Everything Science Grade 11

12.3 CHAPTER 12. FORCE,MOMENTUM AND IMPULSE

FR = ma

T + Ff = (10)(2)

T + (−150) = 20

T = 170 N

Note: If we had used the same principle and applied it to 15 kg crate, our

calculations would havebeen the following:

FR = ma

Fapplied+ T + Ff = (15)(2)

500 + T + (−300) = 30

T =−170 N

The negative answer here means that the forceis in the direction opposite to the

motion, in other wordsto the left, which is correct. However, the question asks

for the magnitude of theforce and your answer will be quoted as 170 N.

Example 7: Newton II - Man pulling a box

QUESTION

A man is pulling a 20 kgbox with a rope that makes an angle of 60◦with the horizontal. If he

applies a force of 150 Nand a frictional force of15 N is present, calculate the acceleration of

the box.

20 kg

60 ◦

150 N

�

15 N

SOLUTION

Step 1 : Draw a force diagram

The motion is horizontal and therefore we will only consider the forces in a

horizontal direction. Remember that vertical forces do not influence horizontal

motion and vice versa.