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4.4 Everyday Forces. Weight, Normal Force and Friction pp. 141 - 149 Mr . Richter. Agenda. Check and Review HW Warm Up Notes Today and Tomorrow: Weight Normal Force Friction Static Kinetic Overcoming Friction Problem-Solving Practice. Upcoming Dates: Take Home Quiz Due Tomorrow
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4.4 Everyday Forces Weight, Normal Force and Friction pp. 141 - 149 Mr. Richter
Agenda • Check and Review HW • Warm Up • Notes Today and Tomorrow: • Weight • Normal Force • Friction • Static • Kinetic • Overcoming Friction • Problem-Solving Practice • Upcoming Dates: • Take Home Quiz Due Tomorrow • Methods Rough Draft Wednesday • This can be written directly into your notebook. • Chapter 4 Test Friday
Objectives: We Will Be Able To… • Explain the difference between mass and weight. • Find the direction and magnitude of normal force. • Describe air resistance as a form of friction. (Tomorrow) • Use coefficients of friction to calculate frictional force. (Tomorrow)
Warm-Up: • A child pulls a wagon along behind him. Is the force the ground applies up on the wagon… • greater than the force of gravity pulling down on the wagon? • less than the force of gravity pulling down on the wagon? • equal to the force of gravity pulling down on the wagon? • Draw a FBD to help you consider the problem, then choose one answer and discuss at your table. We will discuss as a class momentarily.
Weight The force due to the acceleration of gravity.
Weight • Weight is the magnitude of the force due to gravity acting on an object. • Because force is simply mass*acceleration, the formula to calculate weight is Fg=mg. • g is 9.81 m/s2 • the units of weight are Newtons • 1 lb = 4.4 N
Weight • Is your weight different on the moon? What about your mass? • Weight changes depending on the acceleration of gravity. If gravity changes, weight changes. • Mass is the amount of molecules in an object. Mass is not dependent on gravity, so it will not change on the moon.
The Normal Force • We know that when objects are in equilibrium vertically (Σfy = 0), there must be some force to balance out gravity. • This force is the normal force. • More technically: the normal force is a force exerted by one object on another in a direction perpendicular to the surface of contact. • Normal = perpendicular.
The Normal Force • The normal force is • ALWAYS perpendicular to the surface • NOT ALWAYS in the opposite direction of gravity • Gravity always pulls straight down, the normal force can be at an angle.
Warm-Up • Is it easier to start an object in motion or to keep an object in motion? • Choose one and then discuss at your table.
Friction • Friction is the force that opposes an applied force. Friction opposes motion. • Friction can come in many forms. For example: • surfaces • fluids (like air and water) • electrical (called resistance) • Objects experiencing no applied force experience no friction.
Static Friction • When an object experiences an applied force and still does not move, it experiences static friction. • Static friction (Fs) is the force that opposes motion of two objects whose surfaces are at rest. • Static = not moving. • The force of static friction is equal and opposite to the applied force. • Fs= -Fapplied
Kinetic Friction • When an object is moving, it still experiences friction, but the force of friction is less than the applied force (hence acceleration!). • Kinetic friction (Fk) is the force that opposes the motion of two objects whose surfaces are moving past one another. • Kinetic = moving. • Kinetic friction is always less than the maximum static friction.
Coefficients of Friction • The force of friction is proportional to the normal force. • The amount of force keeping the surfaces in contact will affect how hard it is to move an object. • Friction also depends on which surfaces are in contact with each other. • A hockey puck sliding on ice works better than trying to slide on sandpaper. • The coefficient of friction (μ) represents the ratio between the normal force and the force of friction between two surfaces.
Coefficients of Friction • Static Friction: Fs = μsFn • Kinetic Friction: Fk= μkFn p. 144
Practice Problem • A 24-kg crate initially at rest on a horizontal floor requires 75 N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor.
Warm-Up • It takes 450 N of force to slide a 120 kg refrigerator across a linoleum floor at a constant speed. • Find the coefficient of friction between the friction between the refrigerator and the floor. • Is this static or kinetic friction?
Plan of Attack • Draw a diagram and list your given info. • Find sum of the forces in the y-direction. Often but not always zero. Σy = ? • Find sum of the forces in the x-direction. • Zero if the object is in equilibrium (at rest or constant velocity) • Non-zero if asked to find acceleration of an object. • Solve and check for reasonableness.
Practice Problem • A student moves a box of books by attaching a rope to the box and pulling with a force of 90.0 N at an angle of 30.0°. The box of books has a mass of 20.0 kg, and the coefficient of kinetic friction between the box and the sidewalk is 0.50. Find the acceleration of the box.
1. Draw a diagram. List given info. Fn • Fa = 90.0 N • θ=30.0° • m = 20.0 kg • μk= 0.50 Fa Fk 30.0° Fg
2. Sum of the forces in the y-direction • Σfy = 0. The box does not rise off of nor sink into the sidewalk. Fn Fa Fay Fk 30.0° Fg
3. Sum of the forces in the x-direction. Non-zero, to find acceleration. Fn Fa Fk 30.0° Fax Fg
4. Solve. Find Acceleration a = 0.12 m/s2
Wrap-Up: Did we meet our objectives? • Explain the difference between mass and weight. • Find the direction and magnitude of normal force. • Describe air resistance as a form of friction. • Use coefficients of friction to calculate frictional force.
Homework • p145 #1-3 • p 147 #1,4