# what is a force?

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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. What is a force?. 5.1 How are forces created?. Forces are created in many ways. - PowerPoint PPT Presentation

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• PERSONAL\\screen\Internal\TeacherII\Teacher II.ppt

• What is a force?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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• Two Kinds of ForcesContact-actual touching 2. Long-Rangeforces acting at a distanceFrictional ForceTensional ForceNormal ForceAir Resistance ForceApplied ForceSpring Force

A GravitationalB. ElectromagneticC. Nuclear

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• 5.1 How forces actOne way forces act is the result of direct contact.A contact force is transmitted by matter directly touching other matter such as wind acting to slow a parachute.

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• 5.1 How forces actThe force of gravity between Earth and Moon appears to be what people once called action-at-a-distance.Today we know that the gravitational force is carried from the Earth to the Moon by a force field.

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• Four Elemental ForcesAll 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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• Classify these forces as contact forces or the result of force fields.

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• Effects of ForcesForces can cause objects toChange speedChange velocityChange shape

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• Measuring ForcesTo measure a force, we can use Spring scalesForce gauges

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• What are the units for FORCE?Newtons-abbreviated (N)-metric unit for forcePounds-abbreviated (lb)-English unit for force

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• 5.1 Units of forceThe 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 NewtonsAlthough 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 conversionsThe 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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• 5.1 Contact forces from ropes and springsRopes 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 Spring forcesThe force created by a spring is proportional to the ratio of the extended or compressed length divided by the original (resting) length. If you stretch a spring twice as much, it makes a force that is twice as strong.

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• Vectors (Representing forces)Vectors or scalars are used to represent quantities in physics. We will use vectors to represent forces.A scalar is a line segment, whose length represents only magnitude (amount)A vector is an arrow whose length represents magnitude and the direction it points represents the direction the force acts.

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• 5.1 The force vectorThe direction of a force makes a big difference in what the force does. That means force is a vector, like velocity or position.Arrows are often used to show the direction of forces in diagrams.

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• Drawing VectorsLength represents magnitude, or amountDirection is direction of forceDraw a vector representing a force of 25 Newtons acting to the right.

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• 5.1 Drawing a force vectorThe arrow points in the direction of the force.

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• Drawing VectorsLength represents magnitude, or amountDirection is direction of forceDraw a vector representing a force of 25 Newtons acting to the right.Draw a vector representing a force of 50 N acting to the left

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• Resultant ForceA NET FORCE is the force that results from the addition of two or more forces.VAVBVRSometimes, forces act together. When they do, it is as if they were equal to just one force

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• Tip to Tail MethodPlace the tail of one vector at the tip of the other. The vector sum (also called the resultant) is shown in red. Draw resultant vectors from beginning (tail) first vector to end (tip) of last vector.The resultant is also called the NET FORCE16 N20 N4 N20 N16 N12 N9 N12 N21 N9 N

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• Draw the Vectors..Two workers are moving a refrigerator. One pushes forward with a force of 200 newtons, the other pushes forward with a force of 300 newtons.. The net force they (together) exert on the refrigerator is:?

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• Draw the Vectors..Two students are moving a refrigerator. One pushes forward with a force of 200 newtons, the other pushes backward with a force of 300 newtons. Force is a vector. The total force they (together) exert on the refrigerator is:

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• Forces in CombinationForces acting in same direction are added together to find Net ForceForces acting in opposite directions are subtracted to find Net Force

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• EquilibriumIf the net force acting on an object is equal to zero, the object is said to be in equilibrium, and the forces are balanced

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• Equilibrium

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• Unbalanced ForcesWhen net force acting on an object, the forces are UnbalancedUnbalanced forces cause objects to change their motion, either by changing their speed or velocity

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• What will happen to the load?

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• Gravitational ForceThe attractive force acting between all massesThis is a major force but it is the weakest of all the long range forces!

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• Factors effecting the Stength of a gravitational attractionMasses of two massesDistance between the two massesLarger masses = greater attractionLarger distances between masses = less attraction

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• 5.1 Gravity The force of gravity on an object is called weight. At Earths surface, gravity exerts a force of 9.8 N on every kilogram of mass.

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• 5.1 Weight vs. massWeight 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 gravityA 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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• Solving ProblemsCalculate the weight of a 60-kilogram person (in newtons) on Earth and on Mars.Looking for: weight of person in newtons on both planetsGiven: mass = 60 kg; g = 3.7 N/kg on Mars; implied g = 9.8 N/kg on EarthRelationships: W = m x g Solution: 60 kg x 9.8 N/kg = 588 N60 kg x 3.7 N/kg = 222 NSig. fig. = 600 NSig. fig. = 200 N

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• Would you weigh more or less on a mountain compared to a plain?

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• Ocean TidesThe Moon is primarily responsible for ocean tides on Earth.The Sun contributes to tides also. What are spring tides and neap tides?

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• SunEarthNew MoonFull MoonSpring Tides

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• SunEarthFirst QuarterLast QuarterNeap Tides

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