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# Lecture 03 Dynamics Forces and Motion Along A Line

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• 1. Dynamics: Forces and Motion Along A Line
• 2. Force ( )
- a vector quantity that basically affects the motion of the object
- commonly defined as “push” or “pull”
Units of Force:
SI Unit: newton (N)
Gaussian: dyne
English: pound (lb)
Net Force ( )
- resultant force or sum of all forces acting on the object.
• 3. Net force
Physicists who contributed to the development of force and motion relationship:
Aristotle (384-322 BC)
Galileo (2000 years later, approx. 1678)
Isaac Newton (1687)
- Principia (book published)
- “three laws of motion”
• 4. Newton’s First Law
Law of Inertia:An object at rest or an object in motion at constant velocity will remain at rest or at constant velocity in the absence of a net force.
• The body remains at rest.
• 5. The body moves at constant velocity.
• Discuss what the driver experiences when a car accelerates from rest and then applies the brakes.
(a) The driver is forced to move forward. An object at rest tends to remain at rest.
Understanding the First Law:
(b) Driver must resist the forward motion as brakes are applied. A moving object tends to remain in motion.
• 6. mass (m)
- is a measure of inertia of a body.
- synonymous to quantity of matter. This means that it is constant!
* The more mass a body has, the harder it is to change its state of motion.
• 7. Newton’s Second Law:
Law of Acceleration:Whenever a net force acts on an object, it produces an acceleration with a direction the same to the direction of the net force - an acceleration that is directly proportional to the net force and inversely proportional to the mass.
Unit Conversion:
1 N = 1 kg m/s2; 1 dyne = 1 g cm/s2
1 lb = 1 slug ft/s2
• 8. Pushing the cart with twice the force produces twice the acceleration. Three times the force triples the acceleration.
Acceleration and Force With Zero Friction Forces
• 9. F
F
a/2
a
Pushing two carts with same force F produces one-half the acceleration. The acceleration varies inversely with the amount of material (the mass).
Acceleration and Mass Again With Zero Friction
• 10. Newton’s Third Law
Force of Man on Floor
Force of Ceiling on Man
Force of Wall on Hands
Force of Floor on Man
Force of Hands on Wall
Force of Man on Ceiling
Law of Interaction:Toevery action force there must be an equal and opposite reaction force.
Action and reaction forces act on different objects.
• 11. Action
Reaction
Action
Reaction
Newton’s Third Law
Two More Examples:
Action and Reaction Forces Act on Different Objects. They Do Not Cancel Each Other!
• 12. Weight, Normal Force, and Tension
Weight (w) – gravitational force
w = mg
where m = mass of the object
g = acceleration due to gravity
• For practical purposes on the earth, 1 kg = 2.2 lb since g is relatively constant near the surface.
• 13. Weight is not constant while mass is!
• The force of gravity acts on an object when it is falling.
When the body is at rest on the earth, the gravitational force on it does not disappear.
Why, then, doesn’t the object move?
There must be another force to balance the gravitational force.
• 14. Normal force (FN)
- contact force that acts perpendicular to the common surface of contact.
FN
T
w
w
Tension (T )
- force of a string, cable, wire running on the body