Transcript KD3 Linear Mechanics

KD3 Linear Mechanics
Chapters 4, 5 & 6
Force
• Force- A push or pull which can change
an object’s state of rest or motion
(if the force is unbalanced)
– Measured in Newtons (N)
Force is measured in Newtons
• Newton-force that causes a mass of 1
kg to accelerate at a rate of 1.0 m/s²
• 1 N = .225 lb or 1 lb = 4.45 N
4 Types of Force
• 1. Gravitational Force-An attractive
force that exists between all objects
– Ex) Earth and the Moon
• 2. Electromagnetic Force-The force
involved in electric and magnetic fields
– Ex) Friction
4 Types of Forces Cont..
• Strong Nuclear Force-Holds particles in
the nucleus together
• Weak Force-A form of electromagnetic
force involved in the radio active decay
of some nuclei
Newton’s First Law (Law of Inertia)
• An object at rest stays at rest, an
object in motion stays in motion unless
acted on by an unbalanced force
Inertia
• Inertia - objects resistance to a change
in motion
– NOT a force!
– All objects have inertia
– More massive objects have more inertia
than less massive objects
Newton’s 2nd Law
• The acceleration of a body is directly
proportional to the net force on it and
inversely proportional to the mass
F=ma
Newton’s 2nd Law
Mass vs. Weight
• Mass-The amount of matter “stuff”
from which a body is made
– Measured in kg
– Never changes regardless of gravity
(can change if
removed)
Mass vs. Weight
• Weight- Mass times the acceleration
due to gravity
– Measured in N (kg m/s²)
– Refers to the gravitational force exerted
by a large body (Ex. Earth) on an object
– Varies depending on where you are
(elevation, different planet)
Weight
• Weight is a force
• Measured in Newtons
Force = wt = m g
Example
• A person with a mass of 50 kg has what
weight on Earth?
• Wt. = m g
• Wt. = 50 kg (9.8 m/s2)
• Wt. = 500 N
Example
• A person weighting 160 lbs, has a mass
of what?
(1 lb = 4.45 N)
Law of Universal Gravitation
• A force of attraction (gravity) exists
between any 2 objects
F = G m 1 m2
r²
F = Force (N)
G = 6.67 X 10-11 Nm²/kg²
m = mass (kg)
r = distance (m)
Inverse Square Law
• The gravitational force between any 2
objects varies inversely to the square
of the distance between them
(more distance less attraction)
• The gravitational force between any 2
objects varies directly to the mass of
the objects
(more massive = more attractive)
Law of Universal Gravitation
• What happens to the force when the
mass of 1 object doubles?
• What happens when the masses of both
double?
• What happens when the distance
doubles?
Newton’s 3rd Law
(Action-Reaction)
• When 1 object exerts a force on a 2nd
object, the 2nd object exerts a force on
the 1st that is equal in magnitude but
opposite in direction
Momentum
• Momentum-Inertia in motion
• Vector
• Depends on the mass AND velocity of an
object
– Momentum (p) = mass X velocity
Impulse (Ft)
• Impulse – Change in momentum
• Product of net force and the time
interval over which it acts
–F ∆t = ∆p OR
F ∆t = m ∆v
Impulse (Ft)
• An equal change in momentum can be
achieved by EITHER:
• 1. A large force for a short period of
time
• 2. A small force for a long time
Work and Kinetic Energy
• Work-Product of the force exerted on
an object and the distance the object
moves in the direction of the force
Work
–Work (W) = F d
»OR
–Work (W) = F d (cos Θ)
• Work is measured in Joules (J)
• 1 J = 1 N meter
Power
• Power- The rate of doing
work
•Power = Work/time
• Measured in Watts
• Watt = 1 Joule of energy transferred in
1 second
Energy
• Energy- The ability to do work!
Kinetic Energy
• Kinetic Energy-The energy of an object
due to motion
KE = ½ m v²
Work/Energy Theorem:
• The net work done on an object is equal
to its change in kinetic energy
F d = ½ m v²
Gravitational Potential
• Stored energy of position
• GPE=mgh
– Measured in Joules