Lesson 9: Newton's Second Law

​If we push a cart with more force, the motion of the cart increases. If we push with less force, the motion of the cart decreases. So, force causes a change in the speed and/or direction of objects. When unbalanced forces are applied to an object, it accelerates, or changes speed and/or direction.

​If an object is at rest, then there are either no outside forces acting on it or the

forces ​that are acting on it are

​balanced. Objects which are ​changing

direction or speed ​are being acted

upon by ​unbalanced forces. Some

​moving objects are also ​acted upon

by balanced ​forces.

​

​So, we need to be aware of an object's

​change in motion and which forces

​might be causing that change.

​So, we have two formulas for Newton's Second Law. One is that force = mass x acceleration and the other is acceleration = force / mass. Each of these is useful in different scenarios depending on whether we need to find out the acceleration or the force.

​Circular motion is any motion in which an object is moving along a curved path. There are two primary forces occurring during circular motion: the force of the object moving forward (shown in blue), and the force called centripetal force (shown in red). Each of these forces are constantly causing acceleration in the object.

Centripetal force is the force which acts perpendicular to the direction of motion, toward the center of the curve. The motions of satellites and planets are also circular motion and are subject to centripetal force just like a yoyo being spun around or your keys on your lanyard when you spin them.​