Chapter 7.2 – Projectile Motion

• Set-up a new Cornell Notes Page:– Essential Question: How do we predict the

motion of a projectile?• Warm-up question: Where do you have to

aim to hit the monkey with the banana? Why?

Shoot the Monkey!

Now in Slow-Mo!

What is a projectile?

• A projectile is an object moving in TWO dimensions - Vertical and Horizontal

• A projectile is only accelerating in ONE direction – down

• ONLY acceleration is from gravity.

What is Projectile Motion?

• Projectile Motion is a combination of two types of motion:

• Horizontal– Motion of a ball rolling freely along a

level surface– Horizontal velocity is ALWAYS

constant• Vertical

– Motion of a freely falling object– Acceleration due to gravity = 9.8 m/s2

down– Vertical component of velocity

changes with time

What does projectile motion look like?

• While a projectile is in the air, what happens to the horizontal velocity?

What does projectile motion look like?

• While a projectile is in the air, what happens to the horizontal velocity?

• While a projectile is in the air, what happens to the vertical velocity?

What does projectile motion look like?

• While a projectile is in the air, what happens to the horizontal velocity?

• While a projectile is in the air, what happens to the vertical velocity?

• What is the acceleration of a projectile? Which direction? Constant or changing magnitude?

Examples of Projectile Motion

• Launching a Cannon ball

What does projectile motion look like?

• Is the distance a projectile falls vertically affected by its horizontal velocity?

• Think about rolling a ball off of a table – does it fall farther down if you roll it faster?

• This is the key: The horizontal and vertical motion of a projectile are SEPARATE – they don’t affect each other at all!

• BUT – it takes the same amount of time both ways

• A classic mind-bender: If a bullet is dropped from rest from an elevated position at the same instant that a second bullet is fired horizontally (from the same height), then which bullet will hit the ground first? Assume the bullets behave as projectiles.

Bullet Demo

How do we predict the motion of a projectile?

• We calculate each direction separately!• Horizontal Component• dhorizontal = d0 + ʋ0t + ½ at2

• But a = 0! So this reduces to… • dhorizontal = d0 + ʋ0t

• Why can’t we use any of the other equations?

How do we predict the motion of a projectile?

• Vertical Component• dvertical = d0 + ʋ0t + ½ at2

• ʋ = ʋ0 + at

• ʋ2 = ʋ02 + 2a(d - d0)

How do we predict the motion of a projectile?

• Vertical – Component• But – what is a? If the position of the

ground is 0m, then up is positive, and • a= -9.8m/s2 (called g)• Like this:

• Why don’t we ever need • d = d0 + ½(ʋ + ʋ0)t?

Homework #1

A stone is thrown horizontally at 7.5 m/s from a cliff 87 m high. How far from the base of the cliff does the stone strike the ground?