Chapter 4 – Part 2Force & motion with friction

Sue Ramlo, PhDPart 2 for Panopto / Springboard

Including frictional forces… what is friction? How do we calculate it?

The force of friction always opposes the direction of motion (or of the direction the motion would be in the absence of friction).

Depending on the circumstances, friction may be desirable or undesirable.

Two force of friction types:• ABOUT to move… Maximum..STATIC • Fs max= s FN

• If on incline, only mg, FN, Fs acting then… tan θ = s

• MOVING… kinetic friction force, Fk

• Fk = k FN

• If on incline, only mg, FN, Fk acting & constant speed then… tan θ = k

4.6 FrictionThe coefficients of friction depend on both materials involved.

Consider this system with friction…

20

Re-draw and find Fnet from picture• In motion so Fk m1g sin θ – T + T - m2g - Fk = Fnet

• Here == > Fk = k mg cos θ

• Newton #2 - Fnet = ma Fnet = (m1 + m2) a

• Bring together: m1g sin θ - m2g - k mg cos θ = (m1 + m2) a

• Solve for a…

m1g cos 20°

FN

m1g sin 20°

12

m2gTT

+ direction

Fk

Acceleration of system with friction…

• Solve for a

• Plug in values with units (k=0.030 … VERY small)

• Answer: a = 0.15 m/s2

(about half as much as w/o friction!)

6.5kg)20 cos N) (49 .0300( -(14.7N)20sin (49N)

= a

21

1k21

m + m)20 cos gm ( -gm20sin gm

= a

6.5kg)20 cos )m/s kg)(9.80 (5.0 ( -)m/s kg)(9.80 (1.520sin )m/s kg)(9.80 (5.0

= a2

k22

6.5kgN) (1.4 -(14.7N) (17N)

= a

108. In loading a fish delivery truck, a person pushes a block of ice up a 20° incline at constant speed. The push is 150 N in magnitude and parallel to the incline. The block has a mass of 35.0 kg. (a) Is the incline frictionless? (b) If not, what is the force of kinetic friction on the block of ice?  (a) no (b) 33 N

104. Suppose the slope conditions for the skier shown in Fig. 4.40 are such that the skier travels at a constant velocity. From the photo, could you find the coefficient of kinetic friction between the snowy surface and the skis? If so, describe how this would be done.  yes;

4.4 Newton’s Third Law of MotionFor every force (action), there is an equal and opposite force (reaction).

Note that the action and reaction forces act on different objects.

This image shows how a block exerts a downward force on a table; the table exerts an equal and opposite force on the block, called the normal force N.

4.4 Newton’s Third Law of MotionThis figure illustrates the action-reaction forces for a person carrying a briefcase. Is there a reaction force in (b)? If so, what is it?

52. In an Olympic figure-skating event, a 65-kg male skater pushes a 45-kg female skater, causing her to accelerate at a rate of At what rate will the male skater accelerate? What is the direction of his acceleration?   opposite to her’s