Everything Science Grade 11

12.3 CHAPTER 12. FORCE,MOMENTUM AND IMPULSE

SOLUTION

Step 1 : Analyse the situation

The question asks us toidentify the frictional force. The body is said tobe at

rest on the plane, which means that it is not moving and therefore there is no

resultant force. The frictional force must therefore be balanced by the force F up

the inclined plane.

Step 2 : Choose the correct answer

The frictional force is equal to the component of the weight (Mg) parallel to the

surface, which is equalto Mg sin θ.

Example 10: Newton II - Object ona slope

QUESTION

A force T = 312 N is required to keep a bodyat rest on a frictionlessinclined plane which

makes an angle of 35◦with the horizontal. Theforces acting on the body are shown. Calculate

the magnitudes of forces P and R, giving your answers to three significant figures.

35 ◦

35 ◦

R

T

P

SOLUTION

Step 1 : Find the magnitude ofP

We are usually asked tofind the magnitude of T,but in this case T is givenand we

are asked to find P. We can use the same equation. T is the force that balances the

component of P parallelto the plane (Px) and therefore it has thesame magnitude.

T = P sin θ

312 = P sin 35◦

P = 544 N

Step 2 : Find the magnitude ofR

R can also be determined with the use of trigonometric ratios. The tanor cos

ratio can be used. We recommend that you usethe tan ratio because it does not