Download - Notes 10-2 (Part I)
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Notes 10-2 (Part I)
Friction & Gravity
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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)