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### Projectile Motion

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

• Intro 0:00
• Question 1 0:13
• Question 2 0:45
• Question 3 1:25
• Question 4 2:00
• Question 5 2:32
• Question 6 3:38

### Transcription: Projectile Motion

Hi everyone and welcome back to Educator.com.0000

Today's mini-lesson is going to be on projectile motion as we work through page 1 of the APlusPhysics worksheet on projectile motion and you will find the link to that worksheet down below.0002

Let us dive in with Number 1 -- A volleyball hit into the air has an initial speed of 10 m/s.0013

Which vector best represents the angle above the horizontal that the ball should be hit to remain in the air for the greatest amount of time?0018

Well, obviously, if you want it to remain in the air for the longest amount of time, you want it to have as much vertical velocity so it goes as high as possible, so the answer there must be 1 -- you want it being sent completely vertically.0026

All of the energy being sent into its vertical motion, so it is in the air for the longest amount of time.0039

Now following on from here, on the diagram, use a protractor and a ruler to draw a vector to represent the initial velocity of the projectile, given that it is launched at 250 m/s at an angle of 60 degrees.0045

Begin the vector at (P) and use a scale of 1 cm = 50 m/s.0057

If one centimeter is 50 m/s, our vector is 250 m/s long, our vector should be 5 cm long, so that should look something like this...0062

...where our velocity here is 250 m/s and our angle there -- check with the protractor -- should be 60 degrees.0076

We have a projectile fired from the ground again, same projectile, same velocity, and same angle.0089

Determine the horizontal component of the initial velocity.0094

Well, the horizontal component is going to be the x-component, which will be V × cosine of our angle (60 degrees) or 250 m/s × cos(60 degrees) and cos(60 degrees) is 1/2, so that is going to be 125 m/s.0098

Number 4 -- Explain why the projectile has no acceleration in the horizontal direction.0120

There is nothing to cause a force in the horizontal direction.0126

If we are neglecting air resistance, there is nothing to change its velocity.0130

Once it is going horizontally, gravity is only pulling down, it does not affect the horizontal velocity.0133

So why is there no acceleration?0139

I would write something like gravity only pulls down.0142

Number 5 -- We have two stones, (A) and (B) and they are thrown horizontally from the top of a cliff.0152

Stone (A) has an initial speed of 15 m/s and (B) has an initial speed of 30 m/s.0160

Compared to the time it takes (A) to reach the ground, the time it takes (B) to reach the ground is...?0167

For launching these from a cliff, there goes (A) and (B) has a greater initial velocity, but that has nothing to do with their vertical motion.0172

Regardless of whether you drop an object, throw an object, shoot an object, as long as you have no initial vertical velocity and you have flat ground for them to land on, they will hit the ground at the same time.0186

Similar to the famous thought experiment, where you drop a bullet from a gun and fire the gun horizontally at the same time, assuming that the ground is completely flat, the bullets hit the ground at the same time.0200

Comparing the amount of time it takes -- the same.0212

One more -- This diagram represents the path of a stunt car driven off a cliff, neglecting friction.0218

Compared to the horizontal component of the car's velocity at (A), has some velocity at (A) horizontally, the horizontal component of the car's velocity at (B) is -- well we have hit that a couple of times already just on this page of the worksheet.0226

The horizontal velocity is not going to change. Gravity only pulls down, so in this case, the horizontal components of its velocity is still going to be the same.0238

That completes page 1 of the worksheet on projectile motion.0251

Thanks so much for your time. Make it a great day!0255