# forces lesson 5

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Forces Lesson 5. Forces Explained . When a force is exerted on an object, the object exerts a force back. Newtons Laws. Law I- An object in motion tends to remain in motion unless acted on by an outside force. An object at rest tends to stay at rest unless acted on by an outside force. - PowerPoint PPT Presentation

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• ForcesLesson 5

• Forces Explained When a force is exerted on an object, the object exerts a force back

• Newtons LawsLaw I- An object in motion tends to remain in motion unless acted on by an outside force. An object at rest tends to stay at rest unless acted on by an outside force.

Law II- Force is equal to mass times acceleration

Law III- For every action there is an equal and opposite reaction

• Common Forces: FrictionWill always oppose the motion

Exists when an object is in motion

In a fluid the friction increases when you speed up

• Common Forces: GravityAlways exists

Weight is the force proportional to the mass of an object

Even floating balloons are affected by gravity

• Common Forces: Normal ForceIf an object is resting on a table (force of gravity), the table is exerting a force up on the object (normal force). Normal ForceGravity

• Common Forces: TensionSeen in a string, cable or rope when it is attached to a weightActs along the stringAlways pullsNever pushes TensionGravity

• Common Forces: BuoyancyUpward force exists in any fluid mediumMost important in denser fluids like water

• ThrustForce that typically pushes a vehicle

Commonly seen in airplanes and boats

Sources are often propellers or engines

• TorqueFunction of a force that causes rotational motion Torque = Force x Moment Arm

Door Example: - You apply a torque when you open the door - The further away from the hinge the greater the torque

Torque

• StaticsWhen something is not moving the sum of the forces acting on the object must equal zero.The object is in static equilibrium.The field of statics is extremely important for civil engineers.

Normal ForceGravityNormal Force2 Normal Forces Gravity = 0Force from HoldingGravity

• PressureForce exerted by something divided by the area over which the force is exerted.

• Atmospheric PressureThe atmosphere exerts pressure on each of us everydayBecause Air is a fluidHigher altitudes have less air above them Thereforethe pressure is lower at higher elevations

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