Unit 03b “The Force of Friction”

1. Unit 03b“The Force of Friction”

2. Define Friction • Friction is the force that occurs anytime two surfaces interact. • The force of friction always opposes motion. Define Static Friction • Friction that occurs between motionless objects. Define Kinetic Friction • Friction that occurs between objects in motion. Static Friction is _______than Kinetic Friction. greater

3. What are the two factors that the Force of Friction depends on? • Normal Force (FN) • Coefficient of Friction (μ) Which factor changes when… • Mass is added to a box? The Normal Force (FN) • Sandpaper is taped on the bottom of the box? The Coefficient of Friction (μ)

4. How does the Frictional Force change if a smoother surface is placed between two objects as they are pushed against each other? How does the Frictional Force change if more mass is added to the object being pushed across a floor? The Frictional force increases because the normal force increases. FN Ffriction The Frictional Force decreases because the coefficient of friction would also decrease.μ Ffriction

5. Normal Force FN (N)Newtons No Unit Coefficient of Friction μ

6. Friction Equation FFriction = μFN • Static Friction (Fs) Fs= μsFN • Kinetic Friction (Fk) Fk= μkFN

7. Draw a force diagram for the box on the box on a horizontal surface below. FN Fk Fpull FN = Fw (not moving up or down) Fk = Fpull (moving at a constant velocity or not moving) Fw What do you know about FN? Fk?

8. Draw a force diagram for the box on the inclined plane below. Include: Fw, FN, Fwx, Fwy, Fs FN Fs FWx FWy FW

9. Explain why FN=Fwy and why Fwx=Fk for a box moving at a constant velocity down the inclined plane. FN FN=Fwy because the box is not moving in the y-direction. So the Fnet must be zero, therefore FN and Fwy must be equal and opposite. Fk Fk=Fwx because the box is moving at a constant velocity in the x-direction. So the Fnet must be zero, therefore Fk and Fwx must be equal and opposite. FWx FWy Fw

10. Answer the following Questions The normal force and the coefficient of friction 1. What are the two factors the amount of frictional force depends on? • If the mass of an object is changed, which factor changes? • If the surface between objects is changed, which factor changes? • If a surface is changed from smooth to rough, does the frictional force increase or decrease? • If the weight of an object is changed so that the object is lighter does the frictional force increase or decrease? • If a surface is changed from sandpaper to tile, does the frictional force increase or decrease? The normal force The coefficient of friction increase decrease decrease

11. Easy: What is the coefficient of kinetic friction for a 700N box of bricks being pushed horizontally with a force of 500N if it moves with a constant velocity? Medium: What is the coefficient of static friction of a 50kg boy and wagon if a force of 400N starts him in motion? Hard: What is the coefficient of kinetic friction of a 600N box sliding down a 25o inclined ramp with a constant velocity?