1. Newton’s Second Law of Motion
Chapter 4
Newton’s Second Law
of Motion

2. The 2nd Law Newton’s Laws
The Sum of the Forces acting on a body is proportional to the acceleration that the body experiences
S F  a
S F = (mass) a

3. 2. NEWTON'S 2nd LAW OF MOTION

4. If the net force is perpendicular to the velocity, the direction of the velocity changes.
Acceleration is always in the direction of the net force.

5. Units of Force Force = (kg)(m/s2) = newton or
From the law SF = ma, we get
Force = (kg)(m/s2) = newton or
Force = (slug)(ft/s2) = pound

6. Dynamic Equilibrium Velocity is nonzero and constant Examples:
Driving at constant velocity
Normal up
Friction
Force from road
Weight down

7. Oppose motion or the tendency of motion
Dissipative Forces
Oppose motion or the tendency of motion
Friction
Air Resistance

8. Friction Depends upon... Static friction (no motion)
Materials that are in contact
Forces that press surfaces together (Normal forces)
Static friction (no motion)
Proportional to force used until maximum is reached
Kinetic friction (motion)
Nearly constant for normal speeds

9. Cause of Friction
Friction is caused by the “microscopic” interactions between the two surfaces

10. Friction
Static F f
Kinetic f F

11. Friction Measured
Force meter

12. Air Resistance Terminal Velocity! Depends on
Surface Area
Velocity
When weight = air resistance we have
SF= 0 Þ a = 0
Weight
Air Resistance
Terminal Velocity!