1. Lecture 2: More Forces

2. Springs 10kg Similar to tension, normal Reactive
Changes length according to tension
F = -k Δx
k=50 N/m
10kg

3. Free Body Diagrams Choose Object (book) Label coordinate axis
Identify All Forces
Hand (to right)
Gravity (down)
Normal (table, up)
Friction (table, left)
Normal
friction
hand
Gravity y x
Physics 40

4. Clicker Question: A box is hanging in an elevator by a rope
Clicker Question: A box is hanging in an elevator by a rope. The elevator is accelerating up at 2 m/s2
a= 2 m/s2
Compare the magnitude of the Tension to the magnitude of the Force of gravity
A |Fg| < |T|
B |Fg| = |T|
C |Fg| > |T|
10kg

5. 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

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

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

8. 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

9. Example: pulling a box What is the acceleration of the box? 40N m=10kg

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

11. Clicker Question Two blocks are being pulled along a frictionless floor by a 30N force
10kg
5kg
frictionless
What is the magnitude of the Tension, T, on the rope between the blocks?
A T = 30N
B T < 30N
C T > 30N

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

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