4ExampleA car travels 60. km in 30. min, then turns around and travels 30. in ½ hour. What is the car’s speed over the entire trip? In km/hr? In m/s? What is the car’s average velocity in m/s over the trip?
5Acceleration A change in velocity (speed or direction!!) Rate of change of velocity (a=∆v/∆t)2 important quantities to identify from a v-t graph:The slope of a line (gives us acceleration)The area under a curve (gives us displacement)
6The other important equations….. vf = vi + at d = ½ (vf + vi)t vf2 = vi2 + 2ad d = vit + ½ at2
7An object accelerates uniformly from rest An object accelerates uniformly from rest. If the final velocity of the object after 4.7 s is 15 m/s east, what is it’s displacement?
91. A person initially at point P in the illustration stays there a moment and then moves along the axis to Q and stays there a moment. She then runs quickly to R, stays there a moment, and then strolls slowly back to P. Which of the position vs. time graphs below correctly represents this motion?
102. An object goes from one point in space to another 2. An object goes from one point in space to another. After it arrives at its destination, its displacement is:either greater than or equal toalways greater thanalways equal toeither smaller than or equal toalways smaller thaneither smaller or largerthan the distance it traveled.
113. A marathon runner runs at a steady 15 km/hr. When the runner is 7 3. A marathon runner runs at a steady 15 km/hr. When the runner is 7.5 km from the finish, a bird begins flying from the runner to the finish at 30 km/hr. When the bird reaches the finish line, it turns around and flies back to the runner, and then turns around again, repeating the back-and-forth trips until the runner reaches the finish line. How many kilometers does the bird travel?10 km15 km20 km30 km
12A train car moves along a long straight track A train car moves along a long straight track. The graph shows the position as a function of time for this train. The graph shows that the train:speeds up all the time.slows down all the time.speeds up part of the time and slows down part of the time.moves at a constant velocity
135. The graph shows position as a function of time for two trains running on parallel tracks. Which is true:At time tB, both trains have the same velocity.Both trains speed up all the time.Both trains have the same velocity at some time before tB.Somewhere on the graph, both trains have the same acceleration.
16You hold a ball in your hand at a fixed height and release it You hold a ball in your hand at a fixed height and release it. Its initial velocity is upzerodown
17You hold a ball in your hand at a fixed height and release it You hold a ball in your hand at a fixed height and release it. Its initial acceleration isupzerodown
184. If you drop an object in the absence of air resistance, it accelerates downward at 9.8 m/s2. If, instead, you throw it downward, its downward acceleration after release isless than 9.8 m/s2.9.8 m/s2.more than 9.8 m/s2.
19You are throwing a ball straight up in the air You are throwing a ball straight up in the air. At the highest point, the ball'svelocity and acceleration are zerovelocity is nonzero but its acceleration is zeroacceleration is nonzero, but its velocity is zerovelocity and acceleration are both nonzero
205. A person standing at the edge of a cliff throws one ball straight up and another ball straight down at the same initial speed. Neglecting air resistance, the ball to hit the ground below the cliff with the greater speed is the one initially thrownupwarddownwardneither-they both hit at the same speed
21Example….A rock is thrown with a velocity of 5.0 m/s downward from a cliff of height 60. m. How long does it take the rock to hit the ground? What is the rock’s speed when it hits the ground?
22ExampleAn object is launched directly upward with a velocity of 7.9 m/s.How long does it take to reach the top of the trajectory?What is the object’s velocity at t = 0.5 s? t = 3. s?What is the object’s hang time ( total time in the air)?What is the max height reached?
23Projectile MotionMotion though the air without propulsion Examples:
24Part 1. Motion of Objects Projected Horizontally
34What launch angle will produce the same range as…. a) 75o b) 52o c) 40o
35Acceleration is constant (g = -9.80 m/s2 [downward] ) vx is constant yAcceleration is constant (g = m/s2 [downward] )vx is constantHorizontal and vertical motions are independent of each other, but they have a common timeInitial velocity must be broken into it’s components!x
36X Uniform motion Y Accelerated motion Equations of motion:XUniform motionYAccelerated motionACCELERATIONax = 0ay = g = m/s2VELOCITYvx = vi cos Θvy = vi sin Θ + g tDISPLACEMENTx = vi t cos Θy = vi t sin Θ + ½ g t2
37Question:A projectile is launched with a velocity of m/s [35.0o N of E]Find:Time to the topMaximum heightTotal flight timeRangeWhat angle will produce the same range?
40Question:A projectile is launched from the top of a 60.0 m building with a velocity of 50.0 m/s at an angle of 30.0o with the horizontal.Find:Time to the topMaximum height (relative to the ground)Total flight timeRange
41Relative Velocity Always need a reference point Example: VAB Stated as: “Velocity of Object A relative to Object B”
441. A plane can travel with a speed of 80. km/hr with respect to the air. Determine the resultant speed (aka ‘ground speed’) of the plane if it encounters a 21 km/hr cross wind.
452. A motorboat traveling 6 m/s East across a river encounters a current traveling 3.8 m/s, South. a. What is the resultant velocity of the motor boat?b. If the width of the river is 120. meters wide, then how much time does it take the boat to travel shore to shore?c. What distance downstream does the boat reach the opposite shore?
46An airplane flies west at 300. km/hr. Wind blows North East at 100 An airplane flies west at 300. km/hr. Wind blows North East at 100. km/hr. What is the plane’s velocity relative to the ground?
47vresult = vplane + vwind An airplane flies west at 300. km/hr. Wind blows from the North East at 100. km/hr. What is the planes velocity relative to the ground?Remember:vresult = vplane + vwindSet up a table:VectorX ComponentY-ComponentPlaneWindResult
49The challenger….4. A pilot wishes to fly to a city that is directly 752 km East of her position. Her air speed is 195 km/h and there is a wind from the north of 73 km/h.What direction should she point the plane?What will be her ground speed?How long will it take to get there?
50Answers…Θ = 22o N of E195cos22o = km/hrt = 4.16 hr