Kinematics/Free Fall

By Dan Gallagher, Stephanie Johnson, Kim Grauf, and Ryan Morton

Some Key Terms

• Speed– Measures how fast something is moving

• Velocity– Speed in a certain direction

• Instantaneous• Average

• Acceleration– Change of velocity in a given time

Free Fall

• A state of falling without air resistance (only force is gravity)

• Acceleration is –9.8 m/s2

• Use three of the following equations

Kinematics

• v=d/t

• a= (vf-vi)/t

• d= ½ at2 + vit

• 2ad=vf2-vi2

• A ball is dropped from 7 m above the ground. How fast is it moving 1.2 seconds later?

Kinematics Examples

•A rock is dropped from a cliff that is 50 m high. If you throw a ball down from the cliff 2 seconds later so that is reaches the ground at the same time as the rock, how fast must

the ball be thrown?

-11.76m/s

•36.19 m/s

More Kinematics Examples

• a) How long does it take to reach its maximum height?

• 44.89 seconds

• b) How fast and in what direction is the bullet moving after 3 seconds?

• Up at 410.6 m/s

A bullet is fired straight up into the air with an initial speed of 440 m/s.

The Quiz!• 1.) A race car can slow down with an acceleration of –11

m/s2. If the car is going 55 m/s, how many meters will it take to stop?

d=-137.5 meters

• Givens:• a = -11 m/s2

• vi = 55 m/s

• vf = 0 m/s

• d = ?

Equation—2ad=vf2-vi

2

The Quiz Continues

• Givens: • a = -9.8 m/s2

• t = 8 seconds• vf = ?• vi = 0 m/s

• 2.) A stone falls freely form rest for 8.0 seconds. • A) What is the final velocity?

Equation—a=(vf-vi)/t

vf=-78.4 m/s

#2 continued…

• B) What distance does the stone fall during this time?

• Equation—d=1/2at2+vitGivens:

d = ?

a = -9.8 m/s2

t = 8.0 seconds

vi = 0 m/s

d = -313.6 meters

There’s More?!• 3.) You are driving down a street in a car at 55

km/hr. Suddenly a child runs into the street. If it takes you 0.75 seconds to react and apply the brakes, how many meters will you have moved before you begin to slow down?

• Givens:

• vf = 96 m/s

• t = 8 seconds• a = ?

• vi = 32 m/s

Equation—a = (vf-vi/)t

a = 8 m/s

Last One!

• 4.) Determine the final velocity of 22 m/s accelerated uniformly at the rate of 1.6 m/s2 for 6.8 seconds.

• Givens:• t = 6.8 seconds• a = 1.6 m/s2

• vi = 22 m/s

• vf = ?

Equation—a=(vf-vi)/t

vf = 32.88 m/s

Extra Tips for Final

• Go over your picket fence/Galileo lab

• Go over chapter 2, pgs 10-24

• Worksheet 2-1 + 2-2

• 3 worksheets on Kinematics

• Kinematics take home quiz

The End!