Forces of Friction Contact between bodies with a relative velocity produces friction opposite

Transcription

1 Forces of Friction Contact between bodies with a relative velocity produces friction Friction is proportional to the normal force The force of static friction is generally greater than the force of kinetic friction The coefficient of friction (µ mu ) depends on the surfaces in contact The direction of the frictional force is opposite the direction of motion The coefficients of friction are nearly independent of the area of contact

2 Fig. 4-19, p.101

3 Static Friction ƒ s Static friction acts to keep the object from moving If F increases, so does ƒ s If F decreases, so does ƒ s ƒ s µ n where n = normal force vector uur uur f = s -F

4 Kinetic Friction, ƒ k The force of kinetic friction acts when the object is in motion ƒ k = µ k n Direction of ƒ k opposite to motion: opposes motion Variations of coefficient of friction with speed will be ignored

5 HITT RF Remote Login Procedure: (If you have a really old IR remote you do not need to login.) The radio channel number for this room is 07 (zero, seven). It is STRONGLY recommended to login your remote for every class just to be sure it is on the correct radio channel and working before class. 1. PRESS AND HOLD THE DOWN ARROW KEY until the GREEN light on the remote turns RED. 2. PRESS THE 0 KEY and you will see the RED light flash GREEN. 3. PRESS THE 7 KEY and you will see the RED light flash GREEN. 4. PRESS AND RELEASE THE DOWN ARROW KEY again and you will see the red light search for the receiver, if it BLINKS GREEN MULTIPLE TIMES you are logged in.

6 In-class Quiz 2/3 If you press a book flat against a vertical wall with your hand, in what direction is the friction force exerted by the wall on the book? 1. downward 2. upward 3. out of the wall 4. into the wall 5. Can not be determined

7 In-class Quiz 2/23 A crate is sitting on the center of a flatbed truck. As the truck accelerates to the east, the crate moves with it, not sliding on the bed of the truck. In what direction is the friction force exerted by the bed of the truck on the crate? 1. To the west 2. Upward 3. To the east 4. Downward 5. Can not be determined

8 In-class Quiz 2/23 There are six books in a stack, each with a weight of 5.0 N. The coefficient of friction between all the books is 0.20 as is the coefficient between the table and the bottom book. What horizontal push must I just exceed on the next to bottom book to start sliding the top five books off the bottom one? N N N N

9 Ex. Suppose a block with a mass of 2.50 kg is resting on a ramp. If the coefficient of static friction between the block and ramp is 0.35, what maximum angle can the ramp make with the horizontal before the block starts to slip down?

10 In-class Quiz 2/3 For the block on the ramp the frictional force is pointed-- 1. Normal to the ramp 2. Down the down 3. Up the ramp 4. Out of the page 5. Can not be determined

11 Axes are rotated as usual on an incline The direction of impending motion would be down the plane Friction acts up the plane Opposes the motion Draw Free Body Diagram yada yada yada Block on a Ramp

12 Ex 4.13 m1=4kg m2=7kg µk =0.3 Find acc. and tension.

13 Mechanical Energy Chapter Energy and Work Kinetic (associated with motion) Potential (associated with position) Chemical Energy Electromagnetic Energy Nuclear Energy Energy can be transformed from one form to another But not destroyed--conserved Work/Energy can be used in place of Newton s laws to solve certain problems more simply

14 W Work F is the magnitude of the net force x is the magnitude of the object s displacement r θ is the angle between F (Fcos θ) x This gives no information about the time it took for the displacement to occur the velocity or acceleration of the object Work is a scalar quantity r and x

15 Lecture Quiz 2/3 What are the units of Energy & Work? A. Newton meter B. kg m 2 / s 2 C. foot pound D. Joule E. All of the above

16 Units of Work SI Newton meter = Joule N m = J J = kg m 2 / s 2 US Customary foot pound ft lb no special name Energy: ability to do work

17 Work is Zero when force and displacement are perpendicular Carrying a bucket of water Displacement is horizontal Force is vertical cos 90 = 0 So no work done!

18 Work Can Be Positive or Negative Work done on box is positive when lifting the box Work is negative if lowering the box The force would still be upward, but the displacement would be downward OR Box does positive work on student when lowered

19 Kinetic Energy Energy associated with the motion of an object KE = 1 mv 2 2 Scalar quantity with the same units as work Work is related to kinetic energy

20 Work-Kinetic Energy Theorem When work is done by a net force on an object and the only change in the object is its speed, the work done is equal to the change in the object s kinetic energy W = KE KE = KE net f i Speed will increase if work is positive Speed will decrease if work is negative

21 Work and Kinetic Energy An object s kinetic energy can also be thought of as the amount of work the moving object could do in coming to rest The moving hammer has kinetic energy and can do work on the nail