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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 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? A B

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**Net Force = sum of forces acting on an object**

Forces are vectors Same direction add Opposite direction subtract

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**Balanced vs. Unbalanced Forces**

Unbalanced Forces: the forces are not equal and opposite -the object will change -the object will accelerate Balanced Forces: the forces on an object are equal and opposite. -no change to the motion -object does not 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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1st 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 teachertech.rice.edu/.../Newton/law1.html

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

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

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2nd Law Force = mass x acceleration F = ma

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**3rd Law of Motion Action and Reaction**

For every action there is an equal and opposite reaction

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**How is weight different than mass?**

Weight pull of gravity acting on an object Weight = mass x acceleration due to gravity Mass amount of matter in an object

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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**

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**

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 W = F x d SI unit for work: newton-meter (N·m) or joule (J) If there is no movement, no work is done

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**Power What is power? Power = Work SI Unit – Watt (W)**

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

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**Momentum Momentum = mass x velocity of an object**

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

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

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**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.

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Momentum 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?**

Electromagnetic Nuclear (Strong and Weak) Gravity How are these different from forces talked about in the ? All act over a distance between particles

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**Describe each universal force and what makes them unique.**

Electromagnetic Force only force that can attract and repel Gravitational force An 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 Strong – acts only on neutrons and protons in the nucleus (100x stronger than charges) Weak – attractive force that acts over a short range

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**Force Can be represented by Can be Measured in units of unbalanced**

Causes no change to the object Causes the object to Force unbalanced

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**Force Can be represented by Can be arrows Measured in units of**

unbalanced Newtons balanced Causes the object to Causes no change to the object State of Motion Accelerate

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Section 1: The Nature of Force What is a force? Force Force A push or pull on an object A push or pull on an object Has both Size & Direction Has both.

Section 1: The Nature of Force What is a force? Force Force A push or pull on an object A push or pull on an object Has both Size & Direction Has both.

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