Relative Motion Physics Cedar Hill High School

Galileo’s Dictum

Galileo’s Dictum states that “ if you are in a box moving smoothly, that is, not starting or stopping or jerking or turning but just moving at one speed in one direction, you cant tell if you are in motion; everything in the box happens as if you were at rest.

One of the great findings of scientists is that the laws of motion are the same in frames of reference that move with constant velocity with respect to each other. This is called the Principle of Relativity

Newton’s First Law

Helicopter hovering above a rotating planet. An object in uniform motion will continue in motion unless acted upon by an outside force. Ball being tossed vertically by Mr. Bradshaw while riding on a bicycle. The ball seems to have a parabolic path because it maintains its initial forward momentum as well as up and down motion. Helicopter hovering above a rotating planet. An object in uniform motion will continue in motion unless acted upon by an outside force. Ball being tossed vertically by Mr. Bradshaw while riding on a bicycle. The ball seems to have a parabolic path because it maintains its initial forward momentum as well as up and down motion.

Relative Velocity

Example From Text You are driving North at 80 km/h. A faster car passes you going 90 km/h. To you, traveling the faster car seems to be North at 10 km/h. V se = +80 km/h (velocity of slower car relative to the earth) V fe = 90 km/h (velocity of the faster car relative to the earth) V fs = 10 km/h You are driving North at 80 km/h. A faster car passes you going 90 km/h. To you, traveling the faster car seems to be North at 10 km/h. V se = +80 km/h (velocity of slower car relative to the earth) V fe = 90 km/h (velocity of the faster car relative to the earth) V fs = 10 km/h

Calculation and Rationale Find the velocity of the faster car with respect to the slower car. Write an equation for V fs in terms of the other velocities where the subscripts on the right side of the equation start with f and end with s. V fs = V fe + V es; V fs = 90km/h - 80km/h = 10km/h Since North is the positive direction, we know that Ves = -Vse because the earth appears to be moving south at 80 km/hr. Find the velocity of the faster car with respect to the slower car. Write an equation for V fs in terms of the other velocities where the subscripts on the right side of the equation start with f and end with s. V fs = V fe + V es; V fs = 90km/h - 80km/h = 10km/h Since North is the positive direction, we know that Ves = -Vse because the earth appears to be moving south at 80 km/hr.