Young's Double Slit Experiment

The video below does a pretty good explanation of Young's double slit experient. This experiment demonstates the interference of light and therefore, confirms the wave nature of light. For our class purposes you are only required to watch the video up to 2:15. You can watch the rest of it if you like.

Adding Vectors and Scalars

Scalar quantities are added numerically, for example, 3kg + 5kg = 8kg.

When adding vector quantities, however, the direction must be taken into account.

Adding Forces

Forces are represented by lines drawn to scale. Its direction it the direction of the vector quantity. If we have a force of 5 N east we may use a scale where 1cm represents 1N. Therefore, the 5N force would be represented by a 5cm line.

Below we have three examples. In each case we have to add forces of 3N and 4N. We will calculate the resultant force of each.

Example1:

In this example, the two forces are parallel and in the same direction. Therefore, they produce a resultant force of: 4N + 3N = 7N, to the right. Note, you must state the direction.

Example 2:

In this example, the two forces are in exactly opposite directions. The resultant force is therefore: 4N - 3N = 1N, to the right. Note, you must state the direction.

Example 3:

Example 4:

In these cases, the Principle of Parallelogram of Forces must be applied. This principle states that if the two forces are represented in size and direction by the sides of a parallelogram drawn to scale, the resultant force is represented by the the diagonal drawn from the point where the force acts.

This video should show you how to calculate the resultant vector. If you still do not get it, you can use YouTube.

Result for SBA #12

Below is the table of results for SBA #12:

V/cm3	50.0	45.0	40.0	35.0	30.0	25.0	20.0	15.0
t1/s	0	13.06	26.7	42.19	58.37	75.66	94.38	115.88
t2/s	0	13.94	28.4	44.13	60.84	78.75	98.26	126.62
t/s

Results for SBAs #10 and 11

SBA #10

A = 0.52A
B = 0.26A
C = 0.16A
D = 0.08A
E = 0.52A

SBA #11

Number of Throws	Undecayed Atoms
1	47
2	20
3	12
4	6
5	4

The results for SBA #12 are not good so that will be done again.

Force, Mass and Weight

Force

A force can be defined as a push or a pull. It can change the size, shape or motion of a body. There are several different types of forces, some of which are:

• Gravitational force
• Electrical force
• Magnetic Force
• Nuclear force
  

Measuring Force

The spring balance is used to measure force. The spring balance is based on the fact that the extension of the spring is proportional to the force applied. The unit of force is the newton (N).

Stretching a Spring

We hang a wire spring from a retort stand. A pointer and a small pan are hung at the bottom of the spring. We set up a scale, marked in millimetres vertically by the side.

We add a series of small, equal weights which stretch the spring. We measure the extension i.e. the extra length, of the spring. Typical results are shown in the table below:

Force F/N	0	1.0	2.0	3.0	4.0	5.0
Extension e/cm	0	1.6	3.2	4.8	6.4	8.0

If a graph of extension against load is plotted, a straight line passing through the origin will be obtained. This means that extension if proportional to the load.

Elastic Limit of Springs

If we remove the weights the spring contracts to its original length. However, if we continue to add greater weights, eventually the spring stretches by different amounts. At this point, the extension is no longer proportional to the load.

When we remove the lager weights the spring does not return to its original length. We have gone beyond the elastic limit of the spring. The new graph looks like this:

Hooke's Law

Hooke's law states that the extension of a spring is directly proportional to the force applied, provided the elastic limit has not been exceeded.

We can obtain similar graphs for elastic bands and for straight metal wires. Elastic bands need smaller forces for a measurable extension and metal wires need much greater ones. 

The following video explains more on Hooke's Law:

Mass and Weight

Mass

The mass of an object is the measure of the amount of matter it contains. The mass of an object is also a measure of a resistance to a change in its motion. This resistance is known as the inertia of the body.

We measure mass using a triple beam balance as shown in the picture below. The unit of mass is the kilogram (kg). The mass of an object is constant everywhere.

Weight

The weight of an object is the force of gravity on the object. It acts towards the centre of the earth.

We measure weight using a spring balance as shown in the diagram below. The unit of weight is the newton (N). A mass of 1 kg has a weight of approxiamtely10N on earth. The weight of an object changes is the force of gravity changes.

In outer space there is no gravity, a 1kg mass has no weight. On the moon, where the gravity is about a sixth i.e. 1/6 that on the earth, a mass of 1 kg has a weight of about 1.6 N, as shown in the calculation below

Motion 2

Equations of Uniformly Accelerated Motion

When we have objects moving with a constant acceleration we can also use certain equations to solve problems. In these equations we use the following symbols:

You should be able to recall ALL of the following equations:

This implies that

The above equation can be rewritten as:

You should also know ALL of the following:

Falling Objects

We can drop a large stone and a piece of chalk from a height of 3 or 4 metres and they land simultaneously. However, a stone will reach the ground faster than a piece of paper. If the stone and paper are allowed to fall in a vacuum they fall together.

When objects fall in the air the resistance of the air has a greater effect on the light objects, such as the paper, than on heavy ones, such as the stone. Fluid friction (or viscous drag) always opposes the fall of objects through fluids, i.e. liquids and gases.

Acceleration due to Gravity

If the effects of air resistance are eliminated, or negligible, then all objects fall with the same acceleration. This is called acceleration due to gravity, g.

The acceleration due to gravity has slightly different values in different parts of the world but all values are about 9.81 metres per second squared. We often take g as 10 metres per second squared, as a convenient approximation.

SBAs

Note that all of your SBAs are posted to the blog. Try to get them all through. I will be collecting your books on Thursday, 27th March, 2014 to mark up to wherever you are at. You will collect your books the same day so you can finish up your labs.

The official deadline for your physics SBA books is next week Wednesday i.e. Wednesday, 2nd March, 2014. Try to be all through.

We are nearing the end so put in the effort. It will soon be all over.

Good luck!