Falling Objects

Falling Objects

• All objects, regardless of their mass, fall at 9.8 m/s2 near the surface of the earth.(This law excludes the effects of air resistance.)Ex. If you drop an object in a vacuum, it will be falling at:9.8m/s after one second,19.6m/s after two seconds,29.4m/s after three seconds,

etc…

Air Resistance

• The upward force of air on falling object.

Depends on the surface area of the object.

Think About: Falling with vs. falling without a parachute! Which situation has the most air resistance?

Falling Objects

Check Up:

Which falling object encounters the most air resistance?

a. an elephant b. a feather

Terminal Velocity

• The maximum speed reached by a falling object.

Occurs when the upward push of air resistance equals the downward pull of gravity.

At this point, the object stops accelerating (no net force acts upon it) and falls at a constant speed to the ground.

Fair

Fg

Terminal Velocity

• Depends upon the weight of the object and the amount of air resistance acting upon it.

• The greater the weight of the object, the longer it will accelerate before reaching terminal velocity… therefore, the greater the terminal velocity.

Terminal Velocity

Check Up:

Which falling object is more effected by air resistance?

a. An elephant b. a feather

Calculations

• To calculate the speed of a falling object… (ignoring the effects of air resistance) multiply the acceleration of the object by the number of seconds it has been falling.

V = gt

v = velocity

t = time

g = accel. due to Fg (9.8m/s2)

Try a Sample Problem

• How fast will an object be traveling if it has been falling for 5 seconds?

• Givens: t = 5s

g = 9.8 m/s2

• Equation: v = g x t • Plug-in: v = 5s x 9.8 m/s2

• Answer: 49 m/s :Units

(Notice that s in the numerator will cancel one of the s in the denominator, and leave us with m/s.

Let’s Mix it up a Little…

• If a falling object is traveling at 75 m/s, how long has it been falling?

• Givens: v = 75 m/sg = 9.8 m/s2

• Equation: t = v/g• Plug-In: t = 75 m/s

9.8 m/s2

• Answer: 7.7 s (m/s units cancel…..)