Download Net Force Net force is the sum of the forces acting

Net Force
Net force is the sum of the forces acting on one object.
Sigma , Ʃ, is the Greek symbol used to express addition.
Fnet = Ʃ F
which means....
Fnet = F1 + F2 + ....
ex. Ms. Curtis pushes her trolley eastward, against friction. How would you calculate the net force acting on the trolley?
FBD
Fnet = Ʃ F
= Ff + FA + FN + Fg
motion
However, we can only add linear
vectors like normal integers.
FN
Ff
FA
So, we will break the problem into
the horizontal and the vertical
vectors.
Fg
Fnet y = Ʃ F y
Fnet y = Fg + FN
Fnet x = Ʃ F x
Fnet x = FA + Ff
motion
FN
Ff
FA
Fg
Calculate the net force on the following objects:
FA1
a)
FA1 = 15.8 N [up] FA2 = 12.3 N [up]
Fg = 16.9 N [down] = ­ 16.9 N [up]
FA2
motion
work in one direction
Fg
Fnet y = Ʃ Fy = FA1 + FA2 + Fg
= 15.8 N + 12.3 N + (­ 16.9 N)
= 11.2 N [up]
No horizontal forces here. So all of the given forces
are included in the vertical net force.
FN
motion
b)
Ff
Fa
Fg
Fnet x =
FA = 35.98 N [E]
Fg = 63.90 N [down] = ___
FN = 63.90 N [up]
Ff = 10.17 N [W] = _______
work in [E]
and in [up]
Fnet y = Did you provide equations?
Did you show substitutions?
FN
c)
FA1
FA2
Ff
Fg1
Fg2
Fnet x =
F net y = motion
FN = 12.32 N [up]
FA1 = 9.05 N [N]
FA2 = 10.82 N [N]
Fg2 = 3.87 N [down]
Fg1 = 2.55 N [down]
Ff = 8.33 N [S]
Net force is important because it causes objects to accelerate.
The direction of the net force determines the direction of the acceleration.
Fnet
Fnet
ex.
Fnet
accel
accel
accel
Fnet
accel
Draw an arrow to represent the direction of the acceleration.
Predict if the following objects are speeding up, slowing down, or moving at constant velocity. F net
a)
motion
F net
b)
motion
c)
motion
F net
d)
motion
F net
acceleration
Object is speeding up because the
acceleration is in the same diection as the
motion (or velocity vector).
Newton's Second Law of Motion:
The net force acting on an object is equal to the product of the object's mass and the object's acceleration.
(Note, the net force causes the acceleration. The acceleration does not cause the net force.)
important equation
F net = m a
If the net force is horizontal (ex. east), then the
acceleration will also be horizontal (ex. east).
F net x = m a x
If the net force is vertical (ex. up), then the
acceleration will also be vertical (ex. up).
F nety = m ay
ex. A ball of mass 86 g is pushed by a net force of 9.03 N east. What is the acceleration of the ball?
m = 86 g = 0.086 kg
Fnet = 9.03 N [E]
a = ?
F net x = m a x
a x = F netx
m
2
= 105 m/s2 = 1.1 x 102 m/s [E]
= 9.03 N
0.086 kg
noticed direction of
net force was horizontal
mass must be in kilograms
ex.
What net force is needed stop a 703 kg car, moving east, at a uniform acceleration of 8.663 m/s2?
m = 703 kg
a = ­ 8.663 m/s2 [E]
stopping implies
acceleration is
opposite to the direction
of motion
F netx = m ax
= (703 kg) (­ 8.663 m/s2)
= ­ 6090.089 N
= ­ 6.09 x 103 N [E]
net force acts
in the direction
opposite to the
car's motion
Text Reading
p. 145
p. 161
p. 162
note, book did write
net force.....we will
Try problems on the
Introduction to Newton's Second Law Some of the answers are provided
in another link.
Watch units
carefully!
PHY 521
Newton’s Second Law: An Introduction
1. What is the net force acting on a 20 kg object if the object is accelerating at 0.28 m/s2, west?
2. What is the net force acting on a 20 kg object if the object is accelerating at 5669 km/h2, east?
3. What is the acceleration of a 3 g penny if a net force of 50 N [E] acts on it?
4. What is the acceleration of a supertanker of mass 1.6 x 10 8 kg if a net force of 50 N [E] acts on it?
5. On the Superman roller coaster, you are able to experience an acceleration of 4.5 |g| for an instant. If |g| = 9.81 m/s2, what is the size of the net force acting on a person of mass 65.0 kg ?
(hint........ size of a = | a| = 4.5 ( 9.81 m/s2) = ?? )
6. A person accelerating at 9.81 m/s2, down, as she falls out of a plane experiences a net force of 1.0 x 10 3 N, down. What is her mass?
7. A supertanker of mass 1.0 x 10 8 kg accelerates from rest reaching a velocity of 4.1 km/h south after
traveling 3.5 km south. What is the net force acting on the supertanker?
8. If it takes a human cannonball 1.5 s to exit a 1.6 m long cannon (pointing north), what is the average net acting on the performer if his mass is 65 kg?
force 9. A bicycle of mass 14.6 kg and a rider of mass 50 kg experience a net force of 12 N, west. What is their
2.0 s after they start moving? (hint....What is the total mass being accelerated?)
velocity
Answers:
1. 5.6 N [W]
2. 8.8 N [E]
2
3. 2 x 10 4 m/s [E]
2
–7
4. 3.0 x 10 m/s [E]
3
5. 2.9 x 10 N
6. 1.0 x 10 2 kg
7. 1.9 x 10 4 N [S]
8. 92 N [N]
9. 0.38 m/s [W]
Attachments
Shopping Cart Truck Mishap.mp4