1. Intro problem (p.145 #1-2, p.147 #1-4)
Friction Problems
Intro problem
(p.145 #1-2,
p.147 #1-4)

2. Intro problem
A 50 N force is pushing horizontally on a 10 kg. box. The coefficient of static friction between the floor and the box is 0.43.
Will the box accelerate?
What will be the box’s acceleration?

3. Intro problem
A 50 N force is pushing horizontally on a 10 kg. box. The coefficient of static friction between the floor and the box is 0.43.
If the coefficient of kinetic friction between the box and the floor is 0.25, (keeping the 50 N pushing force), what will the box’s acceleration be now?
What force is needed to keep the box moving at a constant velocity?

4. Intro problem
A 50 N force is pushing horizontally on a 10 kg. box. The coefficient of static friction between the floor and the box is 0.43.
If instead of a horizontal push, the person is pushing down at an angle of 35 degrees above horizontal. What is the minimum force needed to keep the box moving at a constant velocity?

5. P. 145 #1-2
A 24 kg crate is in motion and a horizontal force of 53 N keeps the crate moving with constant velocity. Find μk between the crate and the floor.
A 25 kg chair initially at rest on a horizontal floor requires a 365 N horizontal force to set it in motion. Once the chair is in motion, a 327 N horizontal force is required to keep it moving at a constant velocity. What is μk and μs?

6. P. 147 #1-4
A student moves a box of books down the hall by pulling a rope that is attached to the box. The student pulls with a force of 185N at an angle of 25 degrees above the horizontal. The box has a mass of 35.0 kg and μk between the box and the floor is Find the acceleration of the box.
The student in #1 moves the box up a ramp inclined at 12 degrees with the horizontal. If the box starts from rest at the bottom of the ramp and is pulled at an angle of 25 degrees with respect to the incline and with 185 N of force, what is the acceleration up the ramp? Assume μk = 0.27

7. P. 147 #1-4
A 75 kg box slides down a 25 degree ramp with an acceeleration of 3.60 m/s2.
What is μk between the box and the ramp?
What acceleration would a 175 kg box have on this ramp?
A box of books weighing 325 N moves with a constant velocity across the floor when it is pushed with a force of 425 N exerted downward at an angle of 35.2 degrees below horizontal. Find the μk between the floor and the box.

8. Practice Problem #2 How many seconds until the sled comes to a stop?
You are pushing a 15 kg sled with a force of 25 N, at an angle of 40 degrees above horizontal. The sled has a constant velocity of 5 m/s when you encounter a rough patch of ice. The coefficient of kinetic friction between the rough ice and the sled is Assuming you continue to push with the same force…
How many seconds until the sled comes to a stop?
How far did you get on the rough ice before stopping?

9. Practice Problem #3
Two kids are pulling a sled up a hill (see pic below), both with a force of 20 N. The taller child pulls the wagon at an angle of 30 degrees while the smaller one pulls at an angle of 15 degrees. Are they able to successfully pull the wagon up the hill? What is the acceleration of the sled and in which direction? The coefficient of friction between the sled and ice is 0.15.