Transcript Newton`s Laws

Newton’s Laws
Newton’s Laws
 Newton’s
First Law of Motion states
that an object at rest (or in motion)
will remain at rest (or in motion)
unless acted upon by an unbalanced
force.
 Also
known as the law of inertia
Newton’s Laws

Newton’s Second Law of Motion states
that unbalanced forces cause acceleration.
Example #1- 50 N force up, 50 N force
down
Example #2- 75 N force up, 50 N force
down
Which example causes acceleration?
Case 1-Object at rest
 FL
 If
FL= FR
FR
two equal and opposite forces act
on an object at the same time, the
forces are balanced and the object
will remain at rest.
Case 2- Object at rest
FL

 An
FL<FR
FR
object at rest has two opposite
forces acting on it at the same time.
The force to the right is larger than
the force to the left. The forces are
unbalanced and the object will
accelerate to the right.
Case 3- Object moving
v
FL

 If
FL= FR
FR
an object is moving to the right
with equal and opposite forces acting
on it, the object will continue moving
at constant speed. Therefore, there
will NOT be an acceleration.
Case 4-Object moving
v
FL FL<FR FR

 If
an object is moving to the right
with two forces acting on it, the
greater force to the right, the object
will accelerate because the forces are
unbalanced (it will speed up).
Case 4- Object moving
v
FL

 If
FL>FR
FR
an object is moving to the right
with two forces acting on it, the
greater force to the left, the object
will accelerate because the forces are
unbalanced (it will slow down).
Conclusion
 Unbalanced
forces cause
acceleration!
 Balanced
forces do not cause
acceleration, the object will either
continue to move at constant speed
or stay at rest.
Newton’s 3rd Law
 For
every action, there is an equal
and opposite reaction.
 Ex: As you sit on your chair, your
weight pushes down on the chair
while the chair pushes up on you.
 Ex: When rowing a boat, the oar
pushes on the water while the water
pushes on the oar.
Newton’s Second Law
 The
Equation
Newton’s 2nd Law
Equation: F= m(a)
 F-
Force (N-Newton)
 m- mass (kg)
 a- acceleration (m/s2)
Newton’s 2nd Law
A special case of Newton’s Second Law
is weight.
w
= m(g)
 w- weight (N)
 m- mass (kg)
 g- acceleration due to gravity (m/s2)
 g = 9.8 m/s2 on Earth
Newton’s 2nd Law
 Remember
that mass is the amount
of matter that an object has and is
measured in grams or kilograms.
 Weight
is effected by gravity. It is a
type of force measured in Newtons.
g
is the acceleration due to gravity.
It is not a force.
Newton’s 2nd Law
 Zookeepers
lift a stretcher that holds
a sedated lion. The total mass of the
lion and stretcher is 175 kg. The
upward acceleration of the lion and
stretcher is 0.657 m/s2. What is the
unbalanced force necessary to
produce this acceleration of the lion
and the stretcher?
Newton’s 2nd Law
F
= m (a)
F
= 175 kg (0.657 m/s2)
F
= 115 N
Newton’s 2nd Law
 What
is the net force necessary for a
1600 kg automobile to accelerate
forward at 2 m/s2?
Newton’s 2nd Law
F
= m(a)
F
= 1600 kg (2 m/s2)
F
= 3200 N
Newton’s 2nd Law
A
baseball accelerates downward at
9.8 m/s2. If the gravitational force is
the only force acting on the baseball,
what is the baseball’s mass for a 1.4
N force?
Newton’s 2nd Law
F
= m(a)
m
= F/a
m
= 1.4 N/9.8 m/s2
m
= 0.14 kg
Newton’s 2nd Law
A
sailboat and its crew have a
combined mass of 655 kg. Ignoring
frictional forces, if the sailboat
experiences a net force of 895 N
pushing it forward, what is the
sailboat’s acceleration?
Newton’s 2nd Law
F
= m(a)
a
= F/m
a
= 895 N/655 kg
a
= 1.37 m/s2