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BellworkPick up a free-body diagram sheet and begin working on it.October 21, 2009SWBAT state and calculate weightSWBAT solve for coefficients of static and kinetic friction

CatalystHang on to worksheets from yesterday2Didnt make it all the way through Net Forceneed to move faster through beginning examplesWeight & FrictionWeight the mass and acceleration of an object due to gravityWeight & FrictionWeight the mass and acceleration of an object due to gravity creates weight. aka - FgWeight & FrictionFg = mg

Force of Gravity = mass x gravityWeight & FrictionFg = mg

An object with a mass of 9kg has what weight or Fg?Weight & FrictionFg = mgAn object with a mass of 9kg has what weight or Fg?Fg = (9)(9.81)Weight & Friction

Fg = mgAn object with a mass of 9kg has what weight or Fg?Fg = 88.3NWeight & Friction

What force is resisting the box?Weight & FrictionWhat force is resisting the box?Weight & Friction

This is called the Normal Force (Fn)

Weight & Friction

Normal Force (Fn) The upward force equal to the downward Fg exerted by a surface.Weight & FrictionMoving a RefrigeratorDraw a free body diagram for the following situation.A man pushes against a 85 kg refrigerator with 50 N of force but it doesnt go anywhere. Friction (Ff) The force that opposes the relative motion of two surfaces in contactFriction is proportional to the Fn and depends on the surfaces in contact.

Weight & FrictionStatic friction (Fs) = the resistive force that keeps something from moving when at rest

Static means not movingFs = -FappliedFs = sFn where s is the coefficient of static friction(it has no units)

Weight & FrictionMoving the FridgeDraw a free body diagram for the following situation.A man pushes against a 85 kg refrigerator with 50 N of force but it doesnt go anywhereWhat is the coefficient of static friction between the floor and the fridge?Kinetic FrictionObjects also experience friction while they are moving

For moving object, friction:Points opposite the direction of motionIs parallel to the surface

The equation is Fk=kFn where k is the coefficient of kinetic friction (also no units)Question. Think about pushing a box full of books across the floor. Is it harder to push before or after it starts moving? So would you expect the coefficient of static or kinetic friction to be a greater value? (indicating more Friction)

Weight & Friction A 24kg crate initially at rest on a horizontal floor requires a 75N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor. Figure out a planTry to solve without putting the numbers in first. s = .32

Weight & Friction a 91kg refrigerator is placed on a ramp. The refrigerator begins to slide when the ramp is raised to an angle of 34. What is the coefficient of static frictionFigure out a plan Try to solve without putting the numbers in first. = .67

Weight & Friction A student moves a box of books by attaching a rope to the box and pulling with a force of 90.0N at an angle of 30.0. The box of books has a mass of 20.0kg, and a coefficient of kinetic friction between the bottom of the box and the sidewalk is .50. Find the acceleration of the box.Steps (how are we going to find our answer?)Draw diagramFind Fn using ( Fy = 0)Calculate the force of kinetic friction (Fk = kFn)Use Newtons 2nd Law on the horizontal axis to find acceleration (Fx = max) a = .12m/s2 to the right

Weight & Friction Two students are sliding a 225kg sofa at a constant speed across a wood floor. One student pulls with a force of 225N at an angle of 13 above the horizontal. The other student pushes with a force of 250N at an angle of 23 below the horizontal. What is the coefficient of kinetic friction between the sofa and the floor?Steps (how are we going to find our answer?)Draw diagramFind Fk using ( Fy = 0)Calculate the coefficient of kinetic friction (Fk = kFn)k = .20

Group Practice Problem!Practice WorkPg 145; #1, 2, 3a, 3bPg 147; #1, 3, 4A 24kg crate initially at rest on a horizontal floor requires a 75N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floorWeight & FrictionA block sliding ( = .57) on a surface has a normal force of 13N, what is the friction force?Ff = (.57)(13)Weight & FrictionFf = FnA block sliding ( = .57) on a surface has a normal force of 13N, what is the friction force?Ff = 7.4NWeight & FrictionFf = FnA block sliding ( = .1) on a surface has a friction force of 8N, what is the normal force?Weight & FrictionFf = FnA block sliding ( = .1) on a surface has a friction force of 8N, what is the normal force?8 = (.1)(Fn)Weight & FrictionFf = FnA block sliding ( = .1) on a surface has a friction force of 8N, what is the normal force?Fn = 80NWeight & FrictionFf = FnA 30 kg block sliding ( = .2) on a surface has what friction force?Weight & FrictionFf = Fn

A 30 kg block sliding ( = .2) on a surface has what friction force?Weight & FrictionFf = Fn

A 30 kg block sliding ( = .2) on a surface has what friction force?Fg = mgWeight & FrictionFf = Fn

A 30 kg block sliding ( = .2) on a surface has what friction force?Fg = 30(9.81)Weight & FrictionFf = Fn

A 30 kg block sliding ( = .2) on a surface has what friction force?Fg = 294.3NWeight & FrictionFf = Fn

A 30 kg block sliding ( = .2) on a surface has what friction force?Fg = 294.3NFg = FnWeight & FrictionFf = Fn

A 30 kg block sliding ( = .2) on a surface has what friction force?Fn = 294.3N

Weight & FrictionFf = Fn

A 30 kg block sliding ( = .2) on a surface has what friction force?Fn = 294.3NFf = (.2)(294.3)Weight & FrictionFf = Fn

A 30 kg block sliding ( = .2) on a surface has what friction force?Fn = 294.3NFf = 58.9NWeight & FrictionFf = Fn

What net force is being on the box below?Weight & Friction

What net force is being on the box below? 222NWeight & FrictionIf a 22N force is being applied to a 33kg block sliding ( = .03) on a surface, what is the net force?Weight & FrictionIf a 22N force is being applied to a 33kg block sliding ( = .03) on a surface, what is the net force?Fg = 33(9.81)Weight & FrictionFg = mgIf a 22N force is being applied to a 33kg block sliding ( = .03) on a surface, what is the net force?Fg = 323.7NWeight & FrictionFg = mgIf a 22N force is being applied to a 33kg block sliding ( = .03) on a surface, what is the net force?Fg = 323.7N = FnWeight & FrictionFg = mgIf a 22N force is being applied to a 33kg block sliding ( = .03) on a surface, what is the net force?Ff = (.03)(323.7)Weight & FrictionFf = FnIf a 22N force is being applied to a 33kg block sliding ( = .03) on a surface, what is the net force?Ff = 9.7NWeight & FrictionFf = FnIf a 22N force is being applied to a 33kg block sliding ( = .03) on a surface, what is the net force?Fnet = 22 - 9.7Weight & FrictionFf = FnIf a 22N force is being applied to a 33kg block sliding ( = .03) on a surface, what is the net force?Fnet = 22 - 9.7 = 12.3Weight & FrictionFf = FnTake it a step further, how fast is the block accelerating?Weight & FrictionFnet = maTake it a step further, how fast is the block accelerating?F = 12.3 m = 33 a = ?Weight & FrictionFnet = maTake it a step further, how fast is the block accelerating?F = 12.3 m = 33 a = ?12.3 = 33(a)Weight & FrictionFnet = maTake it a step further, how fast is the block accelerating?F = 12.3 m = 33 a = ?a = .37m/s2Weight & FrictionFnet = maIf a 2003N force is being applied to a 55kg block sliding ( = .47) on a surface, what is the net force? How fast is it accelerating?Weight & FrictionIf a 2003N force is being applied to a 55kg block sliding ( = .47) on a surface, what is the net force? How fast is it accelerating?Fnet = 1749.4NWeight & FrictionIf a 2003N force is being applied to a 55kg block sliding ( = .47) on a surface, what is the net force? How fast is it accelerating?Fnet = 1749.4N a = 31.8m/s2Weight & FrictionDo the Frictional practice Worksheet. Finish for HW