1. Newton’s Three Laws of Motion and Forces

2. Newton’s Three Laws of Motion and Forces
Sections Covered – Honors Physics
Chapter 6: pages
Chapter 7: Section 1: pages
Topics Covered
Force of Gravity and Weight
Force
Normal Force
Mass and Inertia
Friction
Newton’s Three Laws of Motion
*Tension – Atwood Machine
Equilibrium and Non- equilibrium
*Weight in an Elevator
Free Body Diagrams

3. Forces Force - a push or a pull
**Difference between pushing a bowling ball vs. softball?
*Mass

4. What is mass? Good Question!
*Little Kid Definition:
How much stuff something has in it
*Chemistry Definition:
How much matter something contains
Physics Definition:
Mass is a numerical measurement of inertia.
Mass (m)
SI Units: kilograms (kg)

5. What is Inertia? Good Question!
Main Theme of Nature
Nature is lazy, doesn’t like change
Inertia - object’s tendency to resist a change in motion
Objects at rest want to stay at rest
Objects in motion want to stay in motion

6. Newton’s First Law: Law of Inertia
An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion will continue in motion with the same velocity unless acted upon by an unbalanced force.
*So why is its nickname, the law of inertia

7. Newton’s Second Law A force acting on a mass will cause the mass to…..
Accelerate!
Newton’s Second Law
Formula for Force:
F = ma
Force
acceleration
mass
SI Units for Force:
Newton (N)
Force is a VECTOR quantity

8. Newton’s Second Law
For a single force:
F = ma

9. Newton’s Second Law For a single force: F = ma For multiple forces:
Greek Letter – Capital Sigma
Means ‘Sum of…’

10. Newton’s Second Law Two options with multiple forces:
1. Forces are balanced:
Equilibrium
Object is at rest
Object is moving at a constant velocity (a = 0 m/s2)
2. Forces are not balanced:
Non-Equilibrium

11. The Sum of Forces – Working with Multiple Forces
Force is a VECTOR
Can be broken down into COMPONENTS
Sum of the forces in the x-direction F Fy
Sum of the forces in the y-direction Fx

12. How Much is a Newton of Force?
So one Newton is actually pretty small.
One Newton is the force felt when you hold 100 grams in your hand.

13. Example 4: Calculations with Multiple Forces
Two men are trying to pull a car out of a ditch.
A. Write a statement for ΣFx and ΣFy F1 F2
= 40.0 N
F1 is 40.0 N directed at 45.0o above the horizontal
= 30.0 N
45.0o
37.0o
F2 is 30.0 N directed at 37.0o above the horizontal

14. Example 5
Calculate the sum of the forces in the x- and y- direction for the object shown below.
F2 = 33.8 N
θ = 68.0o
F1 = 24.5 N
F3 = 21.0 N
What is the magnitude and angle of the resultant force?

15. Newton’s Third Law
Every action has an equal and opposite reaction.

16. Forces and accelerations go together like….
Important note!
Forces and accelerations go together like…. F a F a
Additional note: when evaluating ΣFx and ΣFy – one direction may be in equilibrium while the other is in non-equilibrium and vice versa

17. Force of Gravity *Acceleration of gravity affects all objects equally
*But what causes accelerations?
*FORCES!
F = ma
acceleration of gravity
Force of Gravity uses the acceleration of gravity (g)
Force of Gravity
F = m g
Ground g
Acceleration of gravity =

18. *Clarification Grab a white board!
Write a definition in your own words for the three terms below. Are there any differences between these terms?
Mass
Weight
Force of Gravity

19. Weight and Mass What is the difference between weight and mass? Mass -
is the numerical amount of inertia (how
much an object resists a change in motion)
Weight -
is the force of gravity on an object
Mass
Weight
# Inertia Fg

20. SO WHAT DO BATHROOM SCALES ACTUALLY MEASURE?

21. What do bathroom scales measure?
On a piece of paper – write 1-2 sentences from your reading – what do they measure?
Scales actually measure the normal force they must exert to hold you up… FN
Equilibrium: FN = Fg
Fg = FN = mg
Since g is a constant value, scales are calibrated to divide it out, leaving you your mass (lbs or kg) Fg

22. Free-Body Diagrams Free-Body Diagrams (FBD) – Force diagram.
Steps to Drawing a FBD F a
Step 1: Draw box
Step 2: Draw forces as arrows, AWAY from the box.
*Let longer arrows represent greater magnitudes
Step 3: Show any accelerations

23. What Keeps the Book from Falling?
Normal Force (FN)
A force exerted by surfaces
Always perpendicularly out of a surface FN FN
An attempt to maintain equilibrium

24. *Normal Force Books on a desk vs. Mrs. Whittaker on a desk
The normal force will only do as much as is needed to maintain equilibrium.
Books on a desk
vs.
Mrs. Whittaker on a desk
The very same desk has to exert a greater normal force to hold up Mrs. Whittaker than the books.

25. Forces of Friction (Ff)
Friction is a resistive force created by contact between two surfaces.
Resistive Force – Force that resists or opposes motion
Forces of Friction are always PARALLEL to the surface.
Example 1
Example 2 Ff m θ v Ff

26. Friction
There are 2.5 kinds of friction

27. Maximum Static Friction (Ffsmax): Break point - instant before the object moves
One specific point/value
Kinetic Friction (Ffk): Friction between two moving objects
= Coefficient of Kinetic Friction
One specific value
Static Friction (Ffs): Friction between two object, not moving
= Coefficient of Static Friction
Static friction can change to maintain equilibrium

28. Coefficient of Static/Kinetic Friction
Variable: μs andμk
Greek Letter: Mu
Unitless ratios between two surfaces
Both values are unique to every substance.

30. Mass on an Incline
There are several tricks to make this genre of problem easier FN FN Ff Ff m Fg
Fgy Fg
Fgx

31. Review of Forces Force – in general, a push or a pull
Force of Gravity (Fg = mg), also is weight
Normal Force (FN) – From surfaces
Force of Friction (Ff) – Both static and kinetic
*Masses on an Incline
Force of Tension (FT) -
Force exerted by a rope, cord, string, etc..