Which statement about an object's relative velocity with respect to another object is true?
A car does 30 m/s on a highway. A truck ahead of it does 25 m/s the same way. The gap shrinks, but slowly. How fast does the car really gain on the truck?
30 m/s − 25 m/s = 5 m/s
The car's relative velocity with respect to the truck is 5 m/s forward. From the truck, the car approaches at running speed.
The same number reads the other way too. From the car, the truck falls back at 5 m/s. One subtraction serves both observers.
Same direction means subtract. The two motions partly cancel, so the relative velocity is small even when both speeds are large.
A motorcycle at 27 m/s gains on a car at 23 m/s, both moving the same way. Calculate the motorcycle's relative velocity with respect to the car.
Write down the values in the question:
motorcycle's speed = 27 m/s
car's speed = 23 m/s, the same way
relative velocity = ?
Write down the equation:
relative velocity = faster speed − slower speed
Substitute in the values:
relative velocity = 27 m/s − 23 m/s
Simplify:
relative velocity = 4 m/s, closing
The same pair, read from the motorcycle: how fast does the car fall back?
From the motorcycle's frame, the car loses ground at the same 4 m/s.
One subtraction serves both observers; only the direction word flips.
The car falls back at 4 m/s.
Now make the move quick. Six items follow: the objects move the same way, so subtract.
A car at 34 m/s follows a car at 28 m/s, moving the same way. Enter their relative velocity in m/s.
A runner at 7.5 m/s chases a runner at 5.5 m/s, moving the same way. Enter their relative velocity in m/s.
A cyclist at 13 m/s overtakes a cyclist at 10 m/s, moving the same way. Enter their relative velocity in m/s.
A train at 40 m/s follows a train at 33 m/s, moving the same way. Enter their relative velocity in m/s.
A boat at 8.0 m/s follows a boat at 6.5 m/s, moving the same way. Enter their relative velocity in m/s.
From a car doing 26 m/s, a car doing 30.5 m/s approaches from behind, the same way. Enter its approach speed in m/s.
A motorcycle at 38 m/s gains on a van at 29 m/s, both moving the same way down a straight road. Enter the motorcycle's relative velocity with respect to the van, in m/s.
A hiker at 1.8 m/s catches up to a hiker at 1.2 m/s on the same trail, moving the same way. Enter their relative velocity in m/s.
Two planes fly the same way on one airway, one at 250 m/s, the other ahead at 236 m/s. Enter the trailing plane's relative velocity with respect to the leading plane, in m/s.