Transcript AP Physics IB

AP Physics I.B
Newton’s Laws of Motion
4.1 Contact and field forces
4.2 Newton’s First Law (the law
of inertia) – an object at rest will
remain at rest, or an object in
motion at a constant velocity will
continue at a constant velocity,
unless acted upon by a net force.
An unlikely trio – Mr. Evans, James
Lovell and Sir Isaac Newton
Net force – the sum of all the
forces acting on an object
If the net force is zero . . .
• The object is not moving or . . .
• The object is moving at a constant velocity
therefore . . .
• The object is in equilibrium
A net force changes velocity
Inertia and mass
4.3 Newton’s Second Law
Some examples
The acceleration of an object is
directly proportional to the net force
and inversely proportional to its
mass.
The more familiar mathematical
form (a new unit)
Net force is a vector in the same
direction as the acceleration
Note! If an object accelerates
then the net force is NOT zero. If
the net force is zero, then the
object is moving with a constant
velocity or it is at rest.
Use free body diagrams to show
all of the forces acting on an
object.
Ex. A barge has a mass of 8.0 EE 3 kg. A force of 1.00 EE 4
N pulls on the barge toward the left while a force of 7.5 EE 3
N pulls in the opposite direction. What is the acceleration of
the barge?
Forces are vectors and may have
components like any other
vectors.
p. 121: 4-7, 9
4. 0.041 s
6. 2900 N
Ex. Two groovy people pulling on a boat between two docks.
Ex. Find the displacement of the boat if the forces are
maintained for 10.0 s and the boat has an initial velocity of 0.50
m/s.
4.5 Newton’s Third Law (highly
misunderstood)
Newton’s Third Law – when one
object exerts a force on a second
object, the second object exerts a
force that is equal in magnitude,
but in the opposite direction to
that of the first
“This third law is confusing!”
Remember – Newton’s Third Law
deals with two forces and two objects,
not two forces on one object.
Ex. Two skaters with masses of 75 kg and 45 kg respectively,
face each other and push away. If the acceleration of the 75
kg skater is 0.73 m/s2, what is the acceleration of the 45 kg
skater?
p. 121: 10-12, 14-16
10. You do – easy one.
12. Note, only acceleration is horizontal. Ans. 1.2
m/s2, left.
14. Find ax and ay. Use kinematics eqns. to find east
and south components of displacement. Use Py.
Th. to find displacement (0.78 m 22º S of E)
16. Hint: total distance traveled by tug and asteroid
is 450 m. (64 s).
The Four (or is it three?)
Fundamental Forces
• The (real) strong nuclear force (short range –
holds the protons and neutrons of an atom
together)
• Electromagnetic forces (10-2 times the strong
force) – long range, holds atoms and molecules
together
• Weak nuclear force (10-6 times the strong force) –
short range, responsible for radioactive decay
-43
• The (really weak) gravitational force (10 times
the strong force) long range
4.7 The gravitational force –
more to say about this later, but
for now . . .
The gravitational force is always
attractive and never repulsive.
Mass and weight
Finding weight the hard way
Ponder the lunar explorer . . .
Now, a much simpler method
4.8 The Normal Force
“As opposed to the abnormal force”
The force a surface exerts on an
object, perpendicular to the surface
Some instructive illustrations
Ex. The tension in a rope applies a force of 100.0 N upward to
a box that has a mass of 10.0 kg. What is the acceleration of
the box?
Apparent weight (how much your
mother or father weighs)
Ex. A rope attached to a box (what else?) with a mass of 10.0
kg applies a force of 40.0 N above the horizontal so that the
blocks slides across a frictionless floor. a) What is the
horizontal acceleration of the box? b) What is the normal
force on the box?
Ex. Two boxes with masses of 12.0 kg and 10.0 kg
respectively are attached with a cord. A second cord pulls
the 10.0 kg box to the right. a) Find the acceleration of each
box and b) the tension in the cord between the boxes.
4.9 Static and Kinetic friction
Friction: we love it, we hate it
The nature of friction
Static friction and the crate
Maximum static friction
• Independent of area (if surfaces are hard
and nondeformable)
• Directly proportional to the normal force
• Depends on the surfaces in contact
• The equation is . . .
Kinetic friction is
• Independent of area
• At slow speeds, independent of speed
• Directly proportional to the normal force
and the coefficient of kinetic friction
• The second equation is . . . (hmmm, looks
familiar . . .)
Ex. A student attaches a rope to a box and pulls with a force of
90.0 N at an angle of 30.0º with the horizontal. The box has a
mass of 20.0 kg and the coefficient of kinetic friction between
the bottom of the box and the floor is 0.500. Find the
acceleration of the box.
Ex. A sled reaches the bottom of a hill with a velocity of 4.0
m/s. It slides horizontally along the snow until it comes to a
stop. What is the distance the sled slides if the coefficient of
kinetic friction between the snow and the sled is 0.0500?
4.10 Tension
“These forces are killing me, give me
an Excedrin.”
The nature of tension
4.11 Equilibrium – the net force
is zero
• So the sum of the horizontal forces is zero
• And the sum of the vertical forces is zero
Ex. A bright physics student finds her car stuck in the mud.
She ties a strong rope to the back of the bumper and the other
end to a tree. She pushes at the midpoint with maximum
effort, which she estimates to be 3.0 EE 2 N. The car just
begins to budge (from the sludge) when the rope makes an
angle of 5.0º. With what force is the rope pulling on the car?
Ex. Find the tension in each cable supporting the 6.0 EE 2 N
cat burglar.
Ex. A cruel and uncaring parent pulls backward on a swing,
where a frightened child screams and cries uncontrollably. The
ropes of the swing makes an angle of 36.0º with the vertical and
the applied force of the parent makes an angle of 20.0º with the
horizontal. The unfortunate child has a pitiful weight of 36.0
N. What is the tension in the ropes and the applied force of the
abusive parent?
p. 123: 46, 50-54, 57
46. a) yours b) 1.67 EE 9 N
50. 4260 N
52. a) 57 600 N b) 20 600 N
54. 310 N
4.12 Non-equilibrium
applications – net force not equal
to zero
Ex. A supertanker is pulled by two tugboats. The cables
connecting the tugs and tanker are at an angle of 30.0º to the
direction of the tanker’s motion. A drive force of 75.0 EE 3 N
powers the tanker forward and the water exerts a resistance
force of 40.0 EE 3 N in the opposite direction. Find the tension
in the two cables if the acceleration of the tanker is 2.00 EE –3
m/s2.
Ex. A crate rests on the bed of a truck which is moving along a
hill at an angle of 10.0º with the horizontal. The coefficient of
static friction between the crate and the bed of the truck is
0.350. What is the minimum acceleration of the truck before
the crate begins to slip?
Ex. Accelerating Blocks (a pretty typical AP problem)
p. 125: 64, 67-68, 70, 74, 76
64. a) 1100 N b) 650 N
68. 1.5 m/s2
70. a) 1.60 EE 3 N
b) 2630 N
74. a) 4.5 m/s2 b) 1170 N
76. 2740 N