1. Sir Isaac Newton (1642 – 1727) Physicist
“If I have seen farther than others, it is because I have stood on the shoulders of giants.”
Sir Isaac Newton (1642 – 1727) Physicist

2. Forces and the Laws of Motion
4-1 Changes in Motion
Force – a push or pull applied to an object
Unbalanced forces cause objects to accelerate.
Cause objects to start moving, stop moving or change direction.
SI Unit of Force is the Newton –
1 N = the amount of force acting on a 1 kg mass, produces an acceleration of 1m/s2
1 N = 1kg x m/s2

3. Forces can act through contact or without physical contact
Contact force – force through touch (contact)
Field Force – electric and magnetic fields, gravity

4. FORCES COME IN PAIRS
When a force is applied to an object, then the object applies a counterforce.
Force is a Vector – magnitude and direction.
Unit is N and direction (up down, south, north ect)
Force Diagrams used to show all forces on object.
Isolate the object – only show forces acting on it.

5. Review: Free Body Diagrams
Free Body Diagrams – FBD’s
FN – normal force
(always perpendicular to the surface the object is touching)
Fg - force of gravity
(weight of object – always directed straight down)
Fa – applied force
(forced applied to object by person or another object)
Ff - force of friction
(usually opposes motion of object)
Review: Free Body Diagrams

6. 4-2 Newton’s First Law
“An object at rest remains at rest and an object in motion remains in motion unless acted upon by an outside force”
Sometimes called “Law of Inertia” – the mass of an object (not it weight) causes it to resist changes in motion.
Inertia – “laziness”

7. Newton’s 1st Law

8. The net external forces are zero.
Mass is a measure of inertia – easier to accelerate an object with less mass.
Objects are in equilibrium when they are either at rest or moving at constant velocity -
The net external forces are zero. 1

10. Fg (force due to gravity) = m x g
Mass and Weight are different! Mass never changes
Weight depends on the gravitational pull.
Weight = mass of an object times the force of gravity (g)
Formula: W = m x g
Weight is a force so the units of weight are in Newton's (N)
Fg (force due to gravity) = m x g

11. Astronauts on the orbiting space station are “weightless” because...
a. there is no gravity in space and they do not weigh anything.
b. space is a vacuum and there is no gravity in a vacuum.
c. space is a vacuum and there is no air resistance in a vacuum.
d. the astronauts are far from Earth's surface at a location where gravitation has a minimal affect.

12. Astronauts on the orbiting space station are “weightless” because...
a. there is no gravity in space and they do not weigh anything.
b. space is a vacuum and there is no gravity in a vacuum.
c. space is a vacuum and there is no air resistance in a vacuum.
d. the astronauts are far from Earth's surface at a location where gravity has a minimal affect- but still there just less

13. Find your mass in kilograms using the conversion: ___lbs = 1 kg
Then Determine how much you weigh in Newton's.
W = mass x gravity (m x g) = ______________ N

14. Newton’s 2nd Law A Net Force on an object will cause it to accelerate.
Formula:
F = m x a
Units of Force: = kg x m/s2 = Newton (N)

15. Space-shuttle astronauts experience accelerations of about 35 m/s2 during takeoff. What force does a 75 kg astronaut experience during an acceleration of this magnitude?
2625 N Or 2600 N (2 sig figs)

16. Use equation for weight (force of gravity):
Rearrange and get

17. Falling and Air Resistance
A. Air resistance decreases the net forces acting on a falling object
1. When air resistance equals downward force on falling object (force of gravity– also called weight) then net force is zero and no further acceleration occurs.

18. terminal speed– when acceleration terminates
we call this maximum speed terminal velocity
Fdrag Fg

20. The Fg (force in Newton’s due to gravity) can be shown as two useful vectors FII and F
mass
FII F ϴ
Fg = mg

21. F perpendicular = FN (normal)

22. Together these 2 forces replace the force of gravity

23. FII = m x g sin Fg = mg F = m x g cos
Using the law of similar triangles and your trigonometric functions we can solve for FII and F
Sin = opp / hyp
= FII / m x g
Cos = adj / hyp
= F / m x g
Hypotenuse = Fg = m x g
mass ϴ
F = m x g cos ϴ
FII = m x g sin
Fg = mg
FII = m x g sin parallel to the plane (x axis)
F = m x g cos the normal force that acts perpendicular to the plane (y axis)

24. The net force is the vector sum of all the forces.

26. 2 Types of Friction: Static and Kinetic
Static friction = friction between surfaces Force required to get something moving
Each surface has its own “coefficient of friction”
Coefficient of Static Friction ratio of the static friction to the normal force (perpendicular force)
μs = Fs / F

27. μk = Fk / F Kinetic Friction (μk) = friction between moving surfaces.
Examples: Sliding, rolling, fluid friction (air or liquid)
Coefficient of Kinetic Friction (μk) = ratio of kinetic friction to the normal force (perpendicular force)
μk = Fk / F

28. Coefficient of Friction – it’s a Decimal – percent of the weight required to move an object

29. Example:
How much force does it take to move a 100 N object that has a μk = .33

30. Example:
How much force does it take to move a 100 N object that has a μk = .33
Given:
Weight: 100 N (FN) = F
µk = .33
Answer:
.33 x 100N = 33 N

32. Newton’s 3rd Law
For every action there is an equal but opposite reaction

33. Forces always exist in PAIRS – or Action – Reaction Pairs.
Occur at same time and are equal but opposite in magnitude.
The forces can act on different objects

34.
Review of Newton’s 3 Laws of Motion
Discrepant event
This is a discrepant event demonstration that can be used to help students begin to understand some applications of Newton's Third law of motion. In this activity, students are asked to predict what would happen if you were sitting on a low friction cart and threw a heavy medicine ball off the back. Students will apply knowledge learned from this demonstration to help them understand basic rocket propulsion.