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Forces Lesson 5

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ForcesLesson 5

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

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.

Law II- Force is equal to mass times acceleration

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

- Will always oppose the motion
- Exists when an object is in motion
- In a fluid the friction increases when you speed up

- Always exists
- Weight is the force proportional to the mass of an object
- Even floating balloons are affected by gravity

- If an object is resting on a table (force of gravity), the table is exerting a force up on the object (normal force).

Normal Force

Gravity

- Seen in a string, cable or rope when it is attached to a weight
- Acts along the string
- Always pulls
- Never pushes

Tension

Gravity

- Upward force exists in any fluid medium
- Most important in denser fluids like water

- Force that typically pushes
a vehicle

- Commonly seen in airplanes and boats
- Sources are often propellers or engines

- Function 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

- When 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.

Force from Holding

Normal

Force

Normal

Force

Gravity

Gravity

2 Normal Forces – Gravity = 0

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

- The atmosphere exerts pressure on each of us everyday
- Because… Air is a fluid

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