Transcript Friction

Friction, Gravity,
Projectiles, Air Resistance,
& Free Fall
Friction: The force that opposes motion
between two surfaces that rub against
each other (opposite direction of the movement)
4 types of friction:
Sliding, Rolling, Fluid, Static
Sliding: one surface slides on another.
Fluid: a surface gliding over liquid or gas.
Rolling: one surface rolling on another
Static: stationary/not moving
The force of friction is dependent
upon these 2 factors:
`
1.
types of surfaces involved
(rough, smooth, wet, etc.)
[ex: rough = more friction, smooth = less friction]
2.
how hard the surfaces push together (mass)
(tight together = high friction,
loose together = low friction)


Gravity: The force that pulls objects toward each other
The strength of the force of gravity is dependent upon
these 2 factors: mass and distance.
1. Masses of the objects involved
(larger = more, smaller = less)
2. Distance between the objects (closer = more, farther away = less)
F = Gm1m2
r2
(G is the universal gravitation constant of 6.6726 x 10-11 m3/kg∙s2)
(m1 and m2 are the masses of the bodies in kilograms)
(r2 is distance between the bodies in meters)

Universal Gravitation:
Gravity video
Inverse Square Law:
_1_
d2
Examples:
twice distance
_1_ = _1_
22
4
three times distance
_1_ = _1_
32
9


Weight: The force of gravity acting on the
mass of an object.

Weight changes dependent upon where
you are; however, mass remains the same.

Example: the moon has 1/6 the gravity of Earth, so a
120 lb person would only weigh 20 lbs on the moon;
however, they would wear the same size clothing.
Projectile: An object that is thrown or shot through the air.
Projectiles follow a curved path (parabola) due to gravity (& some
air resistance for certain objects)
fired & dropped bullet video
Satellites actually fall around the Earth due to gravity & inertia.
Planets orbit the Sun & moons orbit the planets due to gravity &
inertia

Air Resistance: The fluid friction experienced
by objects falling through the air

Free Fall: The motion of a falling object when the
only force acting on it is gravity
free fall video
free fall video 2

Objects in free fall accelerate at 9.8 m/s2 until
terminal velocity [maximum speed] is reached.
Misconceptions video


(v = at) to calculate speed of the fall
Ex. An object falls 4 seconds, its velocity is 39.2 m/s
9.8 m/s2 x 4s = 39.2 m/s
(x = ½at2) to calculate the distance of the fall
Ex. An object dropped from a bridge hits the water in 4
seconds, the height of the bridge is 78.4 m.
½(9.8 m/s2 ∙ 4s2 )
½ (9.8 x 16)
156.8 x 0.5
78.4 m
Acceleration of a falling object due to gravity
rounded off to 10 m/s2 (for easy math)
Time in
seconds
(s)
Acceleration
rate
(m/s2)
Final
velocity
for that
second
(m/s)
Constant
Constant ↑
0
10
0
0
0m
0m
1
10
10
5
5m
5m
2
10
20
15
15 m
20 m
3
4
Average
Distance
velocity
traveled
for that
for that
second
time (m)
period [d]
(m/s)
Constant ↑ Constant ↑
Total distance
traveled
(x)
previous x + d