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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:**

FN – The Normal Force- table pushing up on the box- THIS IS ALWAYS PERPENDICULAR TO THE SURFACE AND EQUAL TO THE Fg. Stationary: F1 = -20 N west F2 = 20 N east 10 kg Fg What is the Fnet acting on this box: ΣF = Fnet = -20 N + 20 N = 0 FORCES ARE BALANCED A Fnet of zero means no change in movement. The box stays stationary.

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**Let’s look at forces acting on this box:**

FN Stationary: F1 = 20 N west F2 = 40 N east 10 kg Fg What is the Fnet acting on this box: ΣF = Fnet = -20 N + 40 N = 20 N east FORCES ARE UNBALANCED Now the Fnet is not zero which means there is a change in movement. This box is not going to remain stationary.

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**Let’s look at forces acting on this box:**

FN Stationary: F1 = 20 N west F2 = 40 N east 10 kg Fg What is the Fnet acting on this box: ΣF = Fnet = 20 N + 40 N = 60 N east FORCES ARE UNBALANCED Again there is a non-zero Fnet which means there is a change in movement.

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**But what if the box had an initial motion:**

FN Moving with a constant velocity: F1 = -20 N west 10 kg F2 = 20 N east Fg What is the Fnet acting on this box: ΣF = Fnet = -20 N + 20 N = 0 FORCES ARE BALANCED 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.

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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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**…Not according to Newton’s 1st Law of Motion:**

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. 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:**

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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? Which turns into Newton’s 2nd equation:

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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:**

FN F1 = -20 N F2 = 40 N 10 kg Fg a = ? Fnet = ΣF = -20 N + 40 N = 20 N m = 10 kg =

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**This leads us to the Fg: THIS IS WEIGHT g = 9.81 m/s2 ~ 10 m/s2**

What’s mass times the acceleration due to gravity? THIS IS WEIGHT

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**Use the weight equation to find your mass:**

This needs to be in N. Use 1 N = .22 lbs

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**Let’s go back and talk about our box again this time analyzing all the forces:**

FN – EQUAL but OPPOSITE to the Fg = 100 N F1 = -20 N F2 = 20 N 10 kg Fg = ma = mg = -100 N Now we are dealing with forces acting in two directions (x and y). ΣFy = Fnety = 100 N N = 0 ΣFx = Fnetx = -20 N + -20N = 0 This box is not moving or changing direction.

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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? F1 = 300 N F2 = -500 N 10 kg 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?**

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6.1 Newton’s First Law pp. 138-140.

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