Everything Science Grade 11

CHAPTER 12. FORCE,MOMENTUM AND IMPULSE 12.2

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Step 3 : Label the forces

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F 1 : Force of car on trailer A(to the right)

F 1

FB: Force of trailer B on trailer A (to the left)

FB

Ff: Frictional force on trailer A (to the left)

Ff

Fg: Downward force of Earth on trailer A

Fg

FN: Upward force of road ontrailer A

FN

Finding the Resultant Force ESBEV

The easiest way to determine a resultant force isto draw a free body diagram. Remember from Chap-
ter 11 that we use the length of the arrow to indicate the vector’s magnitude and the direction of the
arrow to show which direction it acts in.

After we have done this,we have a diagram of vectors and we simply findthe sum of the vectors toget
the resultant force.

4 N 6 N

(a)

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6 N 4 N

(b)

Figure 12.1: (a) Force diagram of 2 forces actingon a box. (b) Free bodydiagram of the box.

For example, two people push on a box from opposite sides with forces of 4 N and 6 N respectively
as shown in Figure 12.1(a). The free body diagram in Figure 12.1(b) shows the object represented by
a dot and the two forcesare represented by arrows with their tails on the dot.

As you can see, the arrows point in opposite directions and have different lengths. The resultant force
is 2 N to the left. This result can be obtained algebraically too, since thetwo forces act along thesame
line. First, as in motionin one direction, choosea frame of reference. Secondly, add the two vectors
taking their directions into account.