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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?

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Projectile motion can be described by the horizontal and vertical components of motion.

+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

+Components of Vectors

A ball’s velocity can be placed into into horizontal and vertical components.

+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.

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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

+5.4 Projectile Motion

Projectile motion can be separated into components.

a) Roll a ball along a horizontal surface, and its velocity is constant because no component of gravitational force acts horizontally.

5.4 Projectile Motion

Projectile motion can be separated into components.

a) Roll a ball along a horizontal surface, and its velocity is constant because no component of gravitational force acts horizontally.

b) Drop it, and it accelerates downward and covers a greater vertical distance each second.

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)