# 10/18/13 - TeacherWeb

Mechanics

Nov 14, 2013 (4 years and 7 months ago)

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10/18/13

If you throw an object

what force/s are on
the object after it leaves your hand?

Only force is gravity (if ignoring air resistance)

Projectiles

Projectiles are any object moving through a
medium (usually the air) that is not supported
by a normal force and without any other
applied force

Jumping, free falling, hitting/kicking/throwing a
ball, etc

Are these projectiles

1.
Football being punted?

2.
Cat jumping out of a tree?

3.
A butterfly flying?

4.
Long distance jumping?

5.
A hot air balloon flying?

6.
Sky diving?

7.
Space ship launching?

Kinematics in Projectile Motion

Projectiles use the same formulas as before in
1D motion.

We just break them down into motion on the x,
the y, and the resultant.

Rules for Projectiles

1.
Ignoring air resistance

all objects fall with
the same acceleration (gravity)

2.
Horizontal motion does not effect vertical
motion

3.
Vertical is used as the timer (how long it can
stay in the air)

4.
Horizontal motion is constant if no air
resistance is used

5.
If the projectile returns to the same height

the time up = the time down

Try this

1.
An elephant and a feather fall in a vacuum

which falls faster?

2.
A bullet is fired and a bullet is dropped from
the same height at the same time

which
hits the ground first?

3.
A kid kicks a ball at an angle of 30 degrees

what is the acceleration in the horizontal?

Falling from rest

If an object falls from rest

use the kinematic
formulas that ignore initial velocity.

Example

A ball rolling 3.0 m/s across a table that is 1.0 m
off the ground:

1)
How long until it hits the ground? (t = ?)

2)
How far from the table will it hit?
(x = ?)

Today

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