Newtons Motion Laws
Force force is a pull or push which generates or tends to generate motion in a body at rest; stops or tends to stop a body in motion; increases and decreases the magnitude of velocity of the moving body or changes or tends to change the shape of the body.
Newton’s first law of motion a body at rest remains in its state of rest and a body in uniform motion remains in tits state of uniform motion unless it is compelled by some external force to change its state.
The inherent property with which the body cannot change its state of rest or of uniform motion unless acted upon by an external force, is called inertia, hence Newton‘s first law of motion may also be called law of inertia.
Internal force if the force applying agent lies inside the system force is internal. internal force cannot provide motion. For example, if you are sitting in a car (stationary but free state), you push the car, the same force, car moves. When you were inside the car, force applying agent was inside the system, therefore, force was internal and car did not move. When the force applying agent (you) had moved outside, the car moves.
Mass in Newtonian mechanics mass is considered to be a measure of inertia of a body and is considered independent of its velocity, it is a scalar quantity and measured in kg (SI system)
Momentum (P) the total quantity of motion contained in a body is called momentum. It is a vector quantity and is measured in kg ms-1.
P = mv.
If two bodies of different masses have same momentum, the lighter has more velocity (or more KE).
Newton’s second law of motion the time rate of change of momentum is directly proportional to external force applied on it and the change in momentum occurs in the direction of forces.
F α dp / dt or F = kdp / dt
Where k is a constant of proportionality. In cge or in SI system k = 1. Thus the equation of motion becomes
F = dp / dt = mdv / dt = ma
In scalar form f = ma is called equation of motion.
Force is a vector quantity. Its unit is Newton (N). Kilogram weight (Kg wt) or kilogram force (kgf) is gravitational unit of force.
1 kg wt – 1 kgf = 9.8 N
Accelerated motion may be produced (a) due to change in its speed (b) due to change in its direction.
Basic forces in nature there are four basic forces in nature. All other forces are manifestation of these forces. The basic forces are gravitational force, electromagnetic force, strong force and weak nuclear force.
Gravitational force: electromagnetic force: weak force: strong force :: 1 : 1038 : 1026 : 1040
In situations like rocked v may be constant (escape velocity) while m changes.
Therefore F = - grad u = - d (u) / dr where u is P.E (potential energy).
Newton’s third law of motion to every action there is an equal and opposite reaction.
F AB = - F BA
That is forces in nature occur in pair, single isolated force is not possible, action and reaction act on different bodies.
Law of conservation of linear momentum if no external force acts, then the total momentum of the system is conserved
F = dp / dt = O or p = constant
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