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Defining 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 0:50
  • Question 3 1:56
  • Question 4 2:24
  • Question 5 3:32
  • Question 6 4:01
  • Question 7 5:36
  • Question 8 6:36

Transcription: Defining Motion

Hi everyone and welcome back to

In this mini-lesson, we are going to go through the first page of the kinematics defining motion worksheet from the APlusPhysics site, which you can find the link for down below.0002

Let us dive right in.0012

A student on her way to school walks four blocks east, three blocks north, and then another four blocks east.0014

Compared to the distance she walks, the magnitude of her displacement from home to school is...?0020

The distance she walks is how far she has traveled, so 4 + 3 + 4 were 11 blocks.0024

The displacement, however, is the straight line distance from where you start to where you finish, so that would be the displacement.0031

A displacement is obviously less than the distance that she walked, so compared to the distance she walks, the magnitude of her displacement is less.0037

Number 2 -- A motorboat which has a speed of 5 m/s in still water, is headed east as it crosses a river flowing south at 3.3 m/s.0050

What is the magnitude of the boat's resultant velocity with respect to the starting point?0058

Well, if it is going 5 m/s east and it is also crossing a river that is flowing at 3.3 m/s south, the boat's resultant velocity with respect to the starting point -- Well to do that, I am just going to add these two vectors.0063

They are already lined up tip to tail, so I can draw a line from the starting point of the first to the ending point of the last.0080

Now to figure out that magnitude, I am going to use the Pythagorean Theorem, so to figure out that magnitude, that is going to be the square root of 5 m/s2 + 3.3 m/s2...0088

...which is going to be 52 + 3.32 and the square root of that gives me about 6 m/s -- Answer 3.0099

Number 3 -- A speedometer in a car does not measure a car's velocity because velocity is a...?0116

Velocity is a vector quantity, so we can get rid of 3 and 4 because velocity is not a scalar and because it is a vector, it has a direction associated with it.0122

Velocity is a vector and has a direction associated with it, a speedometer in a car does not measure that because the speedometer does not tell you the direction, so best answer here is Number 1.0133

Number 4 -- A person observes a fireworks display from a safe distance of 0.75 km, so the distance is going to be 0.75 km, which is 750 m.0146

Assuming that sound travels at 340 m/s in air -- so the velocity or speed of 340 m/s -- what is the time between the person seeing and hearing a fireworks explosion?0160

Well, our relationship there is our velocity is going to be the distance traveled divided by the amount of time, therefore, time is going to be our distance traveled divided by the velocity or 750 m/340 m/s.0173

If I plug that into my calculator, I come up with right around 2.21 m/m or 1/1/seconds, which will leave us with units of seconds -- Answer 2.0192

Number 5 -- On the surface of Earth, a space craft has a mass of 2 × 104 kg.0212

What is the mass of the spacecraft at a distance of 1 earth radius above Earth's surface?0218

Now, that is a trick question there. Remember mass is how much stuff something is made up of.0224

In this case, that is not going to change even though its position is changing. It still has the same mass.0229

So our answer is Number 2, 2 × 104 kg.0235

Number 6 -- An airplane flies with a velocity of 750 km/h, 30 degrees south of East.0242

What is the magnitude of the eastward component of the plane's velocity?0250

Let us draw ourselves a little axis here and it is going to fly 750 km/h at an angle of 30 degrees south of East.0253

If I go south of East, it is probably going to be something right around that and its magnitude is 750 km/h and our angle here is 30 degrees.0267

What is the magnitude of the eastward component of the plane's velocity?0280

If we want the eastward component, well that means we are going to want this component, the component of our 750 km/h vector.0284

The way we get that -- the magnitude of the eastward component of the plane's velocity, which I am going to call the x-component of the velocity is just going to be our total velocity times the cos(30 degrees) or 750 km/h × cos(30 degrees)...0298

...which implies then that our velocity in the east direction is 750 × cos(30) or about 650 km/h -- Answer 2.0318

Number 7 -- One car travels 40 m due East in 5 s and a second car travels 64 m due West in 8 s.0337

During their periods of travel, the cars definitely had the same...?0345

Well, let us take a look -- car 1 travels 40 m due East in 5 s, so that means its velocity is going to be its distance traveled -- let us call that δx -- divided by the time it took or 40 m/5 s = 8 m/s and the velocity will be East.0349

Our second car has a velocity of δx/t, but it is going to travel 64 m due west, so let us call that -64 m/8s, which is going to be -8 m/s East or 8 m/s west.0368

They have the same speed, but different velocities because they are going in different directions, so our correct answer -- they must have the same average speed.0385

Let us check out one more -- State the two general characteristics that are used to define a vector quantity.0395

Vectors are made up of two things -- they must have a magnitude or a size and a vector also has a direction, so magnitude and direction gives you a vector.0402

That covers the first page of our kinematics defining motion worksheet.0419

We will talk to you next time. Make it a great day!0424