Download Chapter 4 Force Lecture Notes

Force
A. Force is the push or pull exerted on an object.
1. Contact force: “I can not touch you
without you touching me”
2. Field force: ‘How does the bird
catch to worm?
a. Explains subatomic particles,
magnetism and gravity.
3. Force is a vector, net force
a. net force is the sum of all forces acting on
an object.
b. net force = zero, is at rest
c. net force
zero, is in motion
Force
B. Unit: Newton (N) where:
1 N = Kg m/s2 means the amount
of force required to accelerate
1Kg @ 1 m/s2
1 pound = 4.45 N
ie: How much N force does a 200 lb man exert?
200(4.45) = 890N
What is the weight expressed in Newtons
Force
Force diagram: show the direction and
magnitude of a force exerted on an object.
At rest:
In motion:
**There is always an unblance in the force
applied in order for an object to move!**
**Gravity is always a constant!**
**Force is a vector with both direction and
magnitude!**
Force Diagrams
Draw a Force Diagram for the following:
1. A book is setting on the lab bench at
rest.
2. A ball rolls off the lab bench then falls to
the floor. Ignore air resistance.
3. A rope is used to lower a box to the
ground with respect to the box.
4. A box is lifted to the top of the top of
the lab bench from the floor with respect
to the box.
Force Samplers
1.
2.
A force pushes on a box to the left with
a force of 100 N. Another set of forces
of 25 N and 125 N pushes on the box
to the right. Draw a diagram and what is
the net force?
A force of 75N pushes on a object to
the west and a second force pushes with
a force of 50 N to the west. Draw a
diagram and find the net force.
Newton’s First Law of Motion
Also called the ‘Law of Inertia’
Inertia: Resistance to changes in motion.
Stated: An object at rest will stay at rest and
an object in motion will tend to stay in
motion unless acted upon by some ‘net
external force’.
Net external force: the sum of all the
vector forces acting on an object.
Newton Second Law of Motion
An object will move in the direction of the
force applied and to the magnitude of that
force.
F = ma
N = Kg m/s2
F = ma use the triangle
Weight: Fw = mg
NOTE: Gravity is -9.81 m/s2. Weight is define
as the downward force on an object. Gravity
is then 9.81 m/s2
Newton Second Law of Motion
Integrate Kinematic Equations into forces via
Newton’s Laws.
Convert: N  Kg  lbs
1 lbs = 4.45 N = 2.2 kg
What is the weight of a 15 kg object?
What is the weight of the same object on
Jupiter? Jupiter gravity is 2.71 x Earth’s.
Newton Third Law of Motion
Action/Reaction:
For every force there is an equal and opposite force
that reacts.
Forces acting on an object at rest:
Fn = force normal, acts opposite gravity
Fg = force due to gravity, opposite Fn
Fa or F‫ = װ‬applied or parallel force
Ff = friction force opposite the motion
F┴ = perpendicular force act at 90o to an incline
Resolution of Forces
Force Integrated w/ Kinematics
We will use the kinematic equation along
with F = ma to solve problems.
Vf = Vi + a∆t
Vf2 = Vi2 + 2a∆d
F = ma
∆d = Vi∆t + 1/2a∆t2
Sampler
What force is needed to stop a 280 Kg
mass that has traveled from 3.2 m/s to 5.6
m/s in a distance of 40 m?
Sampler
A BB loaded into the BB gun travels 0.90 m
down the barrel when the trigger is pulled. If
it takes 0.88 s for the BB to reach the end of
the barrel, what force has been applied to
the BB? The mass of a BB is 0.99g.
Sampler
What is the weight of an object that is
moved from 2.3 m/s to 5.4 m/s in 3.5 s with
a force of 83 N?
Sampler Questions: Newton’s Laws
1. In a tug-of-war, one team pulls to the left
with a force of 15 N and the second
team pulls with a force of 10 N. Both
teams pull only in the horizontal
direction.
a. Which team will win the tug-of-war?
b. Which direction and magnitude will win?
c. Which of Newton’s Laws apply? Explain!
2. Which line on the given graph indicates
a greater force applied with a constant
mass?
Explain!
A
velocity (m/s2)
B
Time (s)
3. In the graph above, where would you
draw line B if the forces acting are equal
for an equal mass?
4. Two forces act on a single object. One force is
156 N and the other 126 N
a. What is the net force if each are pulled in
the same direction, east?
b. What is the net force if each is pulled in the
opposite direction?
c. Which of Newton’s Laws apply?
5. An object setting on a table has a force of
inertia equal to 25 N.
a. If 25 N is applied east, describe the motion?
b. What force would move the object to the
east?
c. Which of Newton’s Law apply?
6. Three forces are acting on a single object.
One force of 150 N pulls east and two
forces of 75 N and of 55 N pulls west:
a. What is the net force acting on the
object?
b. Which of Newton’s Laws best applies?
7. Two separate objects are seen moving at
the same rate of acceleration.
a. What can be concluded about the
force causing their acceleration.
b. Which of Newton’s Law best applies?
8. How can Newton’s Second Law be used
to find the weight of an object?
9. Only horizontal forces apply.
a. An object is pulled with a force of
75N accelerates at 1.25 m/s2. What is the
mass of the object?
b. How much force must be applied to
accelerate a 5 Kg mass at 0.75 m/s2 ?
c. At what rate of acceleration will a
500 Kg mass move with a force of 625N?
10. If you push against the wall with a force
equal to 55 N, what is the force that the
wall is pushing against you?
11. How does Newton’s Third Law explain
why a ball slows down when thrown
directly into the air?
12. A rope pulls on an object from rest to a
final velocity of 2.5 m/s in a distance of 5
m. If the object has a mass of 45 Kg,
what is the force needed to accelerate
the object in this distance?
13. You jump into your car with a Solo cup
of coke in your lap. You begin to drive
off when you remember you forgot
your Physic book. You stop suddenly.
a. What will most likely happen to the
coke in the cup?
b. Which of Newton’s Law best applies
here?
c. What term would best describe what
happen with the coke when you stopped
suddenly?
Resolution of Forces
Net Force: Fnet2 = Fx2 + Fy2
Direction: Tan θ = Fy / Fx
A force acts on an object of 6 N W, 4 N E,
5 N N and 2 N S. What is the net force
acting on the object and in what direction
will the object move?
Resolution of Force
Trigonometric Functions: SOH COH TOA
1. A 5.4 Kg box is pulled horizontally along a
floor with a force of 86 N by a rope fixed at
a 28o angle to the horizontal. What is the
vertical and horizontal force?
2. An object is pulled left with a force of 380 N
while upward force of 280 N, downward
with 156 N and to the right with a force of
450 N. What is the net force acting of the
object and direction.
pp121, 1-4
Frictional Forces
Friction: a force that acts between two
surfaces and in the opposite direction
Ff
Fa = Fp
Friction
Type of Friction:
1. Static Friction: Ffs, force of friction between
two surfaces at rest. Starting friction
2. Sliding Friction: Ffk, force of friction between
two surfaces in motion. Kinetic friction.
Ffk = Fp = ma
** Ffs > Ffk **
Friction
Chstc:
1. Friction acts between objects in
contact
2. Friction is independent of surface area.
3. Friction depends on type of material.
4. Fs = -Fa
Coefficient of Friction (Table 4-2)
A value of defines the relationship between two
surfaces relative to the force normal, Fn.
Fn = Fw =mg
Coefficient of Friction: ratio of Ff to Fn
μ = Ff / Fn
Coefficient of Static Friction: ratio of Fs to Fn
μs = Fs / Fn
Coefficient of Sliding Friction: ratio of Fk to Fn
μk = Fk / Fn
Friction Sampler
1. A man pulls a sled at an angle of 15o from
the horizontal at a constant speed with a
force 25 N. The sled has a mass of 5.4 Kg.
What is the coefficient of sliding friction?
Friction Sampler
2. A 8 Kg box is pulled across a rough surface
at a constant acceleration of 0.25m/s2. What
is the coefficient of sliding friction?
Friction Sampler
3. Describe the motion of an 80 Kg mass with
a coefficient of static friction = 1 ?
TLS: #17-26.
Resolution of Forces 2D w/ Friction
Relationships used:
Fg = Fw = mg
Fy = Fn = FwCos A
Fx = Fp = FwSin A
µf = Fk / Fn
F = ma where the force is the sum of
all forces, ∑F = Fp – Fk so,
Fp – Fk = ma or ∑F = ma
Friction Sampler
A 25 Kg box setting on an incline of 23o is
released and slides down the incline that has a
coefficient of sliding friction or 0.18. What is
the acceleration of the box?
Friction Sampler
A 35 Kg object sets on an incline of 35o when
it begins to slide at a rate of acceleration of
4.34 m/s2 . What is the coefficient of sliding
friction?
#17-21 page 128 and #18-26, page 130