1. Ch. 4 Forces

2. Describe an object’s acceleration in terms of its mass and the net force
acting on it.
Predict the direction and magnitude of the acceleration caused by a known
net force.
3. Identify action-reaction pairs.

4. Newton’s 2nd Law Force is proportional to mass and acceleration.
The acceleration of an object is directly proportional to the net external force acting on the object and inversely proportional to the object’s mass.
The direction of the acceleration is in the same direction as the applied force

5. Newton’s 2nd Law of Motion (N2), the Law of Acceleration
The acceleration of an object is directly proportional to the net force acting on the object:
a = ΣF m

6. .
Comment: The essence of Newton’s 2nd Law is that an object WILL ACCELERATE if and only if the EXTERNAL FORCES acting on it are NOT BALANCED. A Free Body Diagram is used to depict the external forces and Newton’s 2nd Law ties the dynamics to the kinematics. Each external force includes a type, a direction, and a source.

7. F = ma Newton’s 2nd Law Force (F) has units of Newtons (N)
Mass (m) has units of kilograms (kg)
Acceleration (a) has units of meters per second squared (m/s2)

8. In other words…..using the same amount of force….
Small acceleration a
Large Mass
Large acceleration F a
Small Mass

9. Roberto and Laura are studying across from each other at a wide table
Roberto and Laura are studying across from each other at a wide table. Laura slides a 2.2 kg book towards Roberto. If the net external force acting on the book is 2.6 N to the right, what is the book’s acceleration?

11. Newton’s 3rd Law
For every action there is an equal and opposite reaction
Force always exist in pairs, even field forces
Pairs are known as action-reaction pairs

12. Newton’s 3rd Law of Motion (N3), the Law of Interaction
When one object exerts a force on a second object the second object exerts the same magnitude force in the opposite direction on the first object SIMULTANEOUSLY.
Comment: The essence of Newton’s 3rd Law is that all forces are actually pairs in which the objects force each other simultaneously and oppositely REGARDLESS of the MOTION of EITHER. However, since we NEVER SEE FORCES we cannot see this relationship.

13. Action and Reaction pairs
Action-reaction pairs do not imply that the net force on either object is zero.
The action-reaction forces are equal and opposite but either object may still have a net force on it.

14. a = m Earth / Apple (cont.) a m Apple’s Earth’s little mass big mass
The products are the same, since the forces are the same.
a = m m a
Apple’s
little mass
Earth’s
big mass
Earth’s little acceleration
Apple’s big acceleration

15. Identify the action-reaction pair in the following situations.
A person takes a step
A snowball hits someone in the back

16. Identify the action-reaction pair in the following situations.
A baseball player catches a ball
A gust of wind strikes a window

17. The forces acting on a sailboat are 390 N north and 180 N east
The forces acting on a sailboat are 390 N north and 180 N east. If the boat has a mass of 270 kg, what are the magnitude and direction of its acceleration?

18. If a locomotive can exert a constant pull of 7
If a locomotive can exert a constant pull of 7.5x105 N, how long would it take to increase the speed of the train from rest to 85 km/h? (disregard friction)