Post on 03-Dec-2014
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Two-Dimensional Motion and Vectors
Chapter 3 pg. 81-105
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What do you think?
How are measurements such as mass and volume different from measurements such as velocity and acceleration?
How can you add two velocities that are in different directions?
+Introduction to Vectors
Scalar - a quantity that has magnitude but no directionExamples: volume, mass,
temperature, speed
Vector - a quantity that has both magnitude and directionExamples: acceleration, velocity,
displacement, force
+Vector Properties
Vectors are generally drawn as arrows.Length represents the magnitudeArrow shows the direction
Resultant - the sum of two or more vectors
Make sure when adding vectors thatYou use the same unitDescribing similar quantities
+Finding the Resultant Graphically
Method Draw each vector in the proper
direction. Establish a scale (i.e. 1 cm = 2
m) and draw the vector the appropriate length.
Draw the resultant from the tip of the first vector to the tail of the last vector.
Measure the resultant.
The resultant for the addition of a + b is shown to the left as c.
+Vector Addition
Vectors can be moved parallel to themselves without changing the resultant. the red arrow represents the
resultant of the two vectors
+Vector Addition
Vectors can be added in any order. The resultant (d) is the
same in each case
Subtraction is simply the addition of the opposite vector.
Sample Resultant Calculation
A toy car moves with a velocity of .80 m/s across a moving walkway that travels at 1.5 m/s. Find the resultant speed of the car.
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3.2 Vector Operations
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What do you think?
What is one disadvantage of adding vectors by the graphical method?
Is there an easier way to add vectors?
+Vector Operations
Use a traditional x-y coordinate system as shown below on the right.
The Pythagorean theorem and tangent function can be used to add vectors. More accurate and less time-consuming than the
graphical method
+Pythagorean Theorem and Tangent Function
+Pythagorean Theorem and Tangent Function
We can use the inverse of the tangent function to find the angle.
θ= tan-1 (opp/adj)
Another way to look at our triangle
d2 =Δx2 + Δy2 dΔy
Δx
θ
+Example
An archaeologist climbs the great pyramid in Giza. The pyramid height is 136 m and width is 2.30 X 102 m. What is the magnitude and direction of displacement of the archaeologist after she climbs from the bottom to the top?
+Example
Given:
Δy = 136mwidth is 2.30 X 102 m for whole
pyramid
So, Δx = 115m
Unknown:
d = ?? θ= ??
+Example
Calculate:
d2 =Δx2 + Δy2
d = √Δx2 + Δy2
d = √ (115)2 +(136)2
d = 178m
θ= tan-1 (opp/adj)
θ= tan-1 (136/115)
θ= 49.78°
+Example
While following the directions on a treasure map a pirate walks 45m north then turns and walks 7.5m east. What single straight line displacement could the pirate have taken to reach the treasure?
+Resolving Vectors Into Components
+Resolving Vectors into Components
Component: the horizontal x and vertical y parts that add up to give the actual displacement
For the vector shown at right, find the vector components vx (velocity in the x direction) and vy (velocity in the y direction). Assume that that the angle is 35.0˚.
35°
+Example
Given: v= 95 km/h θ= 35.0°
Unknown vx=?? vy = ??
Rearrange the equations
sin θ= opp/ hyp or sin θ=vy/vopp=(sin θ) (hyp) or vy=(sin θ)(v)
cos θ= adj/ hyp or cos θ= vx/vadj= (cos θ)(hyp) or vx= (cos θ)(v)
+Example
vy=(sin θ)(v)
vy= (sin35°)(95)
vy= 54.49 km/h
vx= (cos θ)(v)
vx = (cos 35°)(95)
vx = 77.82 km/h
+Example
How fast must a truck travel to stay beneath an airplane that is moving 105 km/h at an angle of 25° to the ground?
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3.3 Projectile Motion
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What do you think?
Suppose two coins fall off of a table simultaneously. One coin falls straight downward. The other coin slides off the table horizontally and lands several meters from the base of the table. Which coin will strike the floor first? Explain your reasoning.
Would your answer change if the second coin was moving so fast that it landed 50 m from the base of the table? Why or why not?
+Projectile Motion
Projectiles: objects that are launched into the air tennis balls, arrows, baseballs, javelin
Gravity affects the motion
Projectile motion: The curved path that an object follows when
thrown, launched or otherwise projected near the surface of the earth
+Projectile Motion
Path is parabolic if air resistance is ignored
Path is shortened under the effects of air resistance
Components of Projectile Motion
As the runner launches herself (vi), she is moving in the x and y directions.
, cosx i iv v θ=
, siny i iv v θ=
+Projectile Motion
Projectile motion is free fall with an initial horizontal speed.
Vertical and horizontal motion are independent of each other. Vertically the acceleration is constant (10
m/s2 ) We use the 4 acceleration equations
Horizontally the velocity is constant We use the constant velocity equations
+Projectile Motion
Components are used to solve for vertical and horizontal quantities.
Time is the same for both vertical and horizontal motion.
Velocity at the peak is purely horizontal (vy = 0).
+Example
The Royal Gorge Bridge in Colorado rises 321 m above the Arkansas river. Suppose you kick a rock horizontally off the bridge at 5 m/s. How long would it take to hit the ground and what would it’s final velocity be?
+Example
Given: d = 321m a = 10m/s2
vi= 5m/s t = ?? vf = ??
REMEMBER we need to figure out :Up and down aka free fall (use our 4
acceleration equations)Horizontal (use our constant velocity
equation)
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Classroom Practice Problem (Horizontal Launch)People in movies often jump from buildings
into pools. If a person jumps horizontally by running straight off a rooftop from a height of 30.0 m to a pool that is 5.0 m from the building, with what initial speed must the person jump?
Answer: 2.0 m/s
+Projectiles Launched at an Angle
We will make a triangle and use our sin, cos, tan equations to find our answers
Vy = V sin θ
Vx = V cos θ
tan = θ(y/x)
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Classroom Practice Problem(Projectile Launched at an Angle)
A golfer practices driving balls off a cliff and into the water below. The edge of the cliff is 15 m above the water. If the golf ball is launched at 51 m/s at an angle of 15°, how far does the ball travel horizontally before hitting the water?
Answer: 1.7 x 102 m (170 m)