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Chapter 5 5.1 Forces & 5.3 Forces & Equilibrium pp. 108-115 & pp. 124-129

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FORCES ► Where do you see a force happening in the room right now? ► Which object is exerting the force? Which object us receiving the force?

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IMPORTANT DEFINITIONS ► Force – a push or a pull All forces have both a size (a number) and a direction

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5.1 The cause of forces ► A force is a push or pull, or an action that has the ability to change motion. ► Forces can increase or decrease the speed of a moving object. ► Forces can also change the direction in which an object is moving.

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5.1 How are forces created? ► Forces are created in many ways. ► For example, your muscles create force when you swing a baseball bat.

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Four Elemental Forces ► All forces in the universe come from only four basic forces. ► Electromagnetic forces are important to technology. ► Gravity is a universal force.

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5.1 Units of force ► The pound is a unit of force commonly used in the United States. ► For smaller amounts, pounds are divided into ounces (oz.). ► There are 16 ounces in 1 pound.

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5.1 Newtons ► Although we use pounds all the time in our everyday life, scientists prefer to measure forces in newtons. ► The newton (N) is a metric unit of force.

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5.1 Unit conversions ► The newton (N) is a smaller unit of force than the pound (lb). ► If one pound of force equals 4.448 newtons, then a 100 lb person weighs 444.8 newtons.

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CONTACT FORCES ► A contact force acts as a result of direct contact. Contact forces include: ► Tension ► Friction ► Normal Force – the perpendicular force that a surface exerts on an object that is pressing on it ► Air Resistance

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5.1 Contact forces from ropes and springs ► Ropes and springs are often used to make and apply forces. ► Ropes are used to transfer forces or change their direction. ► The pulling force carried by a rope is called tension. ► Tension always acts along the direction of the rope.

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5.1 Gravity ► The force of gravity on an object is called weight. ► At Earth’s surface, gravity exerts a force of 9.8 N on every kilogram of mass.

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5.1 Weight vs. mass ► Weight and mass are not the same. ► Mass is a fundamental property of matter measured in kilograms (kg). ► Weight is a force measured in newtons (N). ► Weight depends on mass and gravity.

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Weight depends on mass and gravity A 10-kilogram rock has the same mass no matter where it is in the universe. On Earth, the10 kg. rock weighs 98 N.. On the moon, the same rock only weighs 16 N.

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5.1 Calculating weight

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► Calculate the weight of a 60-kilogram person (in newtons) on Earth and on Mars. 1. Looking for: …weight of person in newtons on both planets Given: …mass = 60 kg; g = 3.7 N/kg on Mars; …implied g = 9.8 N/kg on Earth 2. Relationships: W = m x g 3. Solution: 60 kg x 9.8 N/kg = 588 N 60 kg x 3.7 N/kg = 222 N Solving Problems

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5.3 Forces and Equilibrium ► The sum of all the forces on an object is called the net force. ► The word net means total but also means the direction of the forces has been taken into account. In what direction will this plane go?

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5.3 Equilibrium When several forces act on the same object: 1. The net force is zero, or 2. The net force is NOT zero.

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5.3 Normal forces ► When the forces are balanced, the net force is zero. ► When the net force on an object is zero, we say the object is in equilibrium.

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BALANCED FORCES ► When the net force on an object is zero, the forces are balanced. ► Balanced forces do not cause a nonmoving object to begin moving. ► Balanced forces will not cause a change in motion of a moving object.

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UNBALANCED FORCES ► When the net force in an object is not zero, the forces on the object are unbalanced.

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UNBALANCED FORCES ► Unbalanced forces produce a change in motion (acceleration) ► Unbalanced forces are needed to cause a nonmoving object to begin to move ► Unbalanced forces are also needed to change the motion of moving objects.

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5.3 Equilibrium and normal forces ► A normal force is created whenever an object is in contact with a surface. ► The normal force has equal strength to the force pressing the object into the surface, which is often the object’s weight. The normal force is sometimes called the support force.

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5.3 The free body diagram ► How do you keep track of many forces with different directions? ► Draw a free-body diagram that contains the objects, like a book on a table.

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5.3 Solving equilibrium problems ► For an object to be in equilibrium, all the forces acting on the object must add up to zero. Is this object in equilibrium?

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Solving Problems Two chains are used to support a small boat weighing 1,500 newtons. One chain has a tension of 600 newtons. What is the force exerted by the other chain?

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1. Looking for: …tension on chain 2 2. Given …weight boat = 1,500N; tension 1 = 600 N Implied: weight and tension are forces 3. Relationships: Net force on boat = zero Solving Problems

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4. Solution: Draw free body diagram Solving Problems Upward force of chains = weight of boat 600 N + tension 2 = 1,500 N tension 2 = 900 N

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