Special Cases of Force

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# Special Cases of Force - PowerPoint PPT Presentation

Special Cases of Force. Projectile Motion. Projectile Motion. A projectile is any object which once a force is used to throw it, hit it , propel it in some fashion, no other force acts on the object except for gravity. Projection can be horizontal, with no initial vertical velocity

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### Special Cases of Force

Projectile Motion

Projectile Motion
• A projectile is any object which once a force is used to throw it, hit it , propel it in some fashion, no other force acts on the object except for gravity.
• Projection can be
• horizontal, with no initial vertical velocity
• vertical, with some initial vertical velocity
This path is a parabola.

If air resistance is negligible then only gravity

affects the path (trajectory) of a projectile.

Characteristics of Projectile Motion
• The motion’s dimensions are vectors.
• The projectile will move along it’s trajectory (path) in both the horizontal (x) direction and the vertical (y) direction at the same time.
• The two directions are independent of each other.
• Only time is the same between the horizontal and vertical motion. It is, after all, only one object.
Independent Motions

Horizontal

Vertical

Real Motion is the Combination of the Horizontal and Vertical Motions

The blue dot show the real motion

The path taken is the trajectory

This is horizontal projection

Initial Velocity – V Along the Trajectory
• Angular projection - The initial velocity is the resultant of adding the two vector quantities together.
• The projection includes an angle of projection
Initial Velocity has an X and Y Component
• The vertical component and the horizontal component are independent of each other.
Vertical velocity = 0

Positive velocity

gets smaller

Negative velocity

gets larger

Horizontal and Vertical Components of Velocity

Vertical velocity decreases at a constant rate

due to the influence of gravity.

It becomes zero.

Then increases in the negative direction

ViCalculating Components

You have learned to calculate components of a vector when we looked at inclined planes.

The components are calculated by using the trig functions.

The initial velocity acts as the hypotenuse of the right triangle.

Vi

Vy

Vx

Continued Calculation
• The vertical velocity and the horizontal velocity are the legs of the triangle.

Calculate the Vy using sin = o/h

Calculate Vx using cos = a/h

Displacement

Time

Horizontal Displacement of Projectiles
• Horizontal projection or projection at an angle
• graph of horizontal displacement v time

As time progresses, the object gets further and further from its starting point.

Height

Time

Vertical Displacement of Projectiles
• Vertical Projection and Projection at an Angle
• graph of vertical displacement (height) v time

Height

Time

Vertical Displacement of Projectiles
• Horizontal projection
• Graph of vertical displacement (height) vs time

The object leaves a horizontal surface and fall to the ground.

Velocity

Time

Horizontal Velocity of Projectiles
• Horizontal Projection and Projection at an Angle

The horizontal velocity of a projectile remains constant from the time it is projected until gravity brings it to the ground.

Remember: We are using an ideal situation where there is no air resistance

Velocity

0

Time

Vertical Velocity of Projectiles
• Vertical Projection and Projection at an Angle

For objects projected directly upward or projected at some angle above the ground, the vertical velocity must begin positive, decrease to zero, and then increase in the negative direction. (Remember, gravity is negative)

Velocity

0

Time

Vertical Velocity of Projectiles
• Horizontal projection

Horizontal projection begins with an initial vertical velocity of zero.

The vertical velocity then increases in a negative direction.

Acceleration

Time

Horizontal Acceleration of Projectiles
• Since we are idealizing the projection, we do not take into account any air resistance.
• We can, therefore, say there is no horizontal acceleration.
Acceleration

0

-9.8

Vertical Acceleration of Projectiles
• Vertical acceleration is the result of the pull of gravity, (-9.8 m/s2)
• This is the same on the way up and on the way down.

Time

Important Notes on Vertical and Angular Projection
• The range (x) is the farthest the object will travel horizontally.
• The maximum height (ymax ) is the farthest the object will travel vertically.
• Y equals zero when it is at its lowest point.
Determining the Range

You can determine the displacement (range) of a projectile any any point along the trajectory.

X = horizontal distance (range)

vx = horizontal velocity

t = time

Determining the Height
• You can determine the height (y) at any point in the trajectory!

y = vertical displacement

vy = initial vertical velocity

g = acceleration due to gravity

t = time

Other Important Notes on Vertical and Angular Projection
• At the highest point of the trajectory, it is the exact midpoint of the time.
• It takes the projectile half of the time to get to the top.
• When the projectile gets to the top, it has to stop going up and start going down, so the velocity in the y-direction at the highest point is zero for a split second.
• As the projectile falls, it is in free fall.
3 Primary Factors Affecting Trajectory
• Projection angle
• aka release angle or take-off angle
• Projection velocity
• aka initial or take-off velocity
• Projection height
• aka above or below landing
Projection Angle
• The optimal angle of

projection is dependent on

the goal of the activity.

• For maximum height, the optimal angle is 90o.
• For maximum distance, the optimal angle is 45o.
The effect of Projection angle on the Range of a projectile

10 degrees

30 degrees

40 degrees

45 degrees

60 degrees

75 degrees

The angle that maximizes Range is = 45 degrees

40

30

20

10

0

80

100

0

10

20

30

40

50

60

90

70

• 10 m/s @ 45 degrees Range ~ 10 m
• 20 m/s @ 45 degrees Range ~ 40 m
• 30 m/s @ 45 degrees Range ~ 90 m
Projectile Problems
• Ignore air resistance.
• ay = g = -9.81 m/s2
• Set up the two dimension separately

Origin xOrigin y

Positive x Positive y

xi = constant yi =

vxi = vyi =

ax = 0 ay = g

Projectile Problems – Two Dimensional Kinematics
• Write general kinematic equations for each direction
• Rewrite them for the problem at hand
• Find the condition that couples the horizontal and vertical motions (usually time)
Equations of Constant Acceleration

This is the only equation to use for the horizontal part of the motion

• x = vxt
• d = ½ (vyf + vyi)t
• d = vyit + ½ gt2
• vvf2 = vvi2 + 2gd

These 3 equations are for the vertical part of the motion

The Monkey and the Banana

A zookeeper must throw a banana to a monkey hanging from the limb of a tree. The monkey has a habit of dropping from the tree the moment that the banana is thrown. If the monkey lets go of the tree the moment that the banana is thrown,will the banana hit the monkey?

Banana’s

Gravity free path

Monkey’s

Gravity free path is “floating” at height of limb

Fall thru same height

It works! Since both banana and monkey experience the same acceleration each will fall equal amounts below their gravity-free path. Thus, the banana hits the monkey.

Homework
• Chapter 6 read to page 152
• Problems: page 164, # 33,35,51,52,53,56
• 57,60.