should you be worried? It depends on the storm’s direction! Speed & Velocity Velocity speed in a given direction can change even when the speed is constant! Calculations Your neighbor skates at a speed of 4 m/s. You can skate 100 m in 20 s. Who skates faster? GIVEN: d = 100 m t = 20 s s=? d s t WORK: s=d÷t s = (100 m) ÷ (20 s) s = 5 m/s You skate faster! Calculations Sound travels 330 m/s. If a lightning bolt strikes the ground 1 km away from you, how long will it take for you to hear it? GIVEN: WORK: s = 330 m/s t=d÷s d = 1km = 1000m t = (1000 m) ÷ (330 m/s) t=? t = 3.03 s d s t Graphing Motion Distance-Time Graph slope = speed A B steeper slope =faster speed Straight line = no motion flat line = constant speed Single point = instantaneous speed Graphing Motion Distance-Time Graph A B Who started out faster? A (steeper slope) Who had a constant speed? A Describe B from 10-20 min. B stopped moving Find their average speeds. A = (2400m) ÷ (30min) A = 80 m/min B = (1200m) ÷ (30min) B = 40 m/min Graphing Motion Distance-Time Graph 400 Distance (m) 300 200 100 0 0 5 10 Time (s) 15 20 Acceleration is indicated by a curve on a Distance-Time graph. Changing slope = changing velocity Graphing Motion Specify the time period when the object was... slowing down 5 to 10 seconds speeding up 0 to 3 seconds Speed-Time Graph 3 Speed (m/s) 2 1 0 0 2 4 6 Time (s) 8 10 moving at a constant speed 3 to 5 seconds not moving 0 & 10 seconds