forces and newton’s 3 laws
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Forces and Newton’s 3 Laws. What is a force?. Push or pull Produce changes in motion or direction. Net force:. The net force is a combined total force acting on an object. F net ΣF - PowerPoint PPT PresentationTRANSCRIPT
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Forces and Newton’s 3 Laws
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What is a force?
• Push or pull
• Produce changes in motion or direction
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Net force:
• The net force is a combined total force acting on an object.
• Fnet
• ΣF
• We represent force by using vectors- arrow symbols that represent magnitude and direction by their length and which way they point.
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Let’s look at forces acting on this box:
10 kg
F1 = -20 N west F2 = 20 N east
Stationary:
What is the Fnet acting on this box:ΣF = Fnet = -20 N + 20 N = 0
A Fnet of zero means no change in movement. The box stays stationary.
FORCES ARE BALANCED
Fg
FN – The Normal Force- table pushing up on the box- THIS IS ALWAYS PERPENDICULAR TO THE SURFACE AND EQUAL TO THE Fg.
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Let’s look at forces acting on this box:
10 kg
F1 = 20 N west F2 = 40 N east
Stationary:
What is the Fnet acting on this box:ΣF = Fnet = -20 N + 40 N = 20 N east
Now the Fnet is not zero which means there is a change in movement. This box is not going to remain stationary.
FORCES ARE UNBALANCED
Fg
FN
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Let’s look at forces acting on this box:
10 kg
F1 = 20 N west F2 = 40 N east
Stationary:
What is the Fnet acting on this box:ΣF = Fnet = 20 N + 40 N = 60 N east
Again there is a non-zero Fnet which means there is a change in movement.
FORCES ARE UNBALANCED
Fg
FN
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But what if the box had an initial motion:
10 kgF1 = -20 N west F2 = 20 N east
Moving with a constant velocity:
What is the Fnet acting on this box:ΣF = Fnet = -20 N + 20 N = 0
The Fnet is zero which means there is not a change in movement or direction. This box is continues to move with a constant velocity.
FORCES ARE BALANCED
Fg
FN
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So what needs to happen to make this box move?
10 kg
UNBALANCED FORCES
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What about when the box is already moving with a constant velocity?
What would cause the box to stop?
10 kg
UNBALANCED FORCES
What would happen if there weren’t unbalanced forces? Would the box ever stop?…
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An object at rest has a natural tendency to stay at rest, or an object in motion will stay in motion, unless a force is acting upon it.
…Not according to Newton’s 1st Law of Motion:
This is also known as the law of INERTIA.
INERTIA is an objects resistance to change in motion.
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Examples of INERTIA:
1. Not wearing your seatbelt- if you get into an accident your body wants to keep moving at the speed you were going.
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2. The famous tablecloth trick:
http://www.youtube.com/watch?v=D-a3kwvY0WM
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The relationship between mass and inertia:
MASS IS A MEASURE OF INERTIA- the more massive the object, the more that object tends to resist changes in its state of motion.
What would be easier to push a small car or a semi?
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Adding mass into the mix brings us to Newton’s 2nd Law of Motion:
If I push both vehicles with the same amount of force which one would accelerate more? Why?
aF
m Which turns into Newton’s 2nd equation: F ma
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F manet kg m/s2
kg • m/s2 = Newton = N
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A medium-sized apple weighs
about one Newton.
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Let’s find the magnitude of the acceleration for this box if the following forces are applied:
10 kg
Fg
FN
F1 = -20 N F2 = 40 N
a = ?
Fnet = ΣF = -20 N + 40 N = 20 N
m = 10 kg
aF
m
20
102 2N
kg
kg ms
kg
=
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This leads us to the Fg:
F mag g = 9.81 m/s2 ~ 10 m/s2
What’s mass times the acceleration due to gravity?
THIS IS WEIGHT w mg
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Use the weight equation to find your mass:
w mg
This needs to be in N. Use 1 N = .22 lbs
mw
g
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Let’s go back and talk about our box again this time analyzing all the forces:
10 kg
F1 = -20 N F2 = 20 N
Fg = ma = mg =
FN – EQUAL but OPPOSITE to the Fg = 100 N
-100 N
Now we are dealing with forces acting in two directions (x and y).
ΣFy = Fnety = 100 N + -100N = 0
ΣFx = Fnetx = -20 N + -20N = 0 This box is not moving or changingdirection.
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Practice with Newton’s 2nd:
1. A tractor pulls a loaded wagon with a constant force of 400 N. If the total mass of the wagon is 200 kg, what is the wagon’s acceleration?
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2. A broken down car is being pushed to the side of the road with a force of 200 N which is causing it to accelerate at .2 m/s2. What is the mass of the car?
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3. The car below was moving with an initial velocity of 50 m/s until F2 was applied to slow the car down. What is the deceleration of the box?
10 kg
F1 = 300 N F2 = -500 N
What is the distance the box travels before it comes to a stop?
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4. A student weighs 600 N. What is his mass?