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Forces

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**Force a push or pull acting on an object**

typically measured in Newtons (kg•m/s2) is a vector can be combined to predict motion net force Soccer Simulation

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**Types of Forces Contact Forces Non-Contact Forces Applied Normal**

Friction Air Resistance Tension Spring Non-Contact Forces Gravity Electromagnetic

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Applied Force any push or pull on an object created from another source (person, animal, another object, etc.)

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Normal Force the support force exerted on an object directly related to weight (gravity) consequence of Newton’s 3rd Law is always perpendicular to the surfaces in contact Gravity 900 Gravity 900 Box Box Friction Normal Force Normal Force

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**is the force (friction)**

exerted by a surface as an object moves across it or attempts to move across it opposes the motion of an object depends on the type of surfaces and the normal force (weight) Types Kinetic Static In which direction is the force (friction) vector pointing? Motion Friction

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**Air Resistance friction due to air molecules**

acts upon objects as they travel through the air opposes the motion of an object most noticeable for objects traveling at fast speeds Examples Space shuttle re-entry Meteorite in Freehold Meteor over Russia

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**Minimizing Air Resistance (Drafting)**

used in variety of competitive events (bicycle and car racing, swimming, etc.) to reduce air resistance Notice how the second biker experiences less air resistance because he is shielded by the first biker. Image taken from: Although it does not work exactly the same way, where is these seen in nature?

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Tension force that is transmitted through a string, rope, cable or wire when it is PULLED tight by forces acting from opposite ends directed along the length of the wire and PULLS equally on the objects on the opposite ends of the wire

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Spring force exerted by a compressed or stretched spring upon any object that is attached to it for most springs, the magnitude of the force is directly proportional to the amount of stretch or compression of the spring Images taken from: If both springs are the same size when not compressed, which spring will apply more force to the ball when released? Explain your reasoning.

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**Gravity natural force of attraction between any two objects factors:**

distance – increased distance less gravitational pull or vice versa mass – increased mass more gravitational pull or vice versa Why does the force of gravity have more of an impact on holding our solar system together compared to holding the parts of an atom together?

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**Electromagnetic Force**

force that moving charges exert on one another results from the repulsion of like charges and the attraction of opposites Notice how the particles with the same charge move apart and the particles with different charges move together. + + + - - - Compare and contrast gravitational force and electromagnetic force.

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**Free Body Diagrams visuals that help show net force Practice**

use a square and draw all forces acting on the object. remember size and direction of vector arrows are important! Practice Fgrav = 5 N Ffric = 3 N Fnorm = 5 N Fapp = 3 N What do you think the symbols w/subscripts represent? What is the net force on this object?

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**You throw a baseball to your friend who is to your left.**

What’s the Net Force Fnorm = 10 N Fapp = 20 N Ffric = 5 N Ffric = 5 N Fapp = 15 N Fgrav = 10 N Fgrav = 10 N You throw a baseball to your friend who is to your left. Your dog pulls you down the street on a skateboard in an eastward direction.

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**What’s the Net Force (An Interesting Case)**

A skydiver is descending with a constant velocity. Consider air resistance. The same skydiver is descending after 30 seconds. Consider air resistance. Fgrav Ffric Fgrav Ffric What has the skydiver reached in this scenario?

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Laws Newton’s 1st Law Newton’s 2nd Law Newton’s 3rd Law

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Newton’s 1st Law objects at rest remain at rest, and objects in motion remain in motion with the same velocity, unless acted upon by an unbalanced force also considered the Law of Inertia How is this illustrated when riding in a car? Can you think of other experiences where this is illustrated?

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Inertia the resistance of an object to a change in the speed or the direction of its motion directly related to mass

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Newton’s 2nd Law the acceleration of an object increases with increased force and decreases with increased mass the direction in which an object accelerates is the same as the direction of the force Formula: F = ma (or a = F/m) Shopping Cart Simulation

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**In this case, the force of the**

Centripetal Force any force that keeps an object moving in a circle directed toward the center of the circle In this case, the force of the ball as it accelerates around the circle is pointing inward, toward the center.

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**Practice Problems - Force**

What net force is needed to accelerate a 24 kg dogsled to a rate of 3 m/s2? 2. A 1.5 kg object accelerates across a smooth table at a rate of 0.5 m/s2? What is the unbalanced force applied to it? 72 kg·m/s2 or 72 N F = ma F = (24 kg) (3 m/s2) = F = (1.5 kg) (0.5 m/s2) F = ma = 0.75 kg·m/s2 or 0.75 N

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Newton’s 3rd Law states that every time one object exerts a force on another object, the second object exerts a force that is equal in size and opposite in direction back on the first object. How was this illustrated during the Scooter Games competition? Can you think of other experiences where this is illustrated?

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**Vector a quantity that has both direction and magnitude (size)**

drawn as an arrow which shows direction and magnitude (length of arrow) consists of two parts: tail and head Head Tail Consider the vectors above. Describe the direction and relative magnitude (force) of each ball based on the vector.

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**What is the hockey puck’s net force?**

Combining Vectors can be combined/added to help determine net force the overall force acting on an object when all of the forces acting on it are combined What is the hockey puck’s net force? Gravity = 14 N Normal Force = 14 N Gravity = 14 N Applied Force = 25 N Friction = 2 N 23 N Applied Force = 25 N Friction = 2 N Normal Force = 14 N

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