## EXAM GUIDELINE GRADE 12 2014

Newton’s laws and application of Newton’s laws
(This section must be read in conjunction with the CAPS, p. 62–66.)
Different kinds of forces: weight, normal force, frictional force, applied force (push, pull), tension (strings or cables)
• Define normal force, N, as the force or the component of a force which a surface exerts on an object with which it is in contact, and which is perpendicular to the surface.
• Define frictional force, f, as the force that opposes the motion of an object and which acts parallel to the surface.
Define static frictional force, fs, as the force that opposes the tendency of motion of a stationary object relative to a surface.
Define kinetic frictional force, fk, as the force that opposes the motion of a moving object relative to a surface.
Know that a frictional force:
o Is proportional to the normal force
o Is independent of the area of contact
o Is independent of the velocity of motion
• Solve problems using fmaxs = μsN where fmaxs is the maximum static frictional force and μs is the coefficient of static friction.
NOTE:
o If a force, F, applied to a body parallel to the surface does not cause the object to move, F is equal in magnitude to the static frictional force.
o The static frictional force is a maximum (fmaxs) just before the object starts to move across the surface.
o If the applied force exceeds fmaxs, a resultant/net force accelerates the object.
• Solve problems using fk = μkN, where fk is the kinetic frictional force and μk the coefficient of kinetic friction.

## Vertical projectile motion

Describe the motion resulting from the velocity vs time graph shown below, assuming an initial height of 0 m, and draw a displacement vs time graph and an acceleration vs time graph:

Chipa Thomas Maimela‘s insight:

The object starts with an initial height of 0 m, moves upwards for 1 s, then slows and falls back to its starting position where it bounces upwards at a higher velocity, and travels upwards (higher than before) for 2 s, then stops and falls back to its starting position where it bounces upwards again at an even higher velocity, travelling over 3 s to an even higher position, before finally falling back to the ground and stopping.

## Force diagrams

The block on a surface but with an applied force, FL, to the left of 10 N and an applied force, F R, to the right of 20 N. The weight and normal also have magnitudes of 10 N.

Chipa Thomas Maimela‘s insight:

It is important to keep the following in mind when you draw force diagrams:

Make your drawing large and clear.

You must use arrows and the direction of the arrow will show the direction of the force.

The length of the arrow will indicate the size of the force, in other words, the longer arrows in the diagram (FR for example) indicates a bigger force than a shorter arrow (FL). Arrows of the same length indicate forces of equal size (FN and Fg). Use “little lines” like in maths to show this.

Draw neat lines using a ruler. The arrows must touch the system or object.

All arrows must have labels. Use letters with a key on the side if you do not have enough space on your drawing.

The labels must indicate what is applying the force (the force of the car) on what the force is applied (on the trailer) and in which direction (to the right)

If the values of the forces are known, these values can be added to the diagram or key.

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## A model of the iron sulphide crystal

Water (H2O), for example, is a compound that is made up of two hydrogen atoms for every one oxygen atom. Sodium chloride (NaCl) is a compound made up of one sodium atom for every chlorine atom. An important characteristic of a compound is that it has a chemical formula, which describes the ratio in which the atoms of each element in the compound occur.

Chipa Thomas Maimela‘s insight:

Compound

A substance made up of two or more different elements that are joined together in a fixed ratio.

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## Components of vectors

Any vector can be resolved into a horizontal and a vertical component. If R  is a vector, then the horizontal component of R is R x and the vertical component is R y.

Chipa Thomas Maimela‘s insight:

A number of vectors acting together can be combined to give a single vector (the resultant). In much the same way a single vector can be broken down into a number of vectors which when added give that original vector. These vectors which sum to the original are called components of the original vector. The process of breaking a vector into its components is called resolving into components.

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