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2 Mechanical Equilibrium
Different Types of Forces• Gravity (also called weight)• Friction• Applied Force • Air Resistance• Tension• Spring Force• Normal Force (also called support force)
• http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/newtlaws/u2l2b.html#spring
2 Mechanical Equilibrium
2.3 Definition of Forces
1. Gravity
2. Tension Force
3. Spring Force
4. Applied Force
5. Friction
6. Air Resistance
7. Normal force
Force of attraction due to mass of an object
Pulled force applied to a rope, wire or cable
Elastic, compressed or stretched force
Any push or pull on an object
Resistance of two objects in contact moving past each other
Resistance of an object moving through the atmosphere
Force that pushes back on one object resting on another
2 Mechanical Equilibrium
For an object at rest on a horizontal surface, the support force (normal force) must equal the object’s weight (gravity).
2.3 Support Force
2 Mechanical Equilibrium
Tension, Weight and Spring
The upward tension in the string has the same magnitude as the weight of the bag, so the net force on the bag is zero.
The bag of sugar is attracted to Earth with a gravitational force of 2 pounds or 9 newtons.
2.1 Force
•1 lb = 4.45 NWeight (of bag)
Tension (in string)
•9 N of weight = 9 N of tension and spring force
Spring force (in scale)
2 Mechanical Equilibrium
The table pushes up on the book with as much force as the downward weight of the book.
2.3 Support Force
Support Force (also called normal force)
Weight (also called gravity)
Fnorm = Fgrav
Net force
2 Mechanical Equilibrium
2.3 Support Force Physics
The book lying on the table compresses atoms in the table and they squeeze upward on the book. The compressed atoms produce the support force.
2 Mechanical Equilibrium
The upward support force is as much as the downward pull of gravity.
2.3 Support Force
2 Mechanical Equilibrium
The upward support force is as much as the downward pull of gravity.
2.3 Support Force
2 Mechanical Equilibrium
think!What is the net force on a bathroom scale when a 110-pound person stands on it?
2.3 Support Force
2 Mechanical Equilibrium
think!What is the net force on a bathroom scale when a 110-pound person stands on it?
Answer: Zero–the scale is at rest. The scale reads the support force, not the net force.
2.3 Support Force
2 Mechanical Equilibrium
think!Suppose you stand on two bathroom scales with your weight evenly distributed between the two scales. What is the reading on each of the scales? What happens when you stand with more of your weight on one foot than the other?
2.3 Support Force
2 Mechanical Equilibrium
think!Suppose you stand on two bathroom scales with your weight evenly distributed between the two scales. What is the reading on each of the scales? What happens when you stand with more of your weight on one foot than the other?
Answer: In the first case, the reading on each scale is half your weight. In the second case, if you lean more on one scale than the other, more than half your weight will be read on that scale but less than half on the other. The total support force adds up to your weight.
2.3 Support Force
2 Mechanical Equilibrium
For an object at rest on a horizontal surface, what is the support force (or normal force) equal to?
2.3 Support Force
•The object’s weight (or gravity)!
2 Mechanical Equilibrium
Tension and Weight
A stretched spring is under a “stretching force” called tension.
Pounds and newtons are units of weight, which are also units of force.
Weight is a force.
2.1 Force
