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Types of Forces Contact Forces Forces at a distance.

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Four Main Forces in Universe Gravity F g. Electromagnetic EM Strong Nuclear Strong or Nuclear Weak Nuclear Weak All Forces fall into one of the above categories.

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Normal Force (reaction force) – perpendicular to the surfaces in contact or touching. Common Forces

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What if you’re leaning on a wall. Where is the normal force?

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Where is it now?

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Sliding Friction – force acts at contact between surfaces. Direction of friction opposes velocity or attempted velocity. F f = F n is constant Which way is friction?

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Which way is the book moving? Right Left The box is not moving.

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Tension – transmitted through string, rope, or wire by forces at each end.

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Air Resistance – air friction. Big reason for bad fuel economy in cars.

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Spring or Elastic Force – Force applied by stretched or compressed spring described by Hooke’s Law.

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Magnetic Force – due to excess electron spin

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Electrical – due to excess charge buildup.

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Weight F g = mg gravity interaction with mass. Weight depends on gravity, Mass does not. What is mass? Amount of matter in object.

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Question: What will be the mass of a 10-kg object on the moon? 1. Zero 2. Less than 10 – kg 3. 10 – kg. 4. Impossible to tell without more information.

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Calculation of Weight F g and mass. g = F g /m F g = mg. F g = gravity on object = weight N. m = mass, kg. g = accl gravity m/s 2. On Earth g = 9.81 m/s 2.

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1. A student weighs 850-N on Earth. What is his mass? F g = mg. F g = m g 850 N 9.81 m/s 2. = 86.6 kg

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Find mass on Earth. m = F g /g m = 650 N/ 9.81 m/s 2. m = 66 kg. g = F g /m g = 12.6 N/ 66 kg. g = 0.2 m/s 2. 2. A 650-N Earth student weighs 12.6 N on planet Tessaz. What is the acceleration of gravity on Tessaz?

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3. Little Jimmy weighs 490-N on Earth. If the acceleration of gravity on MonsterY is 50 m/s 2, what is the force of gravity on Little Jimmy on MonsterY? m = F g /g 490N/9.81 m/s 2. m = 50 kg. F g = mg (50kg)(50m/s 2 ) = 2500N

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3. Make a sketch graph of weight (Y) vs. mass (X). Write the equation for the graph. What is the slope of the line?

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Hwk Sheet 1 - 15.

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Normal Force F n. Contact between surfaces. Always perpendicular to surface. horizontalIf object rests on horizontal surface, then: F n = -W = mg. FnFn

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How does Normal Force Arise? How does a wall know to push back harder when I push with increasing force?

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The object will be at rest only if the surface can exert an equal & opposite force to sustain it, otherwise the object crashes through the surface.

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Friction Between Surfaces

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Friction depends on the nature of the materials in contact and the smoothness of their surfaces.

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. Friction is independent of surface area.. F f is directly proportional the normal force. The ratio of F f, to F n, is the coefficient of friction, . = FfFfFfFf FnFnFnFn has no units. F f = F n. Where is a constant.

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File Cabinet Demo

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depends on two surfaces in contact the type of sliding motion – starting or moving. Does not depend on any other factor. It is a constant.

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Types of Friction Static Friction use st – Present for objects at rest. Need to overcome to start objects moving. Kinetic Friction or sliding use k – Present for objects in motion. Need to overcome to keep objects in motion. Always less than static friction.

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Reference Table

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1. A 10-kg rubber box is at rest on a dry horizontal concrete surface. Calculate: a. the weight of the box. b. the normal force on the box. c. the force needed to start the box moving. d. the force to keep the box moving a constant velocity. 98.1 N Ff = st Fn. = (0.9 )98.1 N = 88.9 N Ff = st Fn. = (0.68 )98.1 N = 66.7 N

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