Greek Gymnasium Grade 2 · 45 minutes · 40 marks
Forces · Pressure
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A box experiences 14 N rightwards and 6 N leftwards. Find the resultant.
Take rightwards as positive.
ΣF=14−6=8 N
Keep SI units and check the sign.
8 N →
A stationary box experiences 7 N rightwards. What horizontal force is needed for equilibrium?
Equilibrium requires zero resultant.
7+F=0 ⇒ F=−7 N
The negative sign denotes leftwards.
7 N ←
A body moves in a straight line at constant velocity. Is its resultant force or its velocity zero?
Velocity does not change, but is not necessarily zero.
Apply Newton's first law in an inertial reference frame.
Balanced forces are compatible with uniform motion.
Zero resultant force, not necessarily zero velocity.
A body experiences 7 N and 6 N rightwards, and 13 N leftwards. Calculate the resultant.
Add all forces algebraically along the same axis.
ΣF=7+6−13=0 N
We cannot infer rest without knowing the initial velocity.
0 N
A normal force of 140 N acts over 0.02 m². Find the pressure.
Use p=Fnormal/A with area in m².
p=140/0.02=7000 Pa
Pa equals N/m², not N.
7000 Pa
A pressure of 7000 Pa acts over 0.03 m². Find the normal force.
Rearrange the definition: F=pA.
F=7000·0.03=210 N
Keep SI units and check the sign.
210 N
At depth 6 m in water, atmospheric pressure is 100000 Pa. Find total pressure with ρ=1000 kg/m³, g=10 m/s².
Total pressure includes atmospheric and hydrostatic contributions.
p=100000+1000·10·6=160000 Pa
Total pressure exceeds atmospheric pressure.
160000 Pa
Convert 7 cm² to m² before using the pressure formula.
Square the length conversion factor.
1 cm²=(0.01 m)²=0.0001 m²; 7 cm²=7/10000 m²
Do not divide by only 100.
0.0007 m²
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