4.4 Everyday Forces

1. 4.4 Everyday Forces Weight, Normal Force and Friction pp. 141 - 149 Mr. Richter

2. 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

3. 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)

4. 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.

5. Weight The force due to the acceleration of gravity.

6. 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

7. 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.

8. The Normal Force

9. 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.

10. 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.

11. 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.

12. Friction

13. 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.

14. 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

15. 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.

16. Static and Kinetic Friction

17. Coefficients of Friction

18. 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.

19. Coefficients of Friction • Static Friction: Fs = μsFn • Kinetic Friction: Fk= μkFn p. 144

20. 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.

21. 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?

22. Everyday Force Problems

23. 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.

24. 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.

25. 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

26. 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

27. 3. Sum of the forces in the x-direction. Non-zero, to find acceleration. Fn Fa Fk 30.0° Fax Fg

28. 4. Solve. Find Acceleration a = 0.12 m/s2

29. 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.

30. Homework • p145 #1-3 • p 147 #1,4