FORCES

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# FORCES - PowerPoint PPT Presentation

FORCES. Force: A push or pull that tends to cause an object to change its movement or shape. Mini-Experiment!. Page 281 in text Find a partner and try the actions shown in the 3 pictures. #1: Pile different number of books on hands – which is harder to hold up?

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### FORCES

Force: A push or pull that tends to cause an object to change its movement or shape

Mini-Experiment!
• Page 281 in text
• Find a partner and try the actions shown in the 3 pictures.
• #1: Pile different number of books on hands – which is harder to hold up?
• #2: Try pushing gently on your partner’s hand from all different directions. Which was the easiest to resist? Which was the hardest?
• #3: Have your partner hold her arm out from the elbow. Push gently, starting close to her elbow and then moving out to her hand. Which was the hardest to resist?
Effect of a Force
• The actual effect of a force will depend on 3 things:

1. Magnitude (size) of force

-Bigger the force, bigger effect on a structure

2. Direction of force

-Up a ramp vs. down a ramp

3. Location where the force is applied

-Push too high on a fridge  tip over

Unit of Force
• Newton (N): standard unit for measuring force
• 1 N = amount of force needed to hold up a mass of 100 g (ie: apple)
External Forces on Structures
• External Force: force applied on a structure by something else
• ex: gravity: pull of the Earth
• 1. Centre of Gravity: imaginary point of a structure where the downward force of gravity acts on it
• Must be supported at this point to be a stable structure.
• Main method to increase stability: place most of the mass of structure close to ground (lowers centre of gravity).
External Forces cont’d.
• 2. Symmetry: a balanced arrangement of mass that occurs on opposite sides of a line
• Force of gravity on either side is the same
• Stable: mass must be distributed equally around the centre of the structure’s base.
External Forces cont’d.
• 3. Load: external force on a structure
• Stay the same for long period of time
• Dynamic Load (Live Load): external force that moves or changes with time
• What the structure must support
• Weather/ground factors
• Engineers use 2 conditions to decide what type of bridge to use
• What the bridge is crossing (ex: water or land)
• What kinds of loads the bridge will be supporting

#1: Beam Bridge: most common

-flat and supported at its two ends

#2: Truss Bridge: lightweight but strong with

triangle-shaped frames along its sides

#4 Arch Bridge: withstand heavy loads

-each piece in arch pushes on piece next

to it  ground, which pushes back

Questions
• Which bridge has the greatest static load?
• Which bridge has the greatest dynamic load?
• Make a list of any components that you see that help to strengthen each type of bridge.
• Check & Reflect p. 295 Q#1-4