Class 9 General Science NCERT Based - 3 - Free MCQ Practice Test with solutions,

For every action, there is an equal and opposite reaction.
The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The  size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs - equal and opposite action-reaction force pairs.

Therefore, momentum can neither be created nor destroyed. Law of conservation of momentum is an important consequence of Newton’s third law of motion. Following are the examples of law of conservation of momentum:

1. Air filled balloons

2. System of gun and bullet

3. Motion of rockets

• Sir Isaac Newton published three laws in the 17th century. In this article, we are going to talk about Newton’s 1st law. This law does an introduction of motion of the object and the force acting on it.

• In other words, it deals with the motion of an object and its relation to force. Newton’s first law states that: a body remains in the state of rest or uniform motion in a straight line unless and until an external force acts on it.

• Putting Newton’s 1st law of motion in simple words, a body will not start moving until and unless an external force acts on it. Once it is set in motion, it will not stop or change its velocity until and unless some force acts upon it once more. The first law of motion is sometimes also known as the law of inertia.

• Newton’s Second Law of Motion Newton’s second law of motion can be formally stated as follows: The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

• The second law of motion gives us a method to measure the force acting on an object as a product of the mass of the object and the acceleration of the object which is the change in velocity with respect to time.

• The difference between the specific gravity and density is that at room temperature and pressure is 1gram per 1 cubic cm is the density of water this density is treated as a standard and density of any other material (usually liquids) is calculated relative to the , this is called relative density or specific gravity.

• Hence, specific gravity is the ratio of the mass of a substance to that of a reference substance, let’s consider the density of honey is approx. 1.42 grams/cm3, so its specific gravity would be 1.42/1 = 1.42.

• Notice that specific gravity is a ratio, therefore specific gravity does not have a unit, and hence specific gravity is a dimensionless physical quantity. The specific gravity of a substance will let us know if it will float or sink, it gives us the idea about relative mass or relative density.

• If the specific gravity of a substance is below 1 then it will float and if it is greater than 1 it will sink. Relative density is the ratio of density of a substance to the density of a given reference material. Thus, it is a unit less quantity.

• Gravitational attraction is caused by the mass of an object. Since Earth is far more massive than the Moon, the gravitational force exerted on the Moon is far greater than that of the Moon on the Earth.

• An example of the difference: while the Moon causes tides on the Earth, the Earth has the Moon locked so that the same face (minus some wobbling) is always visible from the Earth.

• The term potential energy was introduced by the 19th-century Scottish engineer and physicist William Rankine. There are several types of potential energy, each associated with a distinct type of force.

• It is the energy by virtue of an object’s position relative to other objects. We can define potential energy as: The energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors.

• Similarly, in the case of a spring, when it is displaced from its equilibrium position, it gains some amount of energy which we observe in the form of stress we feel in our hand upon stretching it.

• We can define potential energy as a form of energy that results from the alteration of its position or state. Potential Energy Formula: The formula for potential energy depends on the force acting on the two objects.

• For the gravitational force the formula is: W = m×g×h = mgh Where, m is the mass in kilograms g is the acceleration due to gravity h is the height in meters Unit: Gravitational potential energy has the same units as kinetic energy: kg m² / s² Note: All energy has the same units – kg m² / s², and is measured using the unit Joule (J).

• A force must be applied on a body to accelerate an object. Work must be done in order to apply a force. The body will move with an unvarying speed after the work has been done due to the energy provided by it.

• The speed and the mass of the body are factors on which the energy transfer that makes up the kinetic energy depends. The kinetic energy of an object is the energy that it possesses due to its motion.

• Kinetic energy definition is given as: The energy of an object because of its motion or the energy gained by an object from its state of rest to motion. Formula of Kinetic Energy: Following is the formula of kinetic energy: KE = 12mv² Where, KE is the kinetic energy of the object m is the mass of an object v is the velocity of an object Kinetic energy is an example of scalar quantity which means that the quantity has only magnitude and no direction. Unit of Kinetic Energy: The SI unit of kinetic energy is Joule which is equal to 1 kg.m².s^-2. The CGS unit of kinetic energy is erg.

• If you rub your hands together for several seconds, you'll notice that your hands feel warm.

• That warmth is caused by a force called friction. When objects like your hands come in contact and move against each other, they produce friction.

• Friction is defined as: The resistance offered by the surfaces that are in contact with each other when they move over each other.

• Friction works in the opposite direction in which the body is moving making the body slow down. Friction is useful in most of the cases. Friction is also dependent on the external factors. Factors

• Energy is required for the evolution of life forms on earth. In Physics, it is defined as the capacity to do work. We know that energy exists in different forms in nature. You have learned about various forms of energy – heat, electrical, chemical, nuclear, etc. In this article, we will learn about the laws and principles that govern energy.

• This law is known as the law of conservation of energy. What is the Law of Conservation of Energy? The law of conservation of energy states that energy can neither be created nor be destroyed. Although, it may be transformed from one form to another.

Milli Watt is the smallest unit of power.

• Chemical energy is defined as : the energy which is stored in the bonds of chemical compounds (molecules and atoms). It is released in the chemical reaction and mostly produces heat as a by-product, known as an exothermic reaction.

• The examples of stored chemical energy are biomass, batteries, natural gas, petroleum, and coal. Mostly, when the chemical energy is released from a substance, it is transformed into a new substance completely. For instance, when an explosion goes off, the chemical energy in it is transferred to the surroundings as thermal energy, kinetic energy, and sound energy.

Chemical Energy in Everyday Life:

• We know that plants need solar energy to produce sugar from carbon dioxide to water.

Sugar, water and carbon dioxide stay together by chemical bonds that hold the chemicals together.

• For instance, all sugars consist of oxygen, carbon and hydrogen atoms held together by chemical bonds. These atoms do not connect together automatically; rather some energy is required to make them stay together.

• Plants use solar energy to put the hydrogen, the carbon and the oxygen atoms as a whole in the form of sugar. This is a suitable example of energy transformation where energy is transformed from one form to another. Here, solar energy is transformed into chemical energy and prevents it from falling apart.

• It is actually well known that heat is simply a form of energy which transfers hotness from one point to the other. It gets transferred from one point to another by three modes of which radiation is the one.

• The most common illustration for it is electromagnetic radiation. Light gets transferred in the form of electromagnetic radiation. Other examples are solar radiation, nuclear radiation etc Electromagnetic radiation is continuously emitted from all substances because of the molecular as well as atomic vibration accompanying the internal energy.

• Wavelength can be defined as the distance between two successive crests or troughs of a wave. It is measured in the direction of the wave.

• Description: Wavelength is the distance from one crest to another, or from one trough to another, of a wave (which may be an electromagnetic wave, a sound wave, or any other wave).

• Crest is the highest point of the wave whereas the trough is the lowest. Since wavelength is distance/length, it is measured in units of lengths such as metres, centimetres, millimeters, nanometres, etc.

• A sound is a form of energy, just like electricity, heat or light.

• Sound Waves are Longitudinal waves.

• Sound or Audible waves are sensitive to the human ear and are generated by the vibrating bodies like tuning fork, vocal cords etc.

• Infrasonic waves are produced by sources of bigger size such as earthquakes, volcanic eruptions, ocean waves etc. Human ear cannot detect Ultrasonic waves.