2 Mechanical Equilibrium 2 1 Force What is

2 Mechanical Equilibrium 2. 1 Force • What is a force? Measured quantity SI Unit Symbol Force Newton N A force is a push or a pull.

2 Mechanical Equilibrium 2. 1 Force Net Force The combination of all forces acting on an object is called the net force.

2 Mechanical Equilibrium 2. 1 Force Balanced vs. Unbalanced Force Balanced force occurs when the net force = 0 N (the object experiences no change in motion) Unbalanced force occurs when the net force > 0 N (the object experiences a change in motion) • It takes an unbalanced force to change the motion of an object!

2 Mechanical Equilibrium 2. 1 Force Net Force When the girl holds the rock with as much force upward as gravity pulls downward, the net force on the rock is zero. • Fnorm = Fgrav Normal Force Gravity

2 Mechanical Equilibrium 2. 1 Force Net Force The net force depends on the magnitudes and directions of the applied forces. • The blue arrows are called vectors (they indicate size and direction)

2 Mechanical Equilibrium 2. 1 Force Net Force The net force depends on the magnitudes and directions of the applied forces. • The blue arrow in the net force column is a type of vector called the resultant (5 N + 10 N = 15 N)

2 Mechanical Equilibrium 2. 1 Force Net Force The net force depends on the magnitudes and directions of the applied forces.

2 Mechanical Equilibrium 2. 1 Force Net Force The net force depends on the magnitudes and directions of the applied forces. • 10 N – 5 N = net force of 5 N to the right

2 Mechanical Equilibrium 2. 1 Force Net Force The net force depends on the magnitudes and directions of the applied forces.

2 Mechanical Equilibrium 2. 1 Force Net Force The net force depends on the magnitudes and directions of the applied forces. • 5 N – 5 N = net force of 0 N …which is equilibrium!

2 Mechanical Equilibrium 2. 1 Force Vectors can be drawn to scale This vector represents a force of 60 N to the right. N • Vectors show magnitude (size) and direction

2 Mechanical Equilibrium Resultant • The sum of two or more vectors • Can show the magnitude and direction of the net force.

2 Mechanical Equilibrium Resultant (forces in same direction and in opposite directions) 40 N 20 N Net force = 60 N to the right Resultant? 40 N 20 N Net force = 20 N to the right Resultant?

2 Mechanical Equilibrium Resultant (pulling or pushing at an angle)

2 Mechanical Equilibrium Resultant (pulling or pushing at an angle)

2 Mechanical Equilibrium Identify the resultant and the horizontal and vertical forces

2 Mechanical Equilibrium 2. 1 Force How can you change an object’s state of motion? • Apply a net force greater than zero, which is applying an unbalanced force to the object

2 Mechanical Equilibrium An object in mechanical equilibrium is stable, without changes in motion.

2 Mechanical Equilibrium Things that are in balance with one another illustrate equilibrium. Things in mechanical equilibrium are stable, without changes of motion. The rocks are in mechanical equilibrium. • http: //library. thinkqu est. org/27638/leanin g. html

2 Mechanical Equilibrium • http: //sciencefair. math. iit. edu/projects/pisa/ • An object will fall if its center of mass is no longer situated within its base

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. k 12. il. us/GBSSCI/PHYS/Cl ass/newtlaws/u 2 l 2 b. html#spring

2 Mechanical Equilibrium 2. 3 Definition of Forces 1. Gravity Force that pushes back perpendicularly on one object resting on another; support force • 7 • 2 2. Tension Force Pulled force applied to a rope, wire or cable 3. Spring Force of attraction between to objects due 4. Applied Force to their masses, AKA weight. • 1 5. Friction 6. Air Resistance 7. Normal force Resistance of two objects in contact moving past each other • 5 Resistance of an object moving through the atmosphere • 6 Elastic, compressed or stretched force • 3 Any push or pull placed on an object • 4

2 Mechanical Equilibrium 2. 3 Normal Force (support force) For an object at rest on a horizontal surface, the support force (normal force) must equal the object’s weight (gravity).

2 Mechanical Equilibrium 2. 1 Force 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. • 1 lb = 4. 45 N Spring force (in scale) Weight (of bag) • 9 N of weight = 9 N of tension and spring force Tension (in string)

2 Mechanical Equilibrium 2. 3 Support Force (also called normal force) The table pushes up on the book with as much force as the downward weight of the book. Fnorm = Fgrav Net force Weight (also called gravity)

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 2. 3 Support Force The upward support force is as much as the downward pull of gravity.

2 Mechanical Equilibrium 2. 3 Support Force The upward support force is as much as the downward pull of gravity.

2 Mechanical Equilibrium 2. 3 Support Force think! What is the net force on a bathroom scale when a 110 -pound person stands on it?

2 Mechanical Equilibrium 2. 3 Support Force 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 Mechanical Equilibrium 2. 3 Support Force 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 Mechanical Equilibrium 2. 3 Support Force 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 Mechanical Equilibrium 2. 3 Support Force For an object at rest on a horizontal surface, what is the support force (or normal force) equal to? • The object’s weight (or gravity)!

2 Mechanical Equilibrium 2. 1 Force 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.