Negative acceleration for objects moving at constant speed is caused by a change in velocity in the opposite direction of motion. Acceleration is the rate of change of velocity, and it is a vector quantity that takes into account both magnitude and direction.
When an object is moving at constant speed, it means its magnitude of velocity is not changing. However, if there is a change in direction of motion, the velocity vector is changing, resulting in a non-zero acceleration even though the speed remains constant.
Consider an object moving in a circular path at a constant speed. Its velocity vector is continuously changing direction as it moves around the circle. The change in velocity, or acceleration, is directed towards the center of the circle. This inward acceleration is often referred to as centripetal acceleration and is negative because it opposes the outward direction of the object's motion.
In other cases, an object may experience a force that acts in the opposite direction of its motion, causing a decrease in velocity. This decrease in velocity leads to a negative acceleration. For example, if a car is moving forward with a constant speed and the driver applies the brakes, a frictional force acts in the opposite direction of the car's motion, causing it to slow down. The negative acceleration in this case is due to the braking force.
To summarize, negative acceleration for objects moving at constant speed arises from changes in direction or forces opposing the motion, leading to a decrease in velocity despite the speed remaining constant.