1. Free Fall

2. Falling on the Earth
Items that start by going up, stop and then fall again, reversing direction.
More height more speed
The velocity changes, there must be an acceleration.

3. Surface Gravity
At the surface of the Earth, gravity creates a nearly constant acceleration.
This gravitational acceleration is called g.
g = 9.8 m/s2 and is directed downward.
Note that g is an acceleration.
Don’t be fooled by phrases like g-force.
g can be used as a unit of acceleration.

4. Pulling g’s
A top fuel dragster can reach 150 m/s (330 mph) in 3.7 s. What is the acceleration in g’s?
The acceleration is v/t.
a = 40. m/s2
Convert units.
a = 4.0 g
Credit: Lt. Col. W. Thurmond, U.S. Army

5. Up and Down The constant acceleration equations apply for a = -g.
The position is now y, in the vertical direction. v t
Initial upwards velocity v0
Negative slope = -g y t y0
Velocity here is –v0

6. Initial Velocity
A person throws a ball up in the air releasing the ball at 1.5 m. If it lands on the ground after 2.0 s, find the initial velocity.
Simplify the time equation, with
y0 = 1.5 m
y = 0 m
t = 2.0 s.
v0 = 9.1 m/s

7. Exceptions to Free Fall
On Earth’s surface downward acceleration is constant.
Why do we observe that different objects accelerate differently?
Friction reduces the total acceleration.
We think about velocity instead of acceleration.

8. Pulley Acceleration Consider two masses linked by a pulley
m2 is pulled by gravity
m1 is pulled by tension
frictionless surface
The normal force on m1 equals the force of gravity.
The force of gravity is the only external force on m2.
Both masses must accelerate together. FT m1 FT m2
Fg = m2 g

9. Atwood’s Machine
In an Atwood machine both masses are pulled by gravity, but the force is unequal.
The heavy weight will move downward at
( kg)(9.8 m/s2)/( kg) = 1.8 m/s2.
Using y = (1/2)at2, it will take t2 = 2(1.80 m)/(1.8 m/s2)
t = 1.4 s.