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Published byAnnika Shean Modified about 1 year ago

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» A box weighing 100 N is pushed on a horizontal floor. The coefficient of kinetic friction is 0.2. » What acceleration will result if a person applies a horizontal force of 40 N? » Don’t even begin to calculate anything until you have drawn a FBD!!!!!!!

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» Forces are often applied by means of cables or ropes that are used to pull an object. » A force, T, is applied to the right end of a rope attached to a box. » Each particle in the rope in turn applies a force to its neighbor. » As a result the force is applied to the box.

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» How does the tension in the left end of the rope compare with the tension in the right end?

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» Which rope sustains the greater tension? (three people pull just as hard in both cases)

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» When we look at problems with tension, we assume that we are using a “mass-less” rope. » No net force is needed to accelerate a “mass-less” rope, so we can ignore it (otherwise you need to use calculus!)

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» An elevator has a mass of 1000kg. It accelerates upward at 3m/s 2. What is the force, F T, exerted by the cable on the elevator? F W (weight of elevator) F T (tension from the cable) ∑F = F T - F W = ma F T – 1000(9.8) = 1000(3) F T = 12,800 N

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» An elevator has a mass of 1000kg. The elevator is at rest. What is the force, F T, exerted by the cable on the elevator? F W (weight of elevator) F T (tension from the cable) ∑F = F T - F W = ma F T – 1000(9.8) = 0 F T = 9,800 N

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» The ability of a mass-less rope to transmit tension undiminished from one end to the other is NOT affected when the rope passes around a pulley

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» The ability of a mass-less rope to transmit tension undiminished from one end to the other is NOT affected when the rope passes around a simple pulley (one end is stationary)

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» A window washer on a scaffold is hoisting the scaffold up the side of a building by pulling downward on a rope. The pulling force is 540N and the mass of the worker and scaffold is 155kg. What is the acceleration of the unit?

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» An object is in equilibrium when it has zero acceleration. » Zero acceleration means constant velocity. (0 m/s is also constant velocity) » Equilibrium means that the forces are balanced in each direction (x, and y) » ΣF x = 0 » ΣF y = 0

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» Make sure to resolve all vectors into x- and y-components. » ΣF x = 0 » ΣF y = 0

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» An object is NOT in equilibrium if it is accelerating, meaning there is an unbalanced force(s) » Make sure to resolve all vectors into x- and y-components. » ΣF x = ma x » ΣF y = ma y » Use a triangle to find the resultant acceleration

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