Free Fall• Think about an apple falling from a tree. – It starts at rest and gains speed as it falls, or

accelerates. • Gravity causes the apple to accelerate

downward and is said to be in free fall.Free fall: when an object is only affected by

gravity – SI unit: m/s2 ( for acceleration due to gravity) – g = 10 m/s2 on Earth.• The letter g represents the acceleration due to gravity.

– Equation: v = gt• v = velocity or speed (m/s)• g = acceleration due to gravity (10 m/s2 on

Earth)• t = elapsed time (s)

**Hint - as soon as you see any of the following phrases in a word problem, write g = 10 m/s2 for a given: free fall, falling, dropped, thrown**

Remember, the object’s velocity increases by 10 m/s every second it falls

Time falling velocity

0s

1 s

2 s

3 s

4 s

5 s

6s

7s

8s

9s

Check Your UnderstandingWhat would the speedometer reading on a

falling rock be 4.5 seconds after it drops from rest?

v = ?g = 10 m/s2

t = 4.5s

v = gtv = (10 m/s2) (4.5s)

v = 45 m/s

How about 8 seconds? v = ?

g = 10 m/s2

t = 8sv = gt

v = (10 m/s2) (8s)v = 80 m/s

How about 15 seconds? v = ?

g = 10 m/s2

t = 15sv = gt

v = (10 m/s2) (15s)v = 150 m/s

• Now consider an object thrown straight up. It will continue to move straight up, then it comes back down.

• At the highest point, the object changes its direction and the objects instantaneous speed is 0 m/s.

• Whether the object is moving up or down, the acceleration of the object is always 10 m/s2.– It is just -10 m/s/s when the

object is moving up• Because gravity is slowing the

object down instead of speeding it up

• It is decelerating!!

• Because an object in free fall increases the rate of distance covered every second, we cannot use v =d/t to find distance.– Equation: d = ½ gt2

• d = distance (m)• g = acceleration due to gravity (10 m/s2 on

Earth)• t = elapsed time (s)

Use the distance formula to find how far the object will fall at each second

Time falling Distance travelled

0 s

1 s

2 s

3 s

4 s

5 s

6 s

7 s

8 s

9 s

Check Your Understanding

What is the distance an object falls in one second?

d = ?g = 10 m/s2

t = 1 sd = ½ gt2

d = ½ (10)(12)d = 5 m

Practice problem 1

• Copy this down then solve:• If it takes a rock that falls off a cliff 8 seconds

to hit the ground, then what is its speed and how far has it fallen?

Practice problem 2

• Write this down then solve• If a rock falls 320 m, how long did it take for

that rock to fall?

Challenge practice problem

• Think this one through – do you need to do any work to solve it?

• If you throw a ball straight up at 20 m/s, how long will it take for the ball to come to a complete stop before it starts to fall?

The problem with air• When you drop a crumpled piece of paper and a

flat sheet of paper they should fall and hit the ground at the same time, but do they?– The reason the crumpled piece of paper hits the

ground first is because it experiences less air resistance• A frictional force (means it goes against motion) caused by

air• The greater the surface area of an object, the more air

resistance it feels