Download Newton*s Second Law

HOW TO USE
NEWTON’S SECOND LAW
1
Newton’s first law predicts motion
of objects with forces which are
balanced.
Forces are
balanced
Objects at
rest
v = 0 m/s
Remain at rest
Objects in
motion
v ≠ 0 m/s
Stay in motion at
constant velocity
a = 0 m/s2
2
But when the forces are unbalanced
 ACCELERATION results!
 The acceleration depends on:
 NET force


 F  ma
 Mass of the object
 The acceleration is in the same direction as
the net force. Both are vectors!
3
Is there a net force?
Yes or No?
4
Example Problem 1
 Determine the accelerations that
result when a 12-N net force is
applied to a 3-kg object and then
to a 6-kg object.
5
Example Problem 2
 A 1380-kg car is moving due east
with an initial speed of 27.0 m/s.
After 8.00 s, the car has slowed
down to 17.0 m/s. Find the
magnitude and direction of the
net force that produces the
deceleration.
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Force of GRAVITY
 A ball is dropped from the top of a building.
Find the force of gravity on the ball in terms
of the mass of the object m and the
acceleration of gravity g.
 In general, the force of gravity on any
object can be calculated with the formula
=
Fg mg
7
So what’s so hard about
Newton’s Second Law?
Thinking question:
20 N
Is this object moving?
If so, in what direction?
20 N
8
What’s the net force?
9
Find the net force
10
Find the unknown forces
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Now, use Newton’s 2nd Law:
 Kate and Rob exert a 2.45 N
rightward force on a 0.500-kg cart
to accelerate it across a low-
friction track. If the total
resistance force to the motion of
the cart is 0.72 N, then what is the
cart's acceleration?
12
A 200kg crate is being
lifted by a steel cable.
 If the crate is being lifted at a constant velocity
of 2 m/s, what is the tension in the cable?
 If the crate is being lifted with an acceleration
of 3 m/s2 then what is the tension in the cable?
 If the crate is now being lowered with an
acceleration of 1 m/s2 what is the tension now?
13
Another example
 In a Physics lab, Ernesto and
Amanda apply a 34.5 N rightward
force to a 4.52-kg cart to
accelerate it across a horizontal
surface at a rate of 1.28 m/s2.
Determine the friction force
acting upon the cart.
14
The parachute on a race car
that weighs 8820N opens at the
end of a quarter-mile run when
the car is traveling 35 m/s.
What total retarding
force must be supplied
by the parachute to stop
the car in a distance of
1100m?