Everything Science Grade 11

CHAPTER 12. FORCE,MOMENTUM AND IMPULSE 12.3

DEFINITION: Newton’s Second Law of Motion

If a resultant force acts on a body, it will cause the body to accelerate in the direction

of the resultant force. The acceleration of the body will be directly proportional to the

resultant force and inversely proportional to themass of the body. The mathematical

representation is:

F = ma.

See video: VPkiq at http://www.everythingscience.co.za

Applying Newton’s Second Law

Newton’s Second Lawcan be applied to a variety of situations. We will look at the main types of
examples that you needto study.

Example 5: Newton II - Box on a surface 1

QUESTION

A 10 kg box is placed ona table. A horizontal force of 32 N is applied tothe box. A frictional

force of 7 N is present between the surface andthe box.

1. Draw a force diagramindicating all the horizontal forces acting on thebox.


2. Calculate the acceleration of the box.

10 kg

friction = 7 N 32 N

SOLUTION

Step 1 : Identify the horizontalforces and draw a forcediagram

We only look at the forces acting in a horizontal direction (left-right)and not

vertical (up-down) forces. The applied force and the force of frictionwill be

included. The force of gravity, which is a vertical force, will not be included.