forces and circular motion
DESCRIPTION
Forces and Circular motion. I. Force. A. Definition: a push or pull acting on a mass 1. Force is a vector quantity with both magnitude (numeric value) and direction 2. Force can be broken down into horizontal and vertical components 3. Symbol: 4 . Units: . F. Newtons ! (N). - PowerPoint PPT PresentationTRANSCRIPT
FORCES AND CIRCULAR MOTION
A. Definition: a push or pull acting on a mass1. Force is a vector quantity with both magnitude (numeric value) and direction2. Force can be broken down into horizontal and vertical components3. Symbol:4. Units:
I. FORCE
Newtons ! (N)
F
B. Concurrent Forces: many forces acting on the same object at the same time.1. Treat all forces individually to find a resultant force (break into components)2. This resultant of all concurrent forces is called the Net Force
FnetSymbol:
C. Free Body Diagram: represents concurrent forces acting on an object
Two Physics students try pushing a car to see who is stronger. One student pushes west with a force of 500 Newtons. The other pushes East with a force of 700 Newtons.
Draw a free body diagram of the situation.
What is the Net Force?
What way does the car move?
EXAMPLE 1: NET FORCE
Two Physics students are again arguing and this time are in a tug of war. They are pulling on a box. One student pulls 30 degrees toward the northeast with a force of 400 Newtons and the other pulls at 20 degrees toward the Northwest with 500 Newtons.
Draw a free body diagram of the situation.
What is the Net Force?
EXAMPLE 2: NET FORCE
What is net force on a box of mice being pulled with a force of 20 Newtons due West toward a snake pit and another force of 30 Newtons pulling due East toward an alley filled with cats, a 50 Newton force pulling due North toward a cliff, and a 50 Newton force pulling due South toward a large pond? Draw a Free Body Diagram 1st!!!
JOURNAL #1110/1
D. Static Equilibrium: reached when the resultant of all forces acting on an object is ZERO (balanced)
1. At Equilibrium, objects remain at rest or constant velocity.
Fnet = ZeroFnet ≠ Zero
2. Net Force is equal to ZERO in static situations
Fnet =0
What forces MUST be added in order to produce static equilibrium in the free body diagram below?
EXAMPLE: STATIC EQUILIBRIUM
II. DYNAMICS
A. Newton’s First Law: An object maintains a state of equilibrium unless acted on by an unbalanced force. (at rest or constant velocity)
1. Any unbalanced force (Fnet ≠ 0) will produce a change in an object’s velocity…either speed, direction, or both.• the object will ACCELERATE
2. Newton’s First Law is also known as the Law of Inertia• Inertia: the resistance of an object to a change in its motion More Mass = More Inertia
Effects of forces acting on objects (Newtons Laws of Motion)
• Masses resist changes in motion…
What has more inertia? A 10 kg bag of feathers sitting still or a 5 kg gold bar moving along at 10 m/s?
What has more inertia? A 20 kg baseball sitting on a stand, or a 5 kg bowling ball moving along at 30 m/s?
EXAMPLES: INERTIA
B. Newton’s Second Law: the acceleration of an object is directly proportional to the net external force acting on an object and inversely proportional to the object’s mass.
• force is related to mass and acceleration using the famous expression:
• acceleration is produced by force(s)• increasing force will increase the
acceleration
maFnet
1. Units for Force…Yay!! Dimensional Analysis!
a. Newtons are the SI unit of force and are a derived unit (combination of fundamental units)
b. 1 Newton is equal to the force required to accelerate a 1 kilogram mass 1 meter per second squared
maFnet 2smkg N1
2. Increasing mass will increase the force needed to accelerate that mass
maFnet
larger m larger Fnet
*The equation must balance!
mFa net
m a
3. If the force is constant, then increasing the mass of an object will decrease the resulting acceleration
Fnet a
4. Graphing Fnet = ma :
Force (N)
Acce
lera
tion
(m/s
2 ) Direct Relationship: Increasing Force produces more acceleration
A capybara with a mass of 100kg is tackled by a Jaguar with a steady force of 100 N along the ground. Assuming no friction, what is the acceleration of the rodent?
EXAMPLE: NEWTON’S 2ND LAW
C. Newton’s Third Law: when one object exerts a force on a second object, the second object exerts a force on the first that is equal in magnitude, but opposite in direction.
For every action there is an equal and opposite reaction!
• What happens to a boat when you step onto a dock?
Newton’s 3 rd Law!!!
Newton’s 3rd Law also applies in space when making objects move
A. Weight: gravitational force exerted on a small mass by a planet/large body
1. Weight CHANGES based on what planet/object you are on… MASS does NOT CHANGE
2. Symbol:
3. Units:
4. Equation:
III. NATURAL FORCES
mgFg
Newtons ! (N)
How much do you weigh?
EXAMPLE: WEIGHT
The fattest, ugliest Capybara has a mass of 66 kg. What is the weight of the rodent on Earth?
Convert the mass to pounds if 1 kilogram = 2.2 pounds
B. Newton’s Universal Law of Gravitation: Describes the force of attraction between different masses. Any two bodies attract each other with a force that is directly proportional to the product of their masses, and inversely proportional to the square of the distance between them
221
rmGmFg
221
rmGmFg
Fg = Gravitational Force
G = Universal Gravitational Constant = 6.67 x 10 -11
N•m2/kg2
m1 = mass of object 1
m2 = masses of object 2
r = distance between the two masses
On Your Reference Tables!!
(Front Cover)
• Graphical Representation:
What is the force of gravitational attraction between the Earth and the Moon?
m1 = Earth = 5.98 x 1024 kg
m2 = Moon = 7.35 x 1022 kgr = 3.84 x 108 mG = 6.77 x 10 -11 N•m2/kg2
221
rmGmFg
EXAMPLE: NEWTON’S UNIVERSAL LAW OF GRAVITATION
221
rmGmFg 28
222411
)1084.3()1035.7)(1098.5)(1067.6( 2
2
mkgkgkg
mN
2217
2237
1047.1)1093.2(
kgmkgmN
NFg201099.1
2. Gravitational Fields: vectors are used to show gravitational force
A “unit test mass” will accelerate along gravitational field lines, toward the center of the source of gravity