Lesson 11.2: Force and mass determine acceleration

Lesson Focus

Discover how force and mass determine an object's acceleration. This lesson explores Newton's second law and its application to objects in both straight-line and circular motion.

Learning Objectives

• Learn how Newton's second law relates force, mass, and acceleration.
• Understand how centripetal force works to keep objects moving in a circle.

An object's motion changes due to unbalanced forces. Newton's second law explains that acceleration increases with more force but decreases with more mass. The acceleration is always in the same direction as the net force. For example, you must push a full shopping cart harder than an empty one to get it moving quickly.

To precisely describe motion, scientists use the formula Force = mass × acceleration (F = ma). The unit of force is the newton (N). One newton is the force required to accelerate a 1 kg object by 1 m/s². This equation is the mathematical foundation for Newton's second law and helps predict motion.

We can rearrange the motion formula to solve for any variable. To find acceleration, use a = F/m; to find mass, use m = F/a. This is useful for calculating a rocket's increasing acceleration as it burns fuel and loses mass. Knowing the formula lets you solve for any of the three parts of the law.

An object accelerates if its velocity changes, which includes changing direction even if its speed remains constant. A force applied from the side will cause an object to turn. For example, a soccer player gently taps the ball to change its path without changing its speed, demonstrating acceleration through direction change.

An object moves in a circle because a constant, inward-pulling force acts on it. This is called centripetal force. Without this force pulling toward the center, the object would fly off in a straight line. A greater force or smaller mass allows for faster circular motion, connecting back to F = ma.