ForcesForcesLesson 5Lesson 5

Forces Explained Forces Explained

When a force is exerted on an object, the When a force is exerted on an object, the object exerts a force backobject exerts a force back

Newton’s LawsNewton’s LawsLaw I- An object in motion tends to remain in motion unless 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 acted on by an outside force. An object at rest tends to stay at rest unless acted on by an outside force.at rest unless acted on by an outside force.

Law II- Force is equal to mass times accelerationLaw II- Force is equal to mass times acceleration

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

Common Forces: FrictionCommon Forces: Friction Will always oppose the motionWill always oppose the motion

Exists when an object is in motionExists when an object is in motion

In a fluid the friction increases when you In a fluid the friction increases when you speed upspeed up

Common Forces: GravityCommon Forces: Gravity Always existsAlways exists

Weight is the force proportional Weight is the force proportional to the mass of an objectto the mass of an object

Even floating balloons are Even floating balloons are affected by gravityaffected by gravity

Common Forces: Normal ForceCommon Forces: Normal Force If an object is resting on a table (force of gravity), If an object is resting on a table (force of gravity),

the table is exerting a force up on the object (normal the table is exerting a force up on the object (normal force). force).

Normal Force

Gravity

Common Forces: TensionCommon Forces: Tension Seen in a string, cable or rope Seen in a string, cable or rope

when it is attached to a weightwhen it is attached to a weight Acts along the stringActs along the string Always pullsAlways pulls Never pushes Never pushes

Tension

Gravity

Common Forces: BuoyancyCommon Forces: Buoyancy Upward force exists in any fluid mediumUpward force exists in any fluid medium Most important in denser fluids like waterMost important in denser fluids like water

ThrustThrust

Force that typically pushes Force that typically pushes a vehiclea vehicle

Commonly seen in airplanes and boatsCommonly seen in airplanes and boats

Sources are often propellers or enginesSources are often propellers or engines

TorqueTorque Function of a force that causes rotational motionFunction of a force that causes rotational motion Torque = Force x Moment ArmTorque = Force x Moment Arm

Door Example: Door Example: - You apply a torque when you open the door - You apply a torque when you open the door

- The further away from the hinge the greater the torque- The further away from the hinge the greater the torque

Torque

StaticsStatics When something is not moving the sum of the forces When something is not moving the sum of the forces

acting on the object must equal zero.acting on the object must equal zero. The object is in static equilibrium.The object is in static equilibrium. The field of statics is extremely important for civil The field of statics is extremely important for civil

engineers.engineers.

Normal Force

Gravity

Normal Force

2 Normal Forces – Gravity = 0

Force from Holding

Gravity

PressurePressure

Force exerted by something divided by the Force exerted by something divided by the area over which the force is exerted.area over which the force is exerted.

Atmospheric PressureAtmospheric Pressure

The atmosphere exerts pressure on each of us The atmosphere exerts pressure on each of us everydayeveryday Because… Air is a fluidBecause… Air is a fluid

Higher altitudes have less air above them Higher altitudes have less air above them Therefore…the pressure is lower at higher Therefore…the pressure is lower at higher

elevationselevations