1. More Newton’s Laws Applications
Unit 3 Presentation 2

2. Revisit Newton’s 3rd Law
“Every action force has an equal in magnitude but opposite in direction reaction force”
The action and reaction forces act on DIFFERENT objects

3. Newton’s 3rd Law Examples
Action Force
Reaction Force
A gun is fired, propelling a bullet forward.
The gun fired backwards, in the opposite direction of the bullet.
A person pushes on a wall.
The wall pushes back on the person.
A person walks down a street, exerting a frictional force against the ground.
The ground exerts a normal force against the person, propelling them forward.

4. Newton’s 3rd Law Example
A block-block system, like the one pictured below, is pushed by a force of 15 N from the left to the right. Calculate
(a) The acceleration of the block-block system
(b) The magnitude of the action-reaction force in between the two blocks
First, break the two blocks into individual problems and draw two free-body diagrams:
5 kg
15 N
2 kg
Normal Force
Normal Force
AR Force
AR Force
15 N
15 N
2 kg
5 kg
Action/ Reaction Force (same on both blocks)
Gravity
Gravity
2 kg block
5 kg block

5. Newton’s 3rd Law Example (cntd)
Now, consider the free-body diagrams. Remembering to keep the x and y variables separate, set up some equations.
Do we need to consider the y direction forces? NO! The motion is only in the x direction in this example.
Now, using Newton’s 2nd Law on each block in the x-direction only:
Notice, we have 2 equations with 2 unknowns. Lets use substitution and substitute the right-hand equation into the left-hand equation:
Next, solve for the AR Force:

6. Another Newton’s 3rd Law Example
A block-block system, like the one pictured below, is pushed and accelerates at 3.2 m/s2 from the left to the right. Calculate:
(a) The force on the 8 kg block
(b) The magnitude of the action-reaction force in between the two blocks
3 kg
First, break the two blocks into individual problems and draw two free-body diagrams:
8 kg
3.2 m/s2
Normal Force
Normal Force
AR Force
AR Force F F
8 kg
3 kg
Action/ Reaction Force (same on both blocks)
Gravity
Gravity
8 kg block
3 kg block

7. Another Newton’s 3rd Law Example (cntd)
Now, consider the free-body diagrams. Remembering to keep the x and y variables separate, set up some equations.
Do we need to consider the y direction forces? NO! The motion is only in the x direction in this example.
Now, using Newton’s 2nd Law on each block in the x-direction only:
Now, solve the right-hand equation to determine the magnitude of the action-reaction force:
Next, solve for the Force:

8. The Force of Friction Friction is a force that always opposes motion
Opposite in direction to the motion of the object
Friction is directly related to the normal force:
m= Coefficient of Friction (pure number between 0 and 1)
m=0 No Friction (frictionless surface)
m=1 Highest frictional force possible

9. Types of Friction
Static Friction: The force of friction that opposes motion from beginning from rest. Also known as sticking friction.
Kinetic Friction: The force of friction that opposes motion as it is occurring. This only occurs after motion has begun.
Example: When pushing a large block, static friction prevents you from moving the block at all. When you finally start pushing it, kinetic friction takes over and slows the motion.
Always…..static friction is stronger than or equal to kinetic friction in magnitude.

10. Coefficients of Friction
ms =Coefficient of Static Friction
mk =Coefficient of Kinetic Friction
Always….

11. Coefficients of Friction for Common Surfaces
Materials
ms value
mk value
Steel on Steel
0.74
0.57
Aluminum on Steel
0.61
0.47
Copper on Steel
0.53
0.36
Rubber on Concrete
~1.0
0.8
Wood on Wood
0.25 – 0.50
0.2
Glass on Glass
0.94
0.4
Waxed wood on wet snow
0.14
0.10
Waxed wood on dry snow
0.01
0.04
Metal on Metal (lubricated)
0.15
0.06
Ice on Ice
0.03
Teflon on Teflon
Synovial joints in Humans
0.003

12. Frictional Forces Example
Calculate the static and kinetic frictional forces of a 5 kg block of aluminum on a steel table if a force were applied to move it to the right.
First, find the coefficients from the table on the previous slide:
Normal Force
f (friction)
F (applied)
Gravity (Weight)
Noting that there is no acceleration in the y direction and using Newton’s Second Law:

13. Frictional Forces Example (cntd)
Now, lets calculate the static frictional force:
Now, lets calculate the kinetic frictional force: