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Applications of Newton’s Laws
Kimal Honour Djam
Alabama School of Fine Arts
Warm-Up: Newton’s Laws

N1L: An object at rest will remain at rest unless it is acted upon by a
net external force. An object in motion with constant velocity will
continue to move with constant velocity unless it is acted upon by a
net external force.
if

F  0
then v = 0 and  p  0
N2L: If a net external force acts on the body, the body accelerates.
The direction of acceleration is the same as the direction of the
net force. The net force vector is equal to the mass of the body times
the acceleration of the body.
p
 F  t  ma

N3L: For every action there is an equal and opposite reaction.


FA on B   FB on A
Newton’s Law Examples
Newton’s Law Examples
Free body Diagrams
N1L and N2L apply to a specific body.
Decide to which body you are referring! It is not trivial
sometimes.
Only forces acting on the body matter.
To analyze person walking, include the force that the ground
exerts on the person as he walks, but NOT the force that the
person exerts on the ground.
Free-body diagrams are essential to help identify the
relevant forces.
Action-reaction pair NEVER appear in the same free-body
diagram. When a problem involves more than one body: take
this problem apart and draw a separate free-body diagram for
each body.
Free body Diagrams
Free body Diagrams

Homework due 9/12
• Draw a free-body diagram of a block which
slides down a frictionless plane having an
inclination of = 15.0° If the block starts from
rest at the top and the length of the incline is
2.00 m, find
(a) the acceleration of the block and
(b) its speed when it reaches the bottom of the
incline.
Free body Diagrams: Apparent Weight
Free body Diagrams: Apparent Weight
Free body Diagrams: Apparent Weight
Free body Diagrams: Apparent Weight
Normal Force Is Not Always
Equal to the Weight
Homework Due 9/12
• Two blocks connected by a rope of negligible
mass are being dragged by a horizontal force F
. Suppose that F = 68.0 N, m1 = 12.0 kg, m2 =
18.0 kg, and the coefficient of kinetic friction
between each block and the surface is 0.100.
(a) Draw a free-body diagram for each block.
(b) Determine the tension T and the
magnitude of the acceleration of the system.