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How can a force change an object’s velocity without changing its speed?

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A diagram of a cylindrical vessel containing gas, with a movable piston held down by a weight. Countless small dots beneath the piston represent gas particles in rapid, random motion, illustrating Bernoulli's model of gas pressure. — physics As conceived by Daniel Bernoulli in *Hydrodynamica* (1738), gases consist of numerous particles in rapid random motion. He assumed that the pressure of a gas is produced by the direct impact of the particles on the walls of the container.

A force can change an object’s velocity without changing its speed when the force acts perpendicular to the object's direction of motion. Because velocity is a vector—meaning it possesses both magnitude (speed) and direction—a change in either component results in a change in velocity.12

A classic example of this is uniform circular motion. When an object moves along a circular path at a constant speed, its direction is constantly changing as it follows the curve. This change in direction requires an acceleration directed toward the center of the circle, known as centripetal acceleration. The force responsible for this, called centripetal force, acts at a right angle to the object's instantaneous velocity. Because this force does not have a component parallel to the direction of motion, it does not increase or decrease the object's speed, but it continuously alters the object's path by changing its direction.32