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Observing Motion Motion: object’s change in position relative to a reference point. Displacement: change in position of an object. • must always indicate direction. • Distance measures path taken. Speed and Velocity Speed: distance traveled divided by time interval during which motion occurred. describes how fast an object moves. measurements involve distance and time. SI units: meters per second (m/s). constant speed : • when an object covers equal distances in equal amounts of time Speed and Velocity can be studied with graphs and equations. When an object’s motion is graphed by plotting distance on yaxis and time on xaxis, slope of graph is speed. Speed and Velocity Average speed: calculated as distance divided by time. distance speed = time Instantaneous speed: speed at a given point in time. d v= t d v t VELOCITY Metal stakes are sometimes placed in glaciers to help measure a glacier’s movement. For several days in 1936, Alaska’s Black Rapids glacier surged as swiftly as 89 meters per day down the valley. Find the glacier’s velocity in m/s. Remember to include direction. GIVEN: WORK: v=? d= 89 meters t = 1 day v = d/t d v t v = 89 m / 8.6 x 104 s v = 1.0 x 10-3 m/s down the valley VELOCITY Find the velocity in m/s of a swimmer who swims 110 m toward the shore in 72 s. GIVEN: WORK: v=? d= 110 meters t = 72 s v = d/t d v t v = 110 m / 72 s v = 1.5 m/s toward the shore Chapter 10 Speed and Velocity Velocity: speed of an object in a particular direction. Resultant velocity Combination different velocities Acceleration and Motion Acceleration: rate at which velocity changes over time. An object accelerates if its speed, direction, or both change. Uniform circular motion is constant acceleration. Chapter 10 Calculating Acceleration a = final velocity – initial velocity t a = acceleration Vf = final velocity Vi = initial velocity t = time SI units: meters per second per second (m/s/s) or m/s2. ACCELERATION A flowerpot falls off a second-story windowsill. The flowerpot starts from rest and hits the sidewalk 1.5 s later with a velocity of 14.7 m/s. Find the average acceleration of the flowerpot. GIVEN: a=? vf= 14.7 m/s vi = 0 m/s t = 1.5 s WORK: a = final velocity – initial velocity t a = 14.7 m/s – 0 m/s 1.5 s a = 9.8 m/s2 down ACCELERATION A turtle swimming in a straight line toward shore has a speed of 0.50 m/s. After 4.0 s, its speed is 0.80 m/s. What is the turtle’s average acceleration. GIVEN: a=? vf= 0.80 m/s vi = 0.50 m/s t = 4.0 s WORK: a = final velocity – initial velocity t a = 0.80 m/s – 0.50 m/s 4.0 s a = 0.075 m/s2 toward the shore Balanced and Unbalanced Forces Force: action exerted on a body in order to change body’s state of rest or motion. has magnitude and direction. net force • combination of all forces acting on an object. balanced forces: Objects either do not move or move at constant velocity. unbalanced force any change in an object’s state of motion or rest. Chapter 10 The Force of Friction Friction: force that opposes motion between two surfaces that are in contact. opposes applied force. Chapter 10 The Force of Friction Static friction: resists initiation of sliding motion between two surfaces that are in contact and at rest. is greater than kinetic friction. Kinetic friction: opposes movement of two surfaces that are in contact and are sliding over each other. TYPES OF KINETIC FRICTION Sliding friction • When objects slide past each other Fluid friction • When an object moves through a fluid (air, water…) and surface of object Rolling friction • When a round object rolls over a flat surface Chapter 10 Friction and Motion Friction can be helpful or harmful. necessary to roll a vehicle or hold an object. can also cause excessive heating or wear of moving parts. Harmful friction can be reduced. Helpful friction can be increased.