Which motion has a CONSTANT velocity?
Press the brake pedal, and a physicist will tell you the truck is accelerating. That claim sounds backwards, until you see what accelerating means in physics.
In earlier grades you saw that acceleration means a change in velocity. You've also just practiced spotting when a velocity changes. One step remains: attaching the name to every case.
A velocity can change in three ways. The object speeds up, it slows down, or it changes direction.
Slowing down counts. A truck braking for a red light is accelerating, because its velocity is changing.
Which vehicle is accelerating?
Changing direction counts too, even at a steady speed. Because velocity includes direction, a change in direction is a change in velocity even when speed stays constant.
Only an object whose velocity holds steady is not accelerating: same speed, same direction.
Here are three quick cases. A subway train leaves a station, moving faster and faster. Its velocity is changing, so it is accelerating.
A gondola crosses a valley in a dead-straight line at a steady 5 m/s. Its speed holds and its direction holds, so it is not accelerating.
A skateboarder carves through a long curve at a steady speed. Her direction is changing, so she is accelerating.
Motion 1: a motorcycle leans through a curve at a steady 40 km/h. Motion 2: a motorcycle cruising dead straight at a steady 40 km/h. Which motion is accelerating?
A student writes: 'The car cannot be accelerating — its speed never changed all the way around the traffic circle.' What is wrong with the claim?
During one trip a taxi: (1) pulls away from the curb, (2) holds a steady speed down a straight avenue, (3) swings through a corner at that same steady speed, (4) brakes at the airport gate. In how many of the four legs is the taxi accelerating?
In physics, which single condition decides whether an object is accelerating?