Question

in which situation is speed constant while velocity changes?
a a fan blade is spinning at 5 m/s.
b a car slams on its breaks and stops.
c a runner runs on a straight track at 5 m/s.
d a ball begins rolling from 0 m/s to 5 m/s.

Explanation:

Step1: Recall the definitions of speed and velocity

Speed is a scalar quantity (only magnitude), velocity is a vector quantity (magnitude and direction).

Step2: Analyze each option

• Option A:

A fan - blade spinning at \(5\ m/s\). The speed (magnitude) is \(5\ m/s\) (constant). But the direction of the motion of a point on the fan - blade is constantly changing (as it is in circular motion). Since velocity depends on direction, velocity changes.

• Option B:

A car slams on its brakes and stops. The speed changes from some initial value to \(0\ m/s\). So, speed is not constant.

• Option C:

A runner runs on a straight track at \(5\ m/s\). The speed is \(5\ m/s\) (constant) and the direction is also constant (straight - line motion). So, velocity (which has both magnitude and direction) is constant.

• Option D:

A ball begins rolling from \(0\ m/s\) to \(5\ m/s\). The speed is changing (from \(0\) to \(5\ m/s\)). So, speed is not constant.

Answer:

A. A fan blade is spinning at \(5\ m/s\)