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FORCES AND
THE LAWS OF
MOTION
FORCE
• A force is the cause of an acceleration, or the
change in an object’s velocity (speed or
direction). Forces are usually thought of as a
push or a pull.
• The SI unit of force is the Newton (N).
• The Newton is defined as the amount of force
that, when acting on a 1 kg mass, produces an
acceleration of 1 m/s² . Therefore, 1 N = 1
kg· m/s².
FORCES CAN ACT THROUGH
CONTACT OR AS A FIELD
• Contact Forces are forces that arise from the physical contact of two objects. (Push or Pull)
• Field Forces are forces that can exist between objects, even in the absence of physical contact between the objects. Examples: gravity, electromagnetic, weak/strong nuclear
Physics 151: Lecture 5, Pg 10
WEIGHT• The weight of an object is measure
of the gravitational force on the object. It is the result of the interaction of an object’s mass with the gravitational field of another object, such as Earth.
• 1 lb = 4.448 N
• 1 N = 0.225 lb
NEWTON’S FIRST LAW• Newton’s First Law: An object at rest
remains at rest, and an object in motion continues in motion with constant velocity (that is, constant speed in a straight line) unless the object experiences a net external force.
• The tendency of an object to maintain its state of motion is called inertia.
• Newton’s first law is often referred to as the Law of Inertia.
Newton's First Law
• Newton's first law states: An object at rest will remain at
rest, an object in uniform motion will stay in motion -UNLESS acted upon by an outside force
• This is why you should always wear a seat belt!
Outside Force
NEWTON’S FIRST LAW
MASS IS A MEASUREMENT OF
INERTIA• Imagine a basketball and a golf ball at rest side
by side on the ground. Newton’s first law states
that both remain at rest as long as no net external
force acts on them. Now imagine supplying a net
force by striking each ball with a golf club. If the
two are struck with equal force, the golf ball will
accelerate much more than the basketball. The
basketball experiences a smaller acceleration
because it has more inertia than the golf ball.
ACCELERATION IS
DETERMINED BY NET
EXTERNAL FORCE• The net external force is the vector sum of all
the forces acting on an object. It is sometimes
referred to as the resultant force.
• When there is no net force the object is at rest
or constant motion. When there is a net force
there is acceleration in the direction of the the
resultant force.
• The ΣΣΣΣ symbol means the sum of. The net
external force must be found on the x and the y
axis.
EQUILIBRIUM
• Objects that are either at rest or
moving with constant velocity are said
to be in equilibrium.
• Newton’s first law states one condition
that must be true for equilibrium:the
net external force acting on a body in
equilibrium must be equal to zero.
• ΣΣΣΣ Fx = 0 and ΣΣΣΣ Fy = 0
EQUILIBRIUM
EQUILIBRIUM
NON-EQUILIBRIUM
Force of Push
Types of ForcesContact Forces Action-at-a-Distance
Forces
Frictional Force
Tensional Force
Electrical Force
Normal ForceMagnetic Force
Air Resistance Force Applied Force
Gravitational Force
Spring Force
FieldForces
TYPES OF FORCES
• Weight: the magnitude of the force of gravity acting on an object (always a downward force) Fw = mag
• Normal Force: a force exerted by one object on another in a direction perpendicular to the surface of contact.Youmust have a surface to have a normal force.
• Tension: The pull exerted by a string, rope, or cable when attached to a body and pulled taut.
TYPES OF FORCES• Spring: A restoring force, that is, the push
or pull a spring exerts on an object
• Thrust: A general term for the forces that move objects such as rockets, planes, cars, and people
• Push/Pull forces: Physical contact force
• Friction: the resistive force that opposes the relative motion of two contacting surfaces that are at rest with respect to one another.
FORCE DIAGRAMS• A force is a vector. It has magnitude and
direction.
• A force diagram is a diagram of the objects involved in a situation and the forces exerted on the objects.
• A free-body diagram helps analyze a situation.
• In a force diagram, vector arrows represent all the forces acting in a situation. A free-body diagram shows only the forces acting on the object of interest.
FREE-BODY DIAGRAMS
UNBALANCED FORCES
Physics, Friday, November3
Turn in your illustrations in the
basket!
Grab a whiteboard and expo!
FORCE PROBLEMS
ANSWERS
• 0 N
• 5 N; LEFT
FORCE PROBLEMS
ANSWERS
• 0 N
• 15 N; UP
FREE-BODY DIAGRAM
PRACTICE• A book is at rest on a table top. Diagram the forces acting on the book.
• A girl is suspended motionless from a bar which hangs from the ceiling by two ropes.
ANSWERS
FREE-BODY DIAGRAM
PRACTICE• A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book.
• A car is coasting to the right and slowing down. Diagram the forces acting upon the
car.
ANSWERS
QUESTIONS 1 AND 2
• Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose an astronaut in that place throws a rock. The rock will:
• a) gradually stop.
• b) continue in motion in the same direction at constant speed.
• An 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving with the same speed and in the same direction?
ANSWERS 1 AND 2
• B
• 0 N
QUESTIONS 3 AND 4
• Mac and Tosh are arguing in the cafeteria. Mac says that if he throws his jello with a greater speed it will have a greater inertia. Tosh argues that inertia does not depend upon speed, but rather upon mass. With whom do you agree? Why?
• If you were in a weightless environment in space, would it require a force to set an object in motion?
ANSWERS TO 3 AND 4
• Tosh
• YES!!! An object has mass even in space – Newton’s laws still apply
CONCEPTUAL QUESTION 5
• Ben is being chased through the woods by a bull moose which he was attempting to photograph. The enormous mass of the bull moose is extremely intimidating. Yet, if Ben makes a zigzag pattern through the woods, he will be able to use the large mass of the moose to his own advantage. Explain this in terms of inertia and Newton's first law of motion.
Exit Ticket1. Explain the relationship between Weight and Mass
2. What is the net force
on this object:
3. What is the net force on this object:
CONCEPTUAL QUESTION 6
• Several physics teachers are taking some time off to play a little putt-putt golf. The 15th hole at the Hole-In-One Putt-Putt Golf Course has a large metal rim which putters must use to guide their ball towards the hole. Mrs. Davis guides her golf ball around the metal rim. When the ball leaves the rim, which path (1, 2, or 3) will the golf ball follow?
NEWTON’S SECOND LAW
• Newton’s Second Law: The acceleration of an
object is directly proportional to the net external
force acting on the object and inversely
proportional to the object’s mass.
• ΣΣΣΣ F = ma (The sum of forces on a particular axis
equals mass times acceleration)
• A small force on an object causes a small
acceleration, but a larger force causes a larger
acceleration.
PRACTICE PROBLEM
An applied force of 50 N is used to accelerate an object to the right across a frictional surface. The object encounters 10 N of friction. Use the diagram to determine the normal force, the net force, the mass, and the acceleration of the object. (Neglect air resistance.)
ANSWER
• FN: 80 N
(not moving on y-axis so FN = Fw)
• MASS: 8.15 kg (Fw/ag=mass)
• Fnet: 40 N; right
• a: 4.91 m/s² ; right (Fnet/m)
NEWTON’S THIRD LAW
• Newton’s Third Law: If two objects interact, the
magnitude of the force exerted on object 1 by
object 2 is equal to the magnitude of the force
simultaneously exerted on object 2 by object 1,
and these two forces are opposite in direction.
• For every action there is an equal, but opposite
reaction.
NEWTON’S THIRD LAW
• Forces always exist in pairs.
• Action-reaction pair: a pair of
simultaneous equal, but opposite
forces resulting from the
interaction of two objects.
• Field forces also exist in pairs.
http://phyz.org/hewittdrewit/neth.html
INTRO TO NEWTON’S
THIRD LAW
NEWTON’S 3RD LAW
NEWTON’S 3RD LAW
http://www.flippingphysics.com/third-law.html
http://www.flippingphysics.com/third-law-
misconception.html
ACTION-REACTION PAIRS
Newton’s Third Law Pairs
What are the Newton Third Law
Pairs you identified yesterday?
• Station 1: standing from the ground
• Station 2: two spring scales
• Station 3: two lab chairs
• Station 4: Wheels of the car
• Station 5: tennis ball
QUESTION 7
•While driving, Anna observed a bug striking the windshield of her car. Obviously, a case of Newton's third law of motion. The bug hit the windshield and the windshield hit the bug. Which of the two forces is greater: the force on the bug or the force on the windshield?
ANSWER 7
• The same force
QUESTION 8• A gun recoils when it is fired. The recoil is the result of action-reaction force pairs. As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards. The acceleration of the recoiling gun is ...
a.greater than the acceleration of the bullet.
b.smaller than the acceleration of the bullet.
c.the same size as the acceleration of the bullet.
ANSWER 8
• B; The forces are equal, but the mass of the bullet is smaller than the gun and the acceleration is inversely proportional to the mass
FRICTION
• Friction occurs because the surface of any object is rough.
• Even surfaces that appear smooth are really covered with microscopic hills and valleys.
http://www.mrteverett.com/physics/newton's%20first%20law%20of
%20motion.asp
FRICTION
• Friction opposes the applied force
• Kinetic friction is the resistive force that opposes the relative motion of two contacting surfaces that are moving past one another.
• Static friction is the resistive force that keeps a stationary object at rest
• Kinetic friction is less than static friction.
FRICTION
• The force of friction is proportional to the normal force.
• The coefficient of friction (µk or s)is the ratio of the force of friction to the normal force acting between two objects.
• FF = µ FN
• µ = FF / FN
• Friction depends on the surfaces in contact.
FRICTION
TYPES OF KINETIC FRICTION• Sliding Friction:
type of friction that
occurs when objects
slide past each
other
• Rolling Friction:
type of friction
when a rounded
object rolls over a
flat surface.
• Rolling < Slidinghttp://www.school-for-champions.com/science/friction.htm
REDUCING UNWANTED FRICTION
• Use low-friction materials
• Lubricants: substances
that are applied to surfaces
to lower the friction
between them.
• Ex) oil, wax, greasehttp://en.wikipedia.org/wiki/Cookware_and_bakeware
BALL BEARINGS
• A bearing can reduce friction by virtue of its
shape, by its material, or by introducing and
containing a fluid between surfaces.
http
://en
.wik
ipe
dia
.org
/wik
i/Be
arin
g_
(me
ch
an
i
ca
l)
INCREASING HELPFUL FRICTION• Helpful friction is
increased by making surfaces rougher.
• Friction is also greater if the force pushing the surfaces together is increased.
Putting sand on icy roads. http://quizlet.com/8107954/chapter-10-motion-flash-cards/Paper weights
increase force and therefore increase friction.
Shoe treads increase friction by increasing roughness.
EXIT TICKET
• Explain why the force normal and the force
of gravity on a book sitting on a table are
NOT a Newton’s third law force pair
(action-reaction pair).