Preparing for Lower Sixth Physics #10: Speed, velocity and relative velocity at Fort Dunlop

 

This is a view from the M6 driving through Birmingham. I remember Fort Dunlop from my childhood when it was a tyre factory. Now it is office space and hotel accommodation but it has always been an impressive building. The traffic was light and we were doing 70 mph. That was our SPEED. If I add in a direction and say we were doing 70 mph NORTH then I can call it my VELOCITY. Speed only has a size (a MAGNITUDE) but velocity has a MAGNITUDE and a DIRECTION. Speed is a SCALAR quantity (magnitude only) and velocity is a VECTOR quantity (magnitude and direction). So the red car on the other side of the road has a speed of 70 mph and a velocity of 70 mph south. You could say the velocity was -70 mph. Because the ground between us is closing up due to motion in both directions, it looks to me as though the red car is doing 140 mph. And it looks to the red car as if I am approaching at 140 mph. This is called the relative velocity.