1. Today
Finish Newton’s Laws
More Forces

2. Friction(Static) No motion between surfaces (can be rolling)
Magnitude of frictional force (parallel to surfaces) is proportional to the normal force.
fstatic <= ms N ms coefficient of Static friction
Be Careful!
Static friction <=, can be any value up to msN
It is reactive just like Normal, Tension 42

3. Example: speeding truck
What is the magnitude of the force of friction on box?
m=10kg
µs = 0.4
a= 2m/s2

4. Example: speeding truck
What is the maximum acceleration of the truck?
m=10kg
µs = 0.4
a= 2m/s2

5. Friction (kinetic) Similar name, but behaves very differently!
fkinetic = mk N mk coefficient of Kinetic friction
The equals sign means we always know the magnitude
Direction is always opposite of motion

6. Example: pulling a box
40N
m=10kg
µk=0.3

7. Newton’s 3rd law Forces come in pairs
Same type (gravity, normal, friction)
Different objects
Always same magnitude, opposite direction

8. Example: pulling 2 blocksT
10kg
5kg
frictionless
Solve for acceleration
Solve for tension

9. Example: pulling 2 blocksT
10kg
5kg
µk= 0.2
Solve for acceleration
Solve for tension