To change the direction of an object moving at a constant speed, you need to apply a force that is perpendicular to its current direction of motion. Here's why:

* Newton's First Law of Motion: An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

* Force and Acceleration: A force causes an object to accelerate. Acceleration is the rate of change of velocity, which can be a change in speed, a change in direction, or both.

Examples:

* Pushing a box: If you push a box horizontally, you apply a force in the same direction as its motion, causing it to accelerate faster. If you push it sideways, you apply a force perpendicular to its motion, causing it to change direction.

* Turning a car: The steering wheel applies a force to the wheels, causing them to turn and changing the car's direction.

Important Note: The force doesn't have to be very strong to change direction. Even a small force applied over a long period of time can change an object's direction.