Lesson 9: Newton's Second Law

Newton's Second Law: The Formula for Force

Force = Mass x Acceleration

​Forces Change Motion

​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.

​Balanced and Unbalanced Forces

​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.

The Two Formulas

​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

​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.

​Circular Motion

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.​

Multiple Choice

Centripetal force is indicated by the __________ ray.

• black

• green

Multiple Choice

True or False:

According to Newton's First Law, all moving objects must have some force causing their acceleration.

• True

• False

Multiple Choice

Which of the following is the correct formula for force?

• f = ma

• f = m/a

• f = a/m

• f = a + m

Multiple Select

If an object is accelerating, then it is changing _____________ and/or ______________.

• speed

• direction

• color

• sound

Multiple Choice

True or False:

If an object is not accelerating, then there are no external or internal forces acting on it or the forces acting on it are balanced.

• True

• False