1. Modeling and applying it
Friction
Modeling and applying it
§ 4.9

2. Poll Question Which requires more force? Dragging a box at 1 m/s.
Dragging the same box at 2 m/s.
Both require the same force.

3. Poll Question
Which pair of identical bricks experiences the greatest force of friction?
In tandem.
Stacked.
Same for both.

4. Board Work
A typical coefficient of static friction between car tires and wet pavement is 0.40.
What is the greatest acceleration at which a car can brake on level wet pavement?
Would it be more or less on a slope?
Would it matter if the slope is uphill or downhill?

5. Board Work
I push a 0.75-kg book across a table top at an initial speed of 1.3 m/s. It stops in 0.80 m. What is the coefficient of friction between the book and the table?

6. Scenario Problem
An 18-wheel 20-ton truck and a 4-wheel 1.5-ton car travel the same speed on the same stretch of road. M v m v

7. SCI 340 L11_traction
Board Work
An 18-wheel 20-ton truck and a 4-wheel 1.5-ton car travel the same speed on the same stretch of road.
a. Make symbolic expressions for the normal force exerted by the road on each vehicle (or by each vehicle on the road).
In general: N = F.
What is N here?

8. Board Work
An 18-wheel 20-ton truck and a 4-wheel 1.5-ton car travel the same speed on the same stretch of road. Both lock their tires, made of the same rubber.
b. Make symbolic expressions for the force of friction exerted by the road on each vehicle (or by each vehicle on the road).
In general: f = mN.
What do we know about mcar and mtruck?

9. Board Work
An 18-wheel 20-ton truck and a 4-wheel 1.5-ton car travel the same speed on the same stretch of road. Both lock their tires, made of the same rubber.
c. Make symbolic expressions for the acceleration of each vehicle.
In general, a = SF/m. What is SF on each?

10. SCI 340 L11_traction
Question
To slide a box across the floor, does it take less force F to push it or to pull it? F1 q F2 q
Explain using written words. 10

11. Board Work An object is at rest on an incline.
Draw a free-body diagram for it. a
Write formulas for all the (vector) forces acting on it.

12. Group Board Work
An object is on an incline. If the coefficient of static friction between the object and the incline is ms, what must be the angle a of the incline to make the object slip? or
What is the largest angle at which it won’t slip?