Download - Notes 10-2 (Part I)

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Page 1: Notes 10-2 (Part I)

Notes 10-2 (Part I)

Friction & Gravity

Page 2: Notes 10-2 (Part I)

Friction

• A force 2 surfaces exert on each other when they rub against each other

• Acts as an unbalanced force to slow motion down

• The strength of the force of friction depends on the types of surfaces involved and how hard the surfaces push together

• 4 types of friction

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1) Static Friction

• Acts on objects that are not moving• You must exert a force greater than the force

of static friction to make the object move

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2) Sliding Friction

• Occurs when two solid surfaces slide over each other

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3) Rolling Friction

• Occurs when an object rolls across a surface• Rolling friction is less than sliding friction for

similar surfaces

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4) Fluid Friction

• Occurs when a solid object moves through a fluid, such as air, water, oil, etc.

• Fluid friction is usually less than sliding friction

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Gravity

• Force that pulls towards the center of the earth

• Newton realized that gravity acts everywhere, not just on earth

• Called the Law of Universal Gravitation• Any 2 objects in the universe attract each

other

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Gravity Between Objects• The force of gravity between objects increases

with greater mass and decreases with greater distance.

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Mass and Weight• Mass is how much matter is in an object• The gravitational force exerted on a person or

object at the surface of a planet is known as weight.

• Weight = Mass x Acceleration due to gravity

Acceleration due to gravity at Earth’s surface = 9.8 m/s2

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Free Fall

• If the only force acting on the object is gravity, it is said to be in free fall

• An object in free fall is accelerating because of the force of gravity at a rate of 9.8 m/s/s

• This means that every second an object is free falling, it increases its velocity 9.8 m/s

• Is this affected by mass? If dropped from the same height at the same time, will a heavier object fall faster?

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Free Fall

• No! If there are no other forces to consider, then the objects will fall at the same rate, regardless of mass

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Air resistance

• A type of fluid friction that acts on objects falling through the air

• An upward force acting on a falling object

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Air Resistance

• Falling objects with a greater surface area experience more air resistance.

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Projectile Motion

• Occurs when an object is thrown horizantally• Gravity will act on the object in the same way

as it does when an object is dropped vertically

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Who can remember Newton’s First Law of Motion?

• An object will remain at rest or moving at a constant velocity unless it is acted upon by an unbalanced force.

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Newton’s 2nd Law

• Acceleration depends on the net force acting on the object and on the objects mass

• Acceleration = Net ForceMass

Or Net Force = Mass x Acceleration

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Calculating Force• A speedboat pulls a 55-kg water-skier. The skier to accelerates at 2.0

m/s2. Calculate the net force that causes this acceleration.

• Read and Understand

• What information have you been given?• Mass of the water-skier (m) = 55 kg• Acceleration of the water-skier (a) = 2.0 m/s2

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Calculating Force• A speedboat pulls a 55-kg water-skier. The skier accelerates at 2.0 m/s2.

Calculate the net force that causes this acceleration.• Plan and Solve• What quantity are you trying to calculate?• The net force (Fnet) = __• What formula contains the given quantities and the unknown quantity?• a = Fnet/m or Fnet = m x a• Perform the calculation.• Fnet = m x a = 55 kg x 2.0 m/s2

• F = 110 kg • m/s2

• F = 110 N

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Calculating Force• A speedboat pulls a 55-kg water-skier. The skier accelerates at 2.0 m/s2.

Calculate the net force that causes this acceleration.

• Look Back and Check

• Does your answer make sense?• A net force of 110 N is required. This does not include the force that

overcomes friction.

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Calculating Force• Practice Problem

• What is the net force on a 1,000-kg object accelerating at 3 m/s2?

• 3,000 N (1,000 kg x 3 m/s2)

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Calculating Force• Practice Problem

• What net force is needed to accelerate a 25-kg cart • at 14 m/s2?

• 350 N (25 kg x 14 m/s2)


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