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Forces
Chapter 5
• Dynamics: The study of the effects of force on
matter (The cause of motion).
– This relationship is the most basic of all scientific
concepts.
• All changes in motion are the result of forces.
– Examples: Blood flow, hitting a baseball, planets orbiting the sun
Isaac Newton (1642-1727)
• By the age of 23, Newton had developed his laws of
motion, which are now basics of physics.
5:1 Forces
• 4 types of forces:
1.
2.
3.
4.
Gravitational
Electromagnetic
Strong nuclear
Weak Nuclear
• Gravitational Force: A force of attraction that exists
between all masses.
– Even though it is very influential to us, it is the weakest of the
forces.
• Electromagnetic Force: Forces between charged particles.
– Exists between stationary charged particles
– Examples: Mechanical and frictional forces are electric forces
between surface atoms of materials in contact
– Moving charged particles produce magnetic force
• Nuclear Force: The strongest of all other forces.
• Strong Nuclear Force: Holds the nucleus of an atom
together despite the large electric force of repulsion
between particles in the nucleus.
• Weak Nuclear Force: Exists inside the nucleus.
– A form of electromagnetic force.
– Involved in the breaking apart of some nuclei (Radiation)
5:2 Newton’s First Law of Motion
• Galileo developed the concept of acceleration by
rolling a ball down an inclined ramp and observing
that it gained velocity uniformly.
– He decided that if the floor were perfectly smooth, the
ball would never stop rolling.
• Inertia: There is a tendency for an object to remain
in the same state of motion.
• Newton studies this idea and derived the laws of
motion
• 1st Law of Motion: An object continues in its state
of rest, or of uniform motion in a straight line,
unless it is acted upon by an unbalanced force.
– “An object in motion stays in motion, while an object at
rest will remain at rest.”
– There is no fundamental difference between an object at
rest and one that is moving with constant velocity.
5:3 Newton’s Second Law of Motion
• 2nd Law of Motion: When an unbalanced force acts
on an object, the object will be accelerated.
F = ma
Where F = force, m = mass, a = acceleration
• Force is a vector quantity.
• Acceleration of applied force is in the direction of
that force.
5:4 Units of Force
• Using the force equation F = ma = 1.00 kg*m/ss
– This is known also as a Newton (N)
• Newton = SI unit of force.
• Example: An unbalanced force gives a 2.00 kg mass
an acceleration of 5.00 m/s2. What is the force
applied on the object?
Practice!
• Pg. 80 #1-6
5:5 Mass and Weight
• Mass: The quantity of matter in an object.
• Concept- A larger force is necessary to give a
larger mass the same acceleration as a smaller
mass. (2nd law of motion)
Consider This:
• The force that must act on a 1.00 kg mass allowed
to fall freely from some point near earth’s surface.
The mass accelerates at 9.81 m/s2.
– Using Newton’s 2nd law, F = ma, the earth exerts a force
of 9.81 N on the 1.00 kg mass.
– Weight: The gravitational force exerted on an object by
a very massive body.
W = mg
Where, W = weight, m = mass, g = acceleration due to
gravity.
– Weight is a vector directed toward the center of the
attracting body.
– Weight varies with location, mass does not.
Example:
• What is the weight of a 7.2-kg mass on the
Earth’s surface?
Practice!
• Pg. 82 #11, 14, 16, 17.
5:6 Two Ways to Measure Mass
• There are 2 different methods used to measure the
mass of an object.
1. Gravitational Mass
A. Balance Beam: Compares the weights of 2 objects
A. When the pans balance, the force of gravity is the same on each pan.
B. Measures gravitational mass.
2. Inertial Mass
A. Can be measured by finding the acceleration caused by a
known force.
B. Can also be measured by finding the period of oscillation of a
mass hanging on a spring.
C. Seldom used because it involves both a frictionless surface
and a difficult measurement of acceleration.
5:7 Friction
• Friction: The force that opposes the motion of 2
objects that are touching each other.
– This is an electromagnetic force and results from
temporary attractions between contact points of the 2
surfaces.
– Always opposes the force of motion
– Static friction (frictions resisting the start of motion) is
larger than sliding friction (force resisting existing
motion).
• Balanced forces:
FN (normal)
Ff (Frictional)
FA (Applied)
Fg (gravitational)
Fw (weight)
• If velocity is constant, then F(NET) = 0N
– When forces are balanced, FN = FW, and Ff = FA.
• If FA is bigger than Ff, the object accelerates.
• Equation:
– Ff = μFN
• This equation expresses the force of friction.
• μ= the coefficient of friction (describes the nature of the
surfaces in contact with one another)
• FN = Normal force (force pushing the surfaces together)
– Always perpendicular to the surfaces in contact.
Examples:
• A smooth wooden block is placed on a smooth
wooden tabletop. A force of 14.0 N is necessary to
keep the 40.0 N block moving at a constant velocity.
A. What is the coefficient of sliding friction for the block
and table?
B. If a 20.0 N weight is placed on the block, what force
will be required to keep the block and weight moving
at a constant velocity across the table.=?
Practice!
• Pg. 86, # 19
5:8 Net Force and Acceleration
• An unbalanced force causes a mass to accelerate.
• The net force is the vector sum of all forces acting
on a body.
FNET = FA + Ff
– Ff tends to have a negative sign here.
• The weight of an object can be one of the forces
acting on an object.
• Consider a 10.0 kg stone lying on the ground. In
order to accelerate it upward, a force greater than
its weight must be applied to it in an upward
direction. The weight of the stone, W, is 98.0 N
down. If someone lifts the stone with a force of 148
N up, what is the net force acting on the stone, and
what is the acceleration of the stone?
Practice!
• Pg. 87 # 21; pg. 88 #26
5:9 Free Fall
• An object in free fall has only the force of gravity
acting on it. Its acceleration is equal to g.
• The frictional force of air molecules striking a
moving object is called air resistance.
• If the air resistance equals the weight, the object
does not accelerate. It moves at its terminal
velocity.
5:10 Newton’s Third Law of Motion
• Also known as the Law of Action and Reaction
– Every action has an equal and opposite reaction force.
• Forces always are in pairs between pairs of objects.
• The action and reaction forces act on different
bodies.