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Published byNathanial Beer Modified about 1 year ago

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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. F net 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 F 1 = -20 N westF 2 = 20 N east Stationary: What is the F net acting on this box: ΣF = F net = -20 N + 20 N = 0 A F net of zero means no change in movement. The box stays stationary. FORCES ARE BALANCED Fg F N – 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 F 1 = 20 N westF 2 = 40 N east Stationary: What is the F net acting on this box: ΣF = F net = -20 N + 40 N = 20 N east Now the F net is not zero which means there is a change in movement. This box is not going to remain stationary. FORCES ARE UNBALANCED Fg FNFN

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Let’s look at forces acting on this box: 10 kg F 1 = 20 N westF 2 = 40 N east Stationary: What is the F net acting on this box: ΣF = F net = 20 N + 40 N = 60 N east Again there is a non-zero F net which means there is a change in movement. FORCES ARE UNBALANCED Fg FNFN

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But what if the box had an initial motion: 10 kg F 1 = -20 N westF 2 = 20 N east Moving with a constant velocity: What is the F net acting on this box: ΣF = F net = -20 N + 20 N = 0 The F net 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 FNFN

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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 1 st 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 2 nd 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 2 nd equation:

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kg m/s 2 kg m/s 2 = 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 FNFN F 1 = -20 NF 2 = 40 N a = ? F net = ΣF = -20 N + 40 N = 20 N m = 10 kg =

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This leads us to the F g : g = 9.81 m/s 2 ~ 10 m/s 2 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: 10 kg F 1 = -20 NF 2 = 20 N Fg = ma = mg = F N – EQUAL but OPPOSITE to the Fg = 100 N -100 N Now we are dealing with forces acting in two directions (x and y). ΣF y = F nety = 100 N + -100N = 0 ΣF x = F netx = -20 N + -20N = 0 This box is not moving or changing direction.

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Practice with Newton’s 2 nd : 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/s 2. 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 F 2 was applied to slow the car down. What is the deceleration of the box? 10 kg F 1 = 300 NF 2 = -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?

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