Peoples Physics Concepts

4.2. Newton’s Second Law http://www.ck12.org

4.2 Newton’s Second Law

• Describe Newton’s Second Law and explain the relationship between acceleration, applied force, and mass.


• Determine the net force when multiple forces act on an object.

The acceleration experienced by an object will be proportional to the applied force and inversely proportional to its
mass. If there are multiple forces, they can be added as vectors and it is thenetforce that matters.

Key Equations

Newton’s Second Law describes his famous equation for the motion of an object

The change of motion is proportional to the motive force impressed; and is made in the direction of the right

(straight)line in which that force is impressed.

The "motion" Newton mentions in the Second Law is, in his language, the product of the mass and velocity of an
object — we call this quantity momentum — sothe Second Law is actually thefamous equation:

~F=∆(m~v)

∆t

=

m∆~v

∆t

=m~a [1]

Force Sums







F~net=∑iFi=m~a Net force is the vector sum of all the forces

Fnet,x=∑iFix=max Horizontal components add also

Fnet,y=∑iFiy=may As do vertical ones

Guidance

To calculate the net force on an object, you need to calculate all the individual forces acting on the object and then

add them as vectors. This requires some mathematical skill.

Example 1

A 175-g bluebird slams into a window with a force of 19.0 N. What is the bird’s acceleration?