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Unit II Forces

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Force = a push or pull on an object measured in Newtons (N) 4 Ways Force Effects an Object 1.Increase v 2. Decrease v 3. Change Direction 4. Change Shape

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What are the forces action on box being pushed?

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Which box is moving? AB

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Net Force = sum of forces acting on an object Forces are vectors Forces are vectors Same direction add Same direction add Opposite direction subtract Opposite direction subtract

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Balanced vs. Unbalanced Forces Balanced Forces: the forces on an object are equal and opposite. -no change to the motion -object does not accelerate Unbalanced Forces: the forces are not equal and opposite -the object will change -the object will accelerate

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Friction: force that opposes motion of an object 1. Static Friction 2. Rolling Friction 3. Sliding Friction 4. Fluid Friction

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Laws of Motion

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1 st Law of Motion An Object in motion will remain in motion, or an object at rest will remain at rest unless acted upon by an unbalanced force An Object in motion will remain in motion, or an object at rest will remain at rest unless acted upon by an unbalanced force teachertech.rice.edu/.../Newton /law1.html

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Inertia The tendency of an object to keep doing what it is already doing The tendency of an object to keep doing what it is already doing

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2 nd Law of Motion Force, mass and acceleration are all related Force, mass and acceleration are all related

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2 nd Law Force = mass x acceleration F = ma

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3 rd Law of Motion Action and Reaction Action and Reaction For every action there is an equal and opposite reaction For every action there is an equal and opposite reaction

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Weight pull of gravity acting on an object Weight pull of gravity acting on an object Weight = mass x acceleration due to gravity Mass amount of matter in an object Mass amount of matter in an object How is weight different than mass?

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Sample Problems 1. A boy pushes a cart of groceries forward with a total mass of 40 kg. What is the acceleration of the car if the net force on the cart is 60 N? 1.5 m/s²

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2. What is the upward acceleration of a helicopter with a mass of 5000 kg if a force of N act on it in an upward direction? 2 m/s²

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3. An automobile with a mass of 1200 kg accelerates at a rate of 3 m/s/s in the forward direction. What is the net force acting on the automobile? 3600 N

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4. A 25 N force accelerates a boy in a wheelchair at 0.5 m/s/s. What is the mass of the boy and the wheelchair? 50 kg

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Sample Problems 5. A girl pushes a box of books forward with a total mass of 80 kg. What is the acceleration of the box if the net force on the box is 40 N? 0.5 m/s²

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6. An truck with a mass of 3600 kg accelerates at a rate of 2 m/s/s in the forward direction. What is the net force acting on the automobile? 7200 N

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7. A 30 N force accelerates a boy on a tricycle at 0.5 m/s/s. What is the mass of the boy and the tricycle? 60 kg

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14.1 Work Definition: the product of distance and the force in the direction an object moves Definition: the product of distance and the force in the direction an object moves W = F x d W = F x d SI unit for work: newton-meter (N·m) or joule (J) SI unit for work: newton-meter (N·m) or joule (J) If there is no movement, no work is done If there is no movement, no work is done

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Power What is power? What is power? The rate of doing workThe rate of doing work Doing work faster requires more powerDoing work faster requires more power Power = Work Power = Work Time Time SI Unit – Watt (W) SI Unit – Watt (W)

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Momentum Momentum = mass x velocity of an object Momentum = mass x velocity of an object Momentum is large if product of mass and velocity is large Momentum is large if product of mass and velocity is large Objects with large momentum’s are hard to stop Objects with large momentum’s are hard to stop

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Momentum Momentum of an object at rest = 0 Momentum of an object at rest = 0 SI Units = kg*m SI Units = kg*m s

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Momentum Law of Conservation of Momentum Law of Conservation of Momentum If no net force acts on the system, the total momentum of the system does not change. In a closed system, momentum is conserved. In a closed system, momentum is conserved.

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Momentum Problem Problem Which has more momentum, a kg golf ball with a velocity of 60 m/s or a 7.0 bowling ball with a velocity of 6 m/s? Remember: Momentum = mass x velocity

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12.4 Universal Forces What are the 4 universal forces? What are the 4 universal forces? ElectromagneticElectromagnetic Nuclear (Strong and Weak)Nuclear (Strong and Weak) GravityGravity How are these different from forces talked about in the ? How are these different from forces talked about in the ? All act over a distance between particlesAll act over a distance between particles

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Describe each universal force and what makes them unique. 1. Electromagnetic Force only force that can attract and repelonly force that can attract and repel 2. Gravitational force An attractive force between 2 massesAn attractive force between 2 masses

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Describe each universal force and what makes them unique. Nuclear force : act within nucleus to hold it together Nuclear force : act within nucleus to hold it together 3.Strong – acts only on neutrons and protons in the nucleus (100x stronger than charges) 4.Weak – attractive force that acts over a short range

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Measured in units of Can be represented by Can be Causes no change to the object Causes the object to Force unbalanced

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Measured in units of Can be represented by Can be Causes no change to the object Causes the object to Force Accelerate unbalanced balanced Newtons arrows State of Motion

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