Pressure and Moments
Pressure – some basic ideasDescribe and draw experiments for each of the
following ideas:
1) Pressure increases with depth
2) Pressure is the same in all directions at a certain depth
Pressure
Explain how the following work:
1) A sharp knife is easier to use then a blunt knife
2) A drawing pin
3) A woman with stilettos on might damage a floor more than an elephant would
Pressure
Pressure depends on two things:
1) How much force is applied, and
2) How big (or small) the area on which this force is applied is.
Pressure can be calculated using the equation:
Pressure (in N/m2) = Force (in N)
Area (in m2)
F
APOR in cm2 and
N/cm2
Some pressure questions
1) Calculate the pressure exerted by a 1000N elephant when standing on the floor if his feet have a total area of 1m2.
2) A brick is rested on a surface. The brick has an area of 20cm2. Its weight is 10N. Calculate the pressure.
3) A woman exerts a pressure of 100N/cm2 when standing on the floor. If her weight is 500N what is the area of the floor she is standing on?
4) (Hard!) The pressure due to the atmosphere is 100,000N/m2. If 10 Newtons are equivalent to 1kg how much mass is pressing down on every square centimetre of our body?
Some examples of pressure
Gas Pressure
Hydraulic systems
Hydraulic systems
Pressure is constant throughout this liquid
Levers
Load
Effort
Pivot
3 types of lever
Load is on other side of pivot (e.g.
seesaw)
Load is “outside” the effort, (e.g.
biceps)
Load is “inside” the effort (e.g. wheelbarrow)
MomentsA moment is a “turning force”, e.g. trying to open or close a door or using a spanner. The size of the moment is given by:
Moment (in Nm) = force (in N) x distance from pivot (in m)
Calculate the following turning moments:
100 Newtons
5 metres
200 Newtons
2 metres
Balancing moments
100 Newtons
200 Newtons
2 metres 2 metres
Total ANTI-CLOCKWISE turning moment = 200x2
= 400Nm
Total CLOCKWISE turning moment = 100x2 =
200Nm
The anti-clockwise moment is bigger so the seesaw will turn anti-clockwise
Balanced or unbalanced?
Stability
1. Centre of mass is within the wheelbase – no problem!
2. Centre of mass is directly above the edge of the wheelbase –car is on the point of toppling
3. Car falls over
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