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Force

Definition:

- A push or a pull
- Force can cause motion
- SI unit -> Newton (N)= 1 kg x 1 m/s2
- Sometimes pound (lb) can be used as a unit for force
- 1 Newton=0.22481 pounds force

Combining Forces

- Balanced Force
- Equal and opposite forces = ‘balanced’
- Balanced forces give no motion

No motion of hands

Combining Forces

- Unbalanced force – more force in one direction (net force)

Net Force

- Bigger force to the right
- Smaller force to the left
- Net force to the right

III. Friction and Gravity

- Friction – a force that opposes motion
- Caused by the interaction between two touching surfaces rubbing against each other
- Four Types

a. Static

- Friction that acts on something that is not moving
- No heat or wear is generated

b. Sliding

- Force resulting when pushing or pulling an object over a surface.

c. Rolling

- Contact is reduced because of rollers or wheels or ball bearings.

d. Fluid friction

- Resistance from a “fluid”
- Fluids are liquids and gases.

Reducing friction

- A smooth surface does not demonstrate as much friction as a rough surface.
- Waxing, oiling, etc. can be used to reduce friction.

Coefficient of Friction

- µ= “coefficient of friction” represents the roughness of the surfaces in contact.
- In reality there two main subtypes:
- µs- Static friction to get it moving
- µk- Kinetic friction to keep it moving
- µs> µk needed for movement to occur

- Gravity
– force of attraction between two objects

---> pulling force

Factors Affecting Gravity

- Mass – more mass, more gravity
- Distance – closer, more gravity

http://www.exploratorium.edu/ronh/weight/

End Check- Complete on separate sheet of paper

- 1. Describe three ways in which a car’s velocity will change
- 2. Identify a one situation involving unbalanced force and one situation involving balanced force. Explain how it each affect the motion of an object.
- 3. Arrange the following pairs of surfaces in order of most friction to least friction
- A shoe sole and a waxed basketball court
- A shoe sole and the frozen surface of a lake
- A shoe sole and the sidewalk

End Check- Complete on separate sheet of paper

- 4. Explain why Venus, which is slightly less massive than Earth, experiences a stronger gravitational pull from the sun than Earth does.
- 5. What is the average acceleration of a scooter that starts at rest and moves straight ahead at 12 m/s in 24 s.
- 6. Which will be moving faster after 3.0 s, a cyclist maintaining a constant velocity of 15 m/s straight ahead or a race car accelerating forward from a stoplight at 4.0 m/s2?

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