Transcript Newton`s Laws

Reading Quiz - Newton’s Laws
1. Which of these laws is not one of
Newton’s laws?
___ 1. Action equals reaction.
___ 2. F = ma.
___ 3. All objects fall with equal
acceleration.
___ 4. Objects at rest stay at rest, etc.
2. The law of inertia
___ 1. is not covered in the reading
assignment.
___ 2. expresses the tendency of bodies to
maintain their state of motion.
___ 3. is Newton’s 3rd law.
3. Is the normal force on a body always
equal to its weight?
___ 1. yes
___ 2. no
___ 3. not covered in the reading
assignment
Newton’s First Law
• Force - a vector quantity: a push, pull or
influence that affects the motion of an object.
• Inertia: property of an object that resists
changes in the state of motion.
• Newton’s First Law: Every object continues
in its state of rest, or of uniform motion in a
straight line, unless it is compelled to change
that state by forces impressed upon it.
Conceptual Questions
1) Beaker and Tablecloth: video demo
2) What happens as a result of the downward
motion and sudden stop of a hammer when
it’s end (not the head) is slammed against a
table?
3) The inertial ball: video demo
Common theme: INERTIA
Newton’s Second Law
• Mass: quantitative measure of the inertia of
an object (kilograms).
• Newton’s Second Law: The acceleration of
an object is directly proportional to the net
force acting on it and is inversely
proportional to its mass. The direction of the
acceleration is in the direction of the net
force acting on the object.
a  mF
or
F  ma
• Note: this is a vector equation meaning
Fx  max
Fy  may
Fz  maz
• Force: an action capable of accelerating an
object. Allows one to quantify forces - the
Newton is the unit for forces. One Newton
is that force that would impart an
acceleration of 1 m/ s2 to an object of mass
one kilogram.
Quantitative Problems
1) A person stands on a bathroom scale in a
motionless elevator. When the elevator
begins to move, the scale briefly reads only
0.75 of the person’s regular weight. Calculate
the acceleration of the elevator, and find the
direction of the acceleration.
2) Starting from rest, a 4.0 kg body reaches a
speed of 8.0 m/s in 2.0 s. What is the net
force acting on the body? What can you say
about the force?
Newton’s Third Law
• A force exerted on an object is always
exerted by another object.
• Newton’s Third Law: Whenever one object
exerts a force on a second object, the second
exerts an equal and opposite force on the
first - action and reaction.
• Note: The pair of action and reaction forces
always act on different objects!
• Weight - the force exerted on an object by
the gravitational attraction of the earth:
W  mg
downwards
• Normal force - when objects are in contact,
forces are exerted by each object on the
other (nature of forces?). The contact force
that is perpendicular to the surface of
contact is called the normal force.
• Note: Weight and normal force are not
action-reaction pairs!
Quantitative Problems
1) A 10.0 kg box is placed on top of a 20.0 kg
box that is resting on top of a table. Determine
the normal force that the table exerts on the
20.0 kg box, and the normal force that the
20.0 kg box exerts on the 10.0 kg box.
2) A 1.0 kg block and a 3.0 kg block are in
contact on a frictionless horizontal surface. A
10.0 N force is applied to the first block. Find
the force this block exerts on the second
block. If the same force is now applied to the
second block, what force is exerted by it on
the first block?
Applications
• Draw a sketch of the problem.
• Free-body diagram - for each body (hence
the word free), draw a diagram with all
forces acting on that body. Apply Newton’s
3rd law where possible.
• Do not include forces exerted by this body
on other bodies (why?).
• Apply Newton’s 2nd law to each body note vector nature.
• Solve the (simultaneous) equations for the
unknowns.
Conceptual Questions
1) A car is towing a trailer at constant speed
on a level road. The force the trailer exerts
on the car
____ a) is less than the force the car exerts on
the trailer
____ b) is equal to the force the car exerts on
the trailer
____c) is equal to the force the road exerts on
the trailer
____d) is equal to the force the trailer exerts
on the road
2) A 250 lb man holding a 30 lb bag of
potatoes is standing on a scale in an
amusement park. He heaves the bag straight
up into the air, and before it leaves his
hands, a card pops out of the slot with his
weight and fortune. It reads
____a) 250 lb
____b) 280 lb
____c) less than 250 lb
____d) more than 250 lb
3) A skydiver is observed to have a terminal speed of 55
m/s in a prone position and 80 m/s in a vertical
position. Which of the following can be concluded
from this observation?
____a) The force of gravity on the skydiver is less in the
prone position
____b) The force of air resistance on the skydiver is
proportional to the speed of the body
____c) The force of air resistance is greater at 55 m/s in
the prone position than at 80 m/s in the vertical
position
____d) The force of air resistance at 55 m/s in the prone
position is the same as at 80 m/s in the vertical
position
Quantitative Problem
A 100 kg basketball player can leap straight
up in the air to a height of 80 cm. How? Let
us analyze this in the following way:
a) The player bends his legs until the upper
part of his body has dropped by 60 cm, then
he begins his jump. Explain in words what
the player is doing from the time he begins
his jump until his feet leave the ground.
b) Draw separate free body diagrams for the
player and for the floor as he is jumping,
but before his feet leave the ground.
c) Is there a net force on the player as he jumps
(before his feet leave the ground)? How can that
be? Explain.
d) What is the speed with which he leaves the
ground?
e) What was his acceleration, assumed to be a
constant, as he jumped?
f) Suppose he jumps while standing on metric
bathroom scales that read in N. What do they read:
i) Before he jumps?
ii) As he is jumping?
iii) After his feet leave the ground?