Grade 9-Forces

FORCES

In previous chapters, we studied how objects move. In this chapter, we will study why objects move as they do. We will study Newton's Laws of Motion, which explain the relationship between acceleration and force. We will also use Newton's Laws for problem solving.

1. What is a Force?

2. Newton's First Law of Motion

3. Newton's Second Law of Motion

4. Newton's Third Law of Motion

5. Mass and Weight

6. Friction

Section 1. What is a Force?

Force can be defined as a push or a pull. (Technically, force is something that can accelerate objects.) For example, when you throw a baseball, you apply a force to the ball. Force is measured by N (Newton). A force that causes an object with a mass of 1 kg to accelerate at 1 m/s is equivalent to 1 Newton.

Section 2. Newton's First Law of Motion

You will have to learn a new terminology here: net force. Net force is the sum of all forces acting on an object. For example, in a tag of war, when one team is pulling the tag with a force of 100 N and the other with 80 N, the net force would be 20 N at the direction of the first team (100 N - 80 N = 20 N).


When you slide your book on floor it will stop soon. When you slide it on icy surface, it will travel further and then stop. Galileo believed that when you slide a perfectly smooth object on a frictionless floor the object would travel forever.
Isaac Newton developed the idea of Galileo further. He concluded that an object will remain at rest or move with constant velocity when there is no net force acting on it. This is called Newton's First Law of Motion, or Law of Inertia

Section 3. Newton's Second Law of Motion

Newton's First Law deals with an object with no net force. Newton's Second Law talks about an object that has net force. It states that when the net force acting on an object is not zero, the object will accelerate at the direction of the exerted force. The acceleration is directly proportional to the net force and inversely proportional to the mass. It can be expressed in formula

F = ma

where:
F is the net force in N,
m is the mass of an object in kg and
a is its acceleration in m/s2.

From this formula, we can say that force is something that accelerates an object.

Section 4. Newton's Third Law of Motion

When you kick the wall in your room, you will probably end up hurting your foot. Newton's Third Law of Motion can explain why: when one object applies a force on a second object, the second object applies a force on the first that has an equal magnitude but opposite direction. In other words, when you kick the wall, the wall kicks you back with equal force. As a result you will get hurt. These forces are called action-reaction forces.
Remember when you kick the wall, you exerts force on the wall. When the wall kicks you back, it exerts force on you. Therefore, the net force on the wall is not zero and the net force on your foot is not zero neither.

Section 5. Mass and Weight

Mass and weight are different in physics. For example, your mass doesn't change when you go to the Moon, but your weight does. Mass shows the quantity, and weight shows the size of gravity.
If you know your mass, you can easily find your weight because

W = mg
where:
W is weight in Newton (N),
m is mass in kg, and
g is the acceleration of gravity in m/s2.
If your mass is 70 kg on Earth, your weight isW=(70 kg)(9.8 m/s2) = 686 N.
Weight is measured by Newton (N).

Section 6. Friction

You will have to learn another vocabulary before you proceed: the normal force. The normal force acts on any object that touches surface (either directly or indirectly). The normal force would be applied on a ball on a table, but not on a ball in the air, for instance. It always acts perpendicularly to the surface. The formula to calculate the normal force is

FN = - mg
where:
FN is the normal force in Newton (N),
m is the mass in kg, and
g is the gravitational force in m/s2.
For example, the normal force acting on a 70 kg-person would beFN = - (70 kg)(-9.8 m/s2) = 686N

Grade 8 Parallel circuit

Parallel circuits

If two or more components are connected in parallel they have the same potential difference (voltage) across their ends. The potential differences across the components are the same in magnitude, and they also have identical polarities. Hence, the same voltage is applicable to all circuit components connected in parallel. The total current I is the sum of the currents through the individual components, in accordance with Kirchhoff's circuit laws. The current in each individual resistor is found by Ohm's law.

Grade 8 - Series Circuit

In electronics, components of an electronic circuit can be connected in series or in parallel. Components connected in series are connected along a single path, so the same current flows through all of the components. Components connected in parallel are connected such that there are multiple independent paths along which the current can flow; in other words, the current is split among the different paths. A circuit composed solely of components connected in series is known as a series circuit; likewise, one connected completely in parallel is known as a parallel circuit.

Series circuits are sometimes called current-coupled or daisy chain-coupled. The current that flows in a series circuit will flow through every component in the circuit. Therefore, all of the components in a series connection carry the same current.

http://http://en.wikipedia.org/wiki/Series_and_parallel_circuits

Grade-7 How hydroeletricity works

So just how do we get electricity from water? Actually, hydroelectric and coal-fired power plants produce electricity in a similar way. In both cases a power source is used to turn a propeller-like piece called a turbine, which then turns a metal shaft in an electric generator , which is the motor that produces electricity. A coal-fired power plant uses steam to turn the turbine blades; whereas a hydroelectric plant uses falling water to turn the turbine. The results are the same.

Here are steps of how hydroelectricity works:
1. A hydropower plant uses falling water as stored energy. Water from the reservoir (A) passes through the penstock (a large pipe that carries water from the reservoir to turbines in the powerhouse) (B) to enter the powerhouse.

2. The flowing water turns the propeller-like water wheel or turbine (C), which is connected by a shaft to the generator (D), which spins and produces electricity.

3. As water leaves the turbine, it is discharged through the draft tube (E), where it enters the tailrace (F) and returns unaltered to the river below the dam.

4. The electricity produced by the spinning generator (D) is conducted to the power transformer (G), where the voltage is increased.