1. 4.2 Newton’s First Law and Net External Forcepp
Mr. Richter

2. Agenda Warm Up Review HW Notes: Inertia Newton’s First Law
Net External Force
Calculating Net External Force
Equilibrium

3. Objectives: We Will Be Able To…
Explain the relationship between the motion of an object and the net external force acting on it.
Determine the net external force on an object.
Describe the behavior of objects in equilibrium.

4. Warm-Up:
If a car is traveling at a constant velocity, are there any forces acting on it? Remember, a force changes an object’s velocity.
Write 2-3 complete sentences answering the prompt in your notes. We will discuss this in a few minutes.

5. Inertia

6. Inertia
Inertia is the tendency of an object to maintain its state of motion.
The more inert an object is, the more stubborn it is. Inertia is a measure of resistance to changes in motion.
What quality of an object contributes to inertia?
MASS! The more mass an object has, the more inertia it has, the more it resists changes in motion.

7. Newton’s First Law (N1L)
Newton’s First Law of Motion is often called the Law of Inertia.
It states:
An object at rest remains at rest, and an object in motion continues in motion with constant velocity (that is, constant speed in a straight line) unless the object experiences a net external force.
In other words: objects keep doing what they’re doing unless a force tells them to do something else.

8. Net External Force

9. Net External Force
An external force is a single force that acts on an object as a result of the interaction between an object an its environment.
Ex: Gravity pulling down on the car.
The net external force is the vector sum of all of the forces acting on the object.
Add up all the force vectors to determine a resultant magnitude and direction.

10. Calculating Net External Force Steps for Success
Draw a diagram.
Pick an x-y coordinate system that is convenient.
Find the x and y components of all vectors (BE CAREFUL WITH ANGLES!):
ΣFx = ? (sum of the forces in the x-direction)
ΣFy = ? (sum of the forces in the y-direction)
Find the net external force vector (Fnet) (this could involve the Pythagorean theorem and trig.)
Check for realism.

11. Practice Problem
Derek leaves his physics book on top of a drafting table that is inclined at a 35° angle. Use the diagram to find the net external force acting on the book, and determine whether or not the book will remain at rest.

12. Draw a Diagram
Ftable = 18 N
Ffriction = 11 N
35°
Fg = 22 N

13. 2. Choose a Convenient Coordinate Systemx
Ftable = 18 N
Ffriction = 11 N
35°
Fg = 22 N

14. 3. Find the x and y components
Ftable = 18 N
ΣFx= -Ffriction + Fg,x ΣFx = cos(55) ΣFx = = 2N Σfy = Ftable – Fg,y Σfy = 18 – 22 sin 55 Σfy = 18 – 18 = 0
Ffriction = 11 N
55°
35°
Fg = 22 N

15. 4. Find the Net External Force
In this case, since there is no net force in the y-direction, the Fnet = Σfx = 2N down the table. This will not always be true.
In most cases, the resultant force needs to be calculated from the x and y components.
Use the Pythagorean Theorem and tan-1
Σfx
Σfy
Fnet θ

16. Equilibrium

17. Equilibrium
An object is said to be in equilibrium if the net external force (in all directions) acting on the object is zero.
In essence, the forces are all balanced, or they cancel each other out.
Objects will continue to accelerate until they reach a state of equilibrium.

18. Equilibrium Can an object be in equilibrium and still be moving?
Yes, an object in equilibrium can still maintain constant velocity, without changing speed or direction, as long as the object is not accelerating.
Examples:
An ice skater gliding.
A car coasting.

19. Wrap-Up: Did we meet our objectives?
Explain the relationship between the motion of an object and the net external force action on it.
Determine the net external force on an object.
Describe the behavior of objects in equilibrium.

20. Homework Due Thursday:
p. 133 #1-4
p. 135 #1-3
Lab Notebook and Background Research due tomorrow.