Download Laws of Motion Notes

Laws of Motion Notes-PreAP Physics
Sir Isaac Newton (1643-1727) Englishman
1st Law: Law of Inertia
An object at rest remains at rest, and an object in motion stays in
motion with constant velocity (constant speed in a straight line) unless
the object experiences a net external force (unbalanced force)
- Inertia is related to the amount of mass in an object
o an object does not have to be moving to have inertia, it only has to
have mass
- Equilibrium: an object at rest or moving with constant velocity is said to
be in equilibrium, all the forces are balanced
2nd Law: ΣF= ma
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.
F α a
1/m α a
**α means proportional
ΣF= ma
ΣF –
m
–
a
-
net force (Newton) N
mass (kilogram) kg
acceleration (m/s2)
** When you double the force you double the acceleration.
** When you double the mass you decrease the acceleration by ½
**When forces are not balanced, the object will accelerate in the direction of
the net force.
** When forces occur in both x and y directions you have to solve for them
separately and then use Pythagorean Theorem and Tan-1 to solve for Net
Force
Find the net force:
10N
3N
Ex 1
10 N
ΣF = (10N + 3N) = 13N
*would accelerate to the right
ΣF = (10N - 3N) = 7N
Ex 2
3N
*would accelerate to the left
15 N
ΣF = (15N - 15N) = 0.0 N
*would remain at rest
Ex 3
15 N
15 N
3N
Ex 3
ΣFy = (15N - 15N) = 0.0 N
ΣFx = (3N - 3N) = 0.0 N
ΣF = √ ΣFy2 + ΣFx2 = 0.0 N
*would maintain constant velocity
3N
15 N
Find the acceleration:
20 N
3 kg
5 kg
3N
20 N
ΣF= ma
20N = (3kg) (a)
A= 20/3 = 6.67 N/kg
ΣF= ma
(20N-3N) = (5kg) (a)
a= 17/5 = 3.4 ms/s2
3rd Law: Force Pairs
For every action force there is an equal in magnitude but opposite in
direction reaction force.
- Force pairs, actions and reactions, come from interactions
- Interaction: two surfaces come in contact with one another
- For example: There is an interaction occurring right now between the
chair and you. Your weight is pushing down on the chair and the chair is
pushing up on you by exactly the same amount. If you pushed harder on the
chair and it couldn’t support this weight it would ‘break’ and you would
accelerate towards the ground because the forces would no longer be
balanced.
Types of Forces
1) Gravitational Force (Fg) (the weight force)
o The force with which the planet attracts objects with mass
o W=mg
(just like F=ma)
w- weight (N)
m- mass (kg)
g- acceleration due to gravity (m/s2)
Weight
-N
-Vector
-force depends
on gravity
vs.
Mass
-kg
-scalar
- measure of matter
You can be weightless but not massless
Mass???
150 lbs x
Weight in Newtons?
W= (68.2)(9.8)= 668N
W = mg
What is the weight of a 10 kg monkey?
W= (10)(9.8) = 98N= Force
1 kg =
2.2 lbs
68.2 kg
2) Normal Force (FN)
o Force that keeps an object from falling through the wall, table, floor,
etc.
o Always points perpendicular away from the surface
3) Spring Force (Fs)
o The force exerted by a spring when it is stretched or compressed
x
Fspring = k x
equilibrium position
k= spring constant (N/m)
x= distance stretched or compressed (m)
x
4) Tension (FT)
o
The force in a rope or a string
o
Always pulls
5)
Friction Force (Ff)
o
The force that opposes motion
Ff = μ F N
Ff - Force of friction (N)
μ - coefficient of friction (Greek letter ‘mu’)
FN - Normal Force (N)
Ex. The coefficient of friction of ice is 0.02. What is the force of friction
acting on a 0.56 kg hockey puck sliding to the right, across the rink?
Ff = μ F N
FN = Fg
Ff = (.02)(0.56 x 9.8)
Ff = 0.11 N
Fg= mg
Laws of Motion Notes-PreAP Physics
Sir Isaac Newton (1643-1727) Englishman
1st Law: Law of Inertia
An object at rest remains at rest, and an object in motion stays in
motion with constant velocity (constant speed in a straight line) unless
the object experiences a net external force (unbalanced force)
-
is related to the amount of mass in an object
o an object does
have to be
to have inertia, it
only has to have
: an object at rest or moving with constant
velocity is said to be in equilibrium, all the forces are
2nd Law: ΣF= ma
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.
F α a
1/m α a
**α means proportional
ΣF= ma
ΣF –
m
–
a
-
** When you double the force you
the acceleration by
** When you double the mass you
the acceleration by
.
**When forces are not balanced, the object will accelerate in the direction of
the
.
** When forces occur in both x and y directions you have to solve for them
separately and then use
and
to solve for Net Force
Find the net force:
10N
3N
Ex 1
10 N
Ex 2
3N
15 N
Ex 3
15 N
15 N
3N
Ex 3
3N
15 N
Find the acceleration:
20 N
3 kg
5 kg
3N
20 N
3rd Law: Force Pairs
For every action force there is an equal in magnitude but opposite in
direction reaction force.
- Force pairs, actions and reactions, come from
- Interaction: two surfaces come in
with one another
- For example: There is an interaction occurring right now between the
chair and you. Your weight is pushing down on the chair and the chair is
pushing up on you by exactly the same amount. If you pushed harder on the
chair and it couldn’t support this weight it would ‘break’ and you would
towards the ground because the forces would no
longer be balanced.
Types of Forces
1) Gravitational Force (Fg) (the weight force)
o The force with which the planet attracts objects with mass
o w =mg
(just like F=ma)
wmgWeight
vs.
Mass
You can be weightless but not massless
Mass???
Weight in Newtons?
W = mg
What is the weight of a 10 kg monkey?
2) Normal Force (FN)
o Force that keeps an object from falling through the wall, table, floor,
etc.
o Always points
from the surface
3) Spring Force (Fs)
o The force exerted by a spring when it is stretched or compressed
Fspring = k x
k=
x=
4) Tension (FT)
o
The force in a rope or a string
o
Always pulls
5)
Friction Force (Ff)
o
The force that opposes motion
Ff = μ F N
Ff μ - coefficient of friction (Greek letter ‘mu’)
FN -
Ex. The coefficient of friction of ice is 0.02. What is the force of friction
acting on a 0.56 kg hockey puck sliding to the right, across the rink?