Everything Science Grade 11

CHAPTER 12. FORCE,MOMENTUM AND IMPULSE 12.3

force equal to the horizontal component of the force of gravity (Fgsin θ). The object will ’stick’to the
slope due to the frictional force between the object and the surface. Asyou increase the angle of the
slope, the horizontal component will also increase until the frictional force is overcome and the object
starts to slide down theslope. The force of gravity will also tend to push an object ’into’ the slope.

Tip

Do not use the abbre-

viation W for weight as

it is used to abbreviate

’work’. Rather use the

force of gravity Fg for

weight.

The vertical componentof this force is equal tothe vertical componentof the force of gravity (Fgcos
θ). There is no movement in this direction as this force is balanced by the slope pushing up against
the object. This “pushing force” is called the normal force (N) and is equal to the force required to
make the component ofthe resultant force perpendicularly into the plane zero, Fgcos θ in this case,
but opposite in direction.

Fg

Surface friction

horizontal component

parallel to the surface

vertical component

θ perpendicular to the surface

Fgsin θ θ

Fgcos θ

θ

Fgsin θ

Fgcos θ

Fg

See simulation: VPkjb at http://www.everythingscience.co.za)

Example 9: Newton II - Box on inclined plane

QUESTION

A body of mass M is atrest on an inclined plane.

N

F

θ

What is the magnitude of the frictional force acting on the body?

A Mg

B Mg cos θ

C Mg sin θ

D Mg tan θ