Download Ch 6 Forces

Chapter 6 Forces
• Kinematics – how objects move
Dynamics – why objects move
What is a Force?
• A push or a pull
Types of Forces
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•
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Gravitational force
Electromagnetic force
Strong nuclear force
Weak nuclear force
Gravity
• Is an attractive force between all matter in
the universe
• Is due to the curvature of spacetime
Newton’s First Law of Motion
Newton’s Second Law of Motion
The Unit of Force
•
•
•
•
F = ma
m is in kg
a is in m/s2
A kg • m/s2 is called a Newton (N)
A)
B)
C)
D)
1000 N
5000 N
20,000 N
50 N
A)1000 N
B)5000 N
C)20,000 N
D)50 N
Newton’s Third Law of Motion
Using Newton’s Laws
• Mass and Weight
• W = mg
• g = -9.80 m/s2
Why is the value for “g”
negative?
a) Gravity is a negative force.
b) Gravity always points down.
c) Gravity always points up.
Why is the value for “g”
negative?
a) Gravity is a negative force.
b) Gravity always points down.
c) Gravity always points up.
Two Kinds of Mass
• Inertial mass
• m = F/a
• Gravitational mass
Friction
• Force that opposes the motion between two
surfaces in contact.
• Direction is always parallel to surface and
opposite the motion.
• All surfaces are rough on a microscopic
scale.
Friction
• The source of friction is the electromagnetic
force.
• Static friction – the force that opposes the
start of motion.
• Sliding friction – the force between surfaces
in relative motion.
• Static friction is always > Sliding friction
Friction
Friction
• W is weight. FN is the “normal” force.
“Normal” means perpendicular to the
surface.
• The force of friction depends
on FN and the nature of the
surfaces in contact.
Friction
• Ff = μFN
• μ (mu) is called the coefficient of friction.
• Can be found experimentally.
Problem Solving Strategy
• Start with a sketch.
• Draw arrows representing the different
forces. Label each with the cause of each
force. (weight, friction etc…)
Net Force Causes Acceleration
• F = ma (F is the net force)
100 N are exerted on a 10 kg
mass.
•
a)
b)
c)
The resulting acceleration is:
100 m/s2
10 m/s2
1000 m/s2
100 N are exerted on a 10 kg
mass.
•
a)
b)
c)
The resulting acceleration is:
100 m/s2
10 m/s2
1000 m/s2
Air Resistance
• Is sometimes called the drag force.
• Is a form of friction.
• When this force = the downward force of a falling
body, we say the object has achieved terminal
velocity.
• In the absence of air resistance all objects fall at
the same rate.
Apollo 15