Motion, Velocity and Acceleration

The study of motion begins with describing how an object's position changes over time. Velocity is the rate of change of position and is a vector, so it specifies both speed and direction. Acceleration is the rate of change of velocity; an object accelerates whenever its speed changes, its direction changes, or both. A car rounding a bend at constant speed is still accelerating because its direction is changing.

For motion with constant acceleration, a set of kinematic equations relates displacement, initial and final velocity, acceleration and time. These equations allow one quantity to be found when the others are known, and they describe familiar situations such as a falling object speeding up under gravity at a steady rate. Plotting position, velocity and acceleration against time reveals the relationships between them, with the slope of a position graph giving velocity and the slope of a velocity graph giving acceleration.

Newton's laws connect this description to its cause. A net force produces an acceleration proportional to the force and inversely proportional to the object's mass, so heavier objects need larger forces to achieve the same change in motion. Motion control in engineering applies these principles to move machine parts smoothly and precisely, planning the required accelerations and velocities to reach a target position without overshoot.

Frequently asked questions

What is the difference between speed and velocity?

Speed is how fast an object moves and has only magnitude. Velocity also includes direction, so a change in direction at constant speed is still a change in velocity.

Can an object accelerate while moving at constant speed?

Yes. If its direction changes, such as a car going round a curve, its velocity changes and it is accelerating even though the speed stays the same.

How is acceleration related to force?

According to Newton's second law, acceleration is proportional to the net force and inversely proportional to mass, so a given force accelerates a lighter object more.