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# Motion in two dimension

الكلية كلية التربية للعلوم الصرفة     القسم قسم الفيزياء     المرحلة 1
أستاذ المادة فؤاد عطية مجيد       29/05/2018 09:11:58
Newton’s Law of Motion
5.1 Force
Force is a central concept in all of physics. It is a vector quantity- must describe the direction in which it acts as well as magnitude. A force is an action that produces a deformation and / or acceleration. The net force on a body is the sum of all forces acting on the body. We sometimes refer to the net force as the total force, the resultant force, or unbalanced force. When several force act simultaneously on an object, the object accelerates only if the net force acting on it, isn’t equal to zero. When the velocity of an object is constant, the object is said to be equilibrium; that is
F ?_net=0 or F ?_(net,x)=0 , F ?_(net,y)=0 and F ?_(net,z)=0
If the particle at rest, then we called static equilibrium, but if the particle moving with a constant velocity then we called dynamic equilibrium.
5.2 Newton’s First Law (Law of Inertia)
Everybody continues in its state of rest, or in uniform motion in a right line (straight line) unless it is compelled to change that state by forces impressed upon it.
F ?=0?v ?=constant
Fixed direction
Fixed magnitude
A body with no forces acting on it is called a force body. Most bodies we normally see, stop when left alone, because
Presence of frictional forces
Air pucks, air- tracks give a hint of the persistence of motion
5.3 Reference Frames
Law is not valid in all frames. Valid in special frames (inertial reference frames)
Test for inertial reference frame:
Take a free body (no forces acting)
If it persists in a state of uniform motion
Any other reference frame in uniform translational motion relative to the first is also an inertial frame. A frame in accelerated motion relative to the first is not an inertial frame. Earth based frame: not inertial ? effect small
5.4 Newton’s Second Law
This law established the relationship between force, acceleration and mass.
“The change in motion (acceleration) is proportional the motive force impressed, and is made in the direction of the right line (straight line) in which the force impressed”
F ?=ma ?=(dp ?)/dt ((dp ?)/dt more correct statement will discuss later)
Valid only in inertial reference frames
a ??F ? in the same direction

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