Gravity & friction

Gravity Force of Attraction

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Gravity Force of Attraction

• What is gravity?

• Definition: Force of attraction between two objects due to their masses.

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Gravity Force of Attraction

• Gravity has an effect on acceleration.

• Acceleration due to gravity: 9.8m/s/s

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All Matter Is Affected By Gravity

• Does all matter experience gravity?

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All Matter Is Affected By Gravity

• All objects are constantly being pulled toward each other. What force causes this pull?

• Why do you not feel the effects?

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All Matter Is Affected By Gravity

• How are all objects around us affected by the mass of the Earth?

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The Law of Universal Gravitation

• Why do all objects fall toward Earth?

• What keeps the planets in motion in the sky?Treated as separate questions

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The Law of Universal Gravitation

Isaac Newton (1642-1727)

•British Scientist

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The Law of Universal Gravitation

• Because of Newton we now have the Law of Universal Gravitation which relates: •Distance•Mass•Gravitational Force

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The Law of Universal Gravitation

• What kind of objects are subject to the Law of Universal Gravitation?

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The Law of Universal Gravitation

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The Law of Universal Gravitation

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Friction

Friction

• Friction: A force that opposes motion.

• Always exists between two surfaces.

• No such thing as a frictionless environment.

• Even in space.

Friction

• Kinds of Friction

• Static - The friction between two objects that are stationary (not moving!)

• Kinetic - The friction between two objects that are moving.

• Rolling, Sliding, Fluid (Air Resistance)

Friction

• Which type of friction is greater, static or kinetic? Why?

Friction

• http://www.youtube.com/watch?v=VMleQwil_KQ&feature=youtu.be

Friction

• Nobody is sure why friction acts the way it does...

• Some theories state that there are tiny imperfections in surfaces that cause friction.

• Some theories state that there is an electrostatic attraction between the two objects.

Friction

Friction

• Friction always acts in the direction opposite to the motion of the object.

• The direction of the force due to friction will be exactly 180° opposite.

• Friction is also proportional to the normal force, which is how we'll be able to calculate it....

Friction

• Ff = µFN

• Ff = Force of Friction (Newtons)

• FN = normal force (Newtons)

• µ =Greek letter “mu”, coefficient of friction between two surfaces (no units)

• µs = Static; µk = Kinetic

Friction

• Obviously, some surfaces have less friction than others…

• A rubber hockey puck against ice has less friction than a car tire on an asphalt road.

• That's why there are also two measurements of friction (static & kinetic) for any combination of surfaces.

Friction

• The static friction that you calculate is a measurement of the maximum it can be. It can be any value up to or equal to that maximum amount.

• Ff static ≤ µs • FN

• The kinetic friction is the value of the friction.

Friction

Friction

• Example: A 12kg piece of wood is placed on top of another piece of wood. There is 35N of maximum static friction measured between them. Determine the coefficient of static friction between the two pieces of wood.

Friction• Example: I have a steel box (mass of 10 kg) sitting on a steel

workbench. I try to push the box out of the way...

• a) Sketch a diagram of the box with vectors showing all forces acting on the box and their direction (free body diagram).

• b) I push against the box with a force of 25 N. Determine if anything will happen.

• c) Determine what will happen if I push with a force of 73 N.

• d) If I push with a force of 100 N , determine if anything will happen.