Falling Objects: Page 88

QUESTION: Does a falling object accelerate all the way down?

OBJECTIVE: Students will be able to explain how falling objects accelerate and why they reach terminal velocity.

VOCABULARY: air resistance terminal velocity projectile

Falling ObjectsWrite these questions onto page 88

SKIP ONE LINE BETWEEN EACH QUESTION

1. Do all objects fall at the same speed?2. What is air resistance?3. When does an object reach terminal velocity?4. Does a projectile fall slower than a dropped object?

How hard does the Earth pull on objects?

The Earth’s gravity pulls on all objects exactly the same.

The acceleration due to the gravity of the Earth (g) is 9.8 meters per second every second.

So all objects should fall at the same speed.

Do all objects fall at the same speed?

What do you think?

No, all objects don’t fall at the same speed.

Follow my instructions for the demonstration.

How do you explain the results?

If gravity pulls on all objects exactly the same, then why don’t they all fall at the same speed?

What is air resistance?

Air resistance is the fluid friction that pushes upward on falling objects. Where is there no air resistance?

Objects that are light and have a large surface area have the most air resistance.

A leaf would have a lot of air resistance, a bowling ball would have much less.

Can we calculate how fast objects fall?It’s almost impossible to do this unless we pretend that there is no air resistance.

Even in high school when you do these types of problems you will pretend objects are falling in a vacuum.

The equation for calculating the speed of a falling object without air resistance is

speed = acceleration due to gravity (g) * time

VacuumNo air

Sample Problem 1

speed = acceleration due to gravity (g) * time

s = gt

What is the speed of a ball dropped from a cliff after it has fallen for 4 seconds?

Speed = 9.8 m/sec2 * 4 seconds

Speed = 39.2 meters/second

Now You Try It

speed = acceleration due to gravity (g) * time

s = gt

What is the speed of a skydiver after he has fallen for 5 seconds?

Speed = 9.8 m/sec2 * 5 seconds

Speed = 49 meters/second

When do Objects Reach their terminal velocity?

Terminal velocity is the fastest speed an object can fall.

Terminal velocity is reached when air resistance pushing upward is equal to gravity pulling downward.

Do projectiles fall slower than dropped objects?

Projectiles fall at exactly the same speed as dropped objects.

Watch the demonstration.

Falling Objects: page 88Check that the answers to these

questions are in your notes.

1. Do all objects fall at the same speed?2. What is air resistance?3. When does an object reach terminal velocity?4. Does a projectile fall slower than a dropped

object?

VOCABULARY

Air resistance- the upward fluid friction on a falling object.

Terminal velocity- the fastest speed an object can fall.

Projectile- an object that is thrown.