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Reading Assignment: (4.1) What did the book section say about the normal force? A.It is a regular force which can occur anywhere (as opposed to abnormal force). B.The normal force is always equal to the weight. C.It is the force exerted on an object by contact with a surface. D.It did not talk about the normal force.

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Forces and total force There is always a physical cause/source for a force: Basic sources of interactions: Gravity Electromagnetic interaction Weak/strong interaction (elementary particles) an interaction with another part of the world

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Forces and total force Derived sources of interactions: contact Friction, drag Normal force Pushing tension (pulling on rope) weight Magnetic or electric forces

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Properties of a force push or pull Acts on an object Requires an agent Is a vector Can be contact or long-range

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Is there a total force acting, and if yes, in which direction? A cloud is seen motionless in the sky.

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Is there a total force acting, and if yes, in which direction? A soccer ball is kicked across the field.

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Is there a total force acting, and if yes, in which direction? A parachute is falling at terminal velocity.

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Is there a total force acting, and if yes, in which direction? An air bubble is rising through oil at constant speed.

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Is there a total force acting, and if yes, in which direction? A pendulum bob swings back and forth between two extrema.

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Forces and total force Find the resulting force on the cart, assuming Blue contributes 100 N, while Red exerts 150 N: 100 N

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Forces and total force Find the resulting force on the cart, assuming Blue contributes 100 N, while Red exerts 150 N: 100 N 150 N 250 N

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Forces and total force Find the resulting force on the cart, assuming Blue contributes 100 N, while Red exerts 150 N: 150 N 50 N 100 N

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Forces and total force Find the resulting force on the cart, assuming Blue contributes 100 N, while Red exerts 150 N: 150 N Sum of red and blue 100 N Missing: force of gravity, contact force with floor (normal force)

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Reading Assignment: (4.2) Which of the following did the book offer as forms of Newtons first law? A.An object in motion stays in motion, and an object at rest stays at rest. B.A body acted on by no net force moves with constant velocity and zero acceleration. C.In an inertial frame of reference, the net force is zero. D.

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For which of these is the net force zero?

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Rope over pulleys How many center mass pieces can you add before masses A and B slide upward over the pulleys? A B

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Example: net force on inclined plane A crate is held in position on an inclined plane by a rope. The force of gravity on the crate is 1000 N, the incline angle is 20º. Since the rope has a critical tension of 800 N, it is justified question on whether this crate will be able to tear the rope apart. Determine the tension in the rope. 20º

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A ball thrown straight up has zero velocity at its highest point. Is the Ball in Equilibrium at this point? Why or why not?

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If the two ends of a rope in equilibrium are pulled with forces of equal magnitude and opposite direction, why is the tension in the rope not zero? If a rising bubble in a soda is in force equilibrium, what would you observe for the motion of the bubble?

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If the two ends of a rope in equilibrium are pulled with forces of equal magnitude and opposite direction, why is the tension in the rope not zero?

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