1 / 11

Chapter 3–4: Relative Motion

Chapter 3–4: Relative Motion. Physics Coach Kelsoe Pages 102–105. Objectives. Describe situations in terms of frame of reference. Solve problems involving relative velocity. Frames of Reference.

vance-drake
Download Presentation

Chapter 3–4: Relative Motion

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 3–4:Relative Motion Physics Coach Kelsoe Pages 102–105

  2. Objectives • Describe situations in terms of frame of reference. • Solve problems involving relative velocity.

  3. Frames of Reference • If you are moving at 80 km/hr north and a car passes you going 90 km/hr, to you the faster car seems to be moving north at 10 km/hr. • Someone standing on the side of the road would measure the velocity of the faster car as 90 km/hr toward the north. • This simple example demonstrates that velocity measurements depend on the frame of reference of the observer.

  4. Frames of Reference • Consider a stunt dummy dropped from a plane. • When viewed from the plane, the stunt dummy falls straight down. • When viewed from a stationary position on the ground, the stunt dummy follows a parabolic projectile path.

  5. Relative Velocity • When solving relative velocity problems, write down the information in the form of velocities with subscripts. • Using our earlier example, we have: • vse = +80 km/hr north (se = slower car with respect to Earth) • vfe = +90 km/hr north (fe = faster car with respect to Earth) • unknown = vfs (fs = fast car with respect to slow) • Write an equation for vfs in terms of the other velocities. The subscripts start with f and end with s. The other subscripts start with the letter that ended the preceding velocity: • vfs = vfe + ves

  6. Relative Velocity • An observer in the slow car perceives Earth as moving south at a velocity of 80 km/hr while a stationary observer on the ground (Earth) views the car as moving north at a velocity of 80 km/hr. In equation form: • ves = -vse • Thus, this problem can be solved as follows: • vfs = vfe + ves = vfe – vse • vfs = (+90 km/hr n) – (+80 km/hr n) = +10 km/hr n • A general form of the relative velocity equation is: • vac = vab + vbc

  7. Sample Problem • Relative Velocity A boat heading north crosses a wide river with a velocity of 10.00 km/hr relative to the water. The river has a uniform velocity of 5.00 km/hr due east. Determine the boat’s velocity with respect to an observer on shore.

  8. Sample Problem Solution • Set up your coordinate system with your givens. • Given: • vbw = 10.00 km/hr due north (velocity of the boat, b, with respect to the water, w) • vwe = 5.00 km/hr due east (velocity of the water, w, with respect to the Earth, e) • Unknown: • vbe = ? • Diagram: • See diagram vwe vbw vbe θ

  9. Sample Problem Solution • Choose an equation or situation: • vbe = vbw + vwe • (vbe)2 = (vbw)2 + (vwe)2 • tan θ = vwe /vbw • Rearrange the equations to isolate the unknowns: • vbe = √(vbw)2 + (vwe)2 • θ = tan-1vwe /vbw

  10. Sample Problem Solution • Substitute the known values into the equations and solve. • vbe = √(10.00 km/hr)2 + (5.00 km/hr)2 • vbe = 11.18 km/hr • θ = tan-1 (5.00 km/hr /10.00 km/hr) • θ = 26.6° east of north

More Related