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**Forces What is a Force? I- Any push or a pull.**

Section 1 What is a Force? I- Any push or a pull. Like velocity and acceleration, a force is described by its strength and by the direction. The strength of a force is measured in the SI unit called the Newton (N) The direction and strength can be represented by an Arrow

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The arrow points to the direction of the force, the length tell you the strength – the longer the arrow the greater its strength. Force moving to the right Big arrow big force Combining Forces :Often more than a single force acts on an object at one time. The combination of all the forces acting on an object is called net force.

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**Balanced Forces are forces that cancel each other out.**

All forces have both size & direction. (vector quantity) When they are balanced they are equal and opposite in direction. Balance objects are in a state of equilibrium.

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Unbalanced Forces Can cause an object to start moving, stop moving, or change direction. Unbalanced forces acting on an object result in a net force and cause a change in the object’s motion. 10 N 10 N = Balanced 10 N 5 N Net force = 5 N Unbalanced

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**Forces and moving objects.**

Engine force Air resistance Net Force = The net force of an object that is balance is zero.

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**Forces on a moving object can be balanced.**

If the forces are equal and opposite. The object is moving at a constant speed. Air resistance Engine force Net Force = 0

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Air resistance Engine force Net Force = If the engine force is greater then the air resistance the object is accelerating

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Air resistance Engine force Net Force = If the engine force is less then the air resistance the object is decelerating.

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**Friction & Gravity Causes of Friction: Types of Friction**

Section 2 Friction & Gravity Causes of Friction: In general smooth surfaces cause less friction then rough surfaces. Types of Friction Static: between surfaces that touch but do not move.

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**Sliding: two solid surface slide over each other.**

Rolling: round object over solid surface. Fluid: liquids or gases solid object moves through fluid. (Try to stream line objects to over come fluid friction)

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**Factors Affecting Gravity:**

Gravity: force that pulls object towards earth. Universal Law of Gravitation: gravity acts between all objects in the universe. Factors Affecting Gravity: Mass and Distance. More Mass = More Gravity Further Apart = Less Gravity

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**Weight & Mass: These two terms are often confused.**

Weight is the measure of the gravitational force on an objects mass. More mass = more weight Less mass = less weight

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**Calculating Weight = F=ma**

F=force, m=mass, and a=acceleration Weight is a force so it is measured in newtons (N).

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Skill Activity Calculation You can determine the weight of an object if you measure it’s mass. Estimate the weight of four objects. Use the balance to find the mass of each object. Multiply each mass by 9.8 m/s2 to find the weight in newtons. (answers should be written to the nearest tenth) 9.8 m/s2 is the rate of acceleration due to gravity on earth.

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**Acceleration due to gravity alone is 9.8 m/s2**

Free Fall: When only the force of gravity is acting on an object. The force of gravity is unbalanced causing the object to accelerate. Acceleration due to gravity alone is 9.8 m/s2 All objects accelerate at the same rate regardless of mass.

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**Objects falling through the air experience fluid friction called air resistance.**

Terminal Velocity is reached when air resistance = the weight of the object. Greatest velocity a falling object can reach.

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**Projectile anything hit, thrown, shot, or hurled forward.**

moves forward and downward at same rate.

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Section 3 Newton’s 1st and 2nd Laws Newton’s 1st Law of Motion: States that an object at rest will remain at rest, an object in motion will remain in motion. Unless an outside force acts on it.

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**This tendency for an object to resist change is called Inertia.**

Depends on mass. More mass = more inertia.

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**Newton’s 2nd Law of Motion:. States that an object**

Newton’s 2nd Law of Motion: States that an object accelerates because a force acts on it. Force

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**The stronger the force the greater the acceleration.**

Big Force

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**The greater the mass of an object, the greater the force must be to accelerate the object.**

Big Mass Big Force

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**Newton defined this in an equation F = ma.**

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**Example: How much force is needed to accelerate a sled 3m/s/s if the sled has a mass of 10kg?**

Force = mass X acceleration. = 10 kg X 3m/s/s = 30kg/m/s/s or 30 N N= Newton. A Newton is the unit used to measure force. 1Newton is equal to 1kg/m/s/s

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**Momentum is the strength of an objects motion.**

It depends on both the mass and velocity of an object. Increase the mass or velocity of an object increases it’s momentum.

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**Newton’s Third Law of Motion: States that for every action there is an equal and opposite reaction.**

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**Whenever one object applies a. force on another object, the**

Whenever one object applies a force on another object, the second object applies a force of equal strength on the first object.

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**Law of Conservation of Momentum: the momentum of any object in a group can change,**

but whatever momentum is lost by one object must be gained by another. The total momentum remains the same, as long as no outside force act on the group.

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**Circular Motion: inward force that keeps objects moving in a circle**

Circular Motion: inward force that keeps objects moving in a circle. Called Centripetal Force Torque causes objects to rotate. The more torque the faster the object will spin. Torque depends on two factors.

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2. The amount of force applied and the distance between the axis {imaginary line through a rotating object}, and the point where the force is applied. Axis Less torque More torque

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**B- Astronomer Johannes. Kepler discovered three**

B- Astronomer Johannes Kepler discovered three Laws of Planetary Motion.

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**1- He realized that the planets**

did not orbit in a circle but in an ELLIPSE or squashed circles, this is 1st Law of Planetary Motion.

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**2- 2nd Law of Planetary. Motion: Says planet. moves faster when they**

2- 2nd Law of Planetary Motion: Says planet moves faster when they are closer to the sun and slower when they are farther away.

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**3- Period of Revolution: length of. time it takes a planet to orbit**

3- Period of Revolution: length of time it takes a planet to orbit the sun once. 3rd Law of Planetary Motion Says the larger the orbit the larger the period of revolution.

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C- Velocities 1-Escape Velocity = 37,000 km/hr the velocity needed to escape the Earths gravitational pull.

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2- Orbital Velocity = 27,000 km/hr the velocity needed to orbit the Earth, without being pulled Earth.

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Vocabulary: Action & Reaction: Forces on an object that are always equal in size and opposite in direction. Axis: Imaginary line around which an object rotates. Balanced: Forces that cancel each other out.

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**Centripetal force: Force that keep objects moving in a circle.**

Equilibrium: When opposing forces on an object balance each other out the object is in a state of equilibrium. Force: Any push or pull that alters the velocity of an object. Friction: Force that opposes motion.

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**Inertia: Tendency of an object to resist a change in motion, also Newton’s 1st Law.**

9.Law of conservation of momentum: The amount of momentum lost by one object in a group, is gained by another object in the group, the total amount of momentum remains the same.

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10.1st Law of motion: An object at rest remains at rest an object in motion remains in motion unless an outside force acts on it. 11.2nd Law of motion: Objects accelerate because a force is applied to it.

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**12.3rd Law of motion: For every action there is an equal and opposite reaction**

Net force: Is zero if the object is balanced, or moving at a constant speed. Momentum: Strength of an objects motion. Torque: Strength of an objects rotation

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**16. Gravity: force that pulls object towards earth.**

17. Microgravity: Condition in which gravity seems to disappear. 18. Terminal Velocity: is reached when air resistance = the weight of the object. Greatest velocity a falling object can reach. 19. Period of revolution: time it takes to go around the sun once.

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**20. Weight: is the measure of the gravitational force on an objects mass.**

21. Torque: the strength of an objects motion. 22. Friction: When two surfaces are in contact with each other. 23.Center of Gravity: Where gravity acts on an object.

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**24. Escape Velocity: velocity an object needs to reach to escape the earths gravitational pull.**

25. Orbital Velocity: the velocity an object needs to reach to orbit the earth. 26. Orbit: the path an object takes to orbit the sun or planet. 27. Ellipse: Oval shape or squashed circle. 28. Projectile: Anything hit, shot, hurled, or thrown forward, it moves downward and forward at the same rate.

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**29. Kepler’s Laws: 1st law = planets orbit in an ellipse**

29. Kepler’s Laws: 1st law = planets orbit in an ellipse. 2nd law = planet moves faster when they are closer to the sun and slower when they are farther away. 3rd law = the larger the orbit the larger the period of revolution. 30. Air Resistance: Fluid friction. 31. Momentum: The strength of an object.

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