Transcript Newton`s 2nd & 3rd Law

Newton’s 2nd & 3rd Laws
A couple things to keep straight
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Mass = the amount of “stuff” something
is made of.
Weight = the measure of the force of
gravity on an object.
A couple of things to remember
You can use Newton’s 2nd Law to solve
for weight.
 Weight = force of gravity acting on a
specific object.
(acceleration due to gravity is -9.8m/s2 )
therefore…
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How to solve for weight
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Fnet = m x a (2nd Law)
Weight = mass of object x acc. due to
gravity
F(grav) = m x acc (due to “g”)
Weight = m x acc (due to “g”)
so: F(grav) = weight
(Good formula to remember = F(grav) = m x g)
Standard Units of Force
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Force measured in Newtons (N)
1 N = 1 kg x m/s2
1 Kg weighs approximately 2.2 lbs
1 kg weighs approximately 10 N
So…10 N = 2.2 lbs
Solving a Force Problem with
Newton’s 2nd Law
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Fnet = sum of all forces
If there is no motion in one direction
(“X” or “Y”), forces are
balanced…nothing to solve.
Must determine direction of motion and
solve for those net forces
Fnet = Fapp - Ff
OR Fnet = Fn – Fg
A question to make you think???
Is the force of gravity the same for all
objects on Earth?
Answer…
NO!!!
Due to mass, however acceleration is the
same!
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A couple things about Newton’s
3rd Law
Forces always occur in pairs
 “For every action there is an equal &
opposite reaction”
Fa on b = -Fb on a
Ex. Car driving down the road. What are
the forces?
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A couple things about Newton’s
3rd Law (cont.)
What forces are acting on the car?
Force of grav. (down) / Normal Force (up)
Force of friction (road on car) / Force of
friction (car on road)
A couple things about Newton’s
3rd Law (cont.)
Drop a softball (mass 0.18Kg)
? What is the force on Earth due to the ball
? What is the Earth’s acceleration
(mass of Earth = 6.0 x 1024 Kg)
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hint: use Fg = mg
Softball Answer
F(grav) = (0.18 kg) x (-9.8 m/s2)
F = -1.8 N
(therefore, force on ball from Earth = 1.8
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Acceleration = 1.8 N = (6.0E24)a
3.1E-25 m/s2