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Newton’s Laws of Motion
Lecture 4
Monday: 26 January 2004
Newton’s Laws of Motion
Newton’s Second Law
in One Dimension
Commonly shortened to “F=ma”.
Correctly, it is :



 F  ma,
 F
a
m
Only forces which act on that object affect the
acceleration of the object.
Forces exert by the object on another object do not.
Using Newton’s 2nd Law to Solve
Problems
1. Identify all forces acting on the object
-Pushes or Pulls
-Frictional forces -Tension in a string
-Gravitational Force (or weight = mg where g is 9.8 m/s2)
- “Normal forces” (one object touching another).
2. Draw a “Freebody Diagram”
-draw the object, show all forces acting on that object as vectors
pointing in the correct direction. Show the direction of the
acceleration.
3. Chose a coordinate system.
4. Translate the freebody diagram into an algebraic expression based on
Newton’s second law.
Consider an elevator moving downward and speeding
up with an acceleration of 2 m/s2. The mass of the
elevator is 100 kg. Ignore air resistance.
What is the tension in the cable?
1. Identify Forces: Tension in cable, weight of the
elevator
v 2. Draw freebody diagram
T
a
W=Fg earthelevator.
Note: No
negative
sign
3. Chose coordinate system: Let up be the +y
direction and down –y. Then :
4. Translate the FBD into an algebraic expression. TW = m(-a) so
T-(100 kg)(9.8 m/s2) = (100 kg)(-2 m/s2)
Newton’s Third Law
•Whenever one object (object A) exerts a force on
another object (object B), the second object exerts a
force back on the first object.
•These forces are ALWAYS equal in magnitude (but
they point in opposite directions).
•Such forces are called “Newton’s third law force
pairs”.
•Not all forces that are equal and opposite are third
law force pairs.
•The forces are on different bodies, so do not add to
zero.