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Pressure can be defined as force acting upon unit area of P=F/A

Force = mass times acceleration (F=Mxa)

here acceleration is acceleration due to gravity 10ms-2

1 bar = 10^5 pascal = 10^5 newton/m2

pascal is SI unit of pressure and newton is SI unit of force

Note: for correct result try to convert all the values in SI units hence area of each tyre in contact is 0.001m2 hence two tyres = 0.002m2

hence 

P=M x a/A

M=P x A/a

M=3 x 10^5 x 0.002/10

M=60 kg

Note: here mass is the total weight of the rider and scooter

If the question would have been such that the excess pressure in the tyres are 3bar then it would have been mass of the rider

Equations Used:

P = F/A.

F = W = m * g (assuming static equilibrium condition)

Algebraic Manipulation:

P*A / g = m

Terms:

Force (F) = Person's Weight distributed across both tires. (N)

Area (A) = Surface Area Contact of both tires.(m2)

Pressure (P) = total pressure in both tires. (Pa)

Solution:

(3 bar * 10^5 Pa/bar)* (0.002 m^2) / (10 m/s^-2) = m

m = 60 kg.

Equations Used:

P = F/A.

F = W = m * g (assuming static equilibrium condition)

Algebraic Manipulation:

P*A / g = m

Terms:

Force (F) = Person's Weight distributed across both tires. (N)

Area (A) = Surface Area Contact of both tires.(m2)

Pressure (P) = total pressure in both tires. (Pa)

Solution:

(3 bar * 10^5 Pa/bar)* (0.002 m^2) / (10 m/s^-2) = m

m = 60 kg.

P = 3 bar =3 X 10^5  Pa

A = 10 cm^2 = 10 X 10^-4 m^2

Pressure is given by

P = F/A

the force on a single tyre is 

F = PA

the force on two tyres will be 

2F = 2PA

this force balances the weight of the person

weight = force by two tyres

mg = 2PA

m = 2PA/g

m = 2 X 3 X 10^5 X 10 X 10^-4  / 10

m = 60 Kg

Hence mass of rider is 60 Kg.

Given that ,

P=3 bar=3*100000 pa or N/m^2

A=2*10 cm^2 =20*10^-4 m^2

P=F/A,

3*100000=mg/A 

m=(300000*20*10^-4)/10

m=60kg

where m is the combination of weight of rider and scooter both

Pressure =Force/Area, so Force =Pressure x Area. Total contact area is 20 cm2. In SI units, this is 20/10000 m2 or 0.002 m2. 3 bar is (3 x 100000) Pa, or N/m2. So Force = 300000 x 0.002 N, or 600N. Mass in kg = 600/10. Since you do not know the mass of the scooter only, which also contributes to the pressure on the ground, so you cannot calculate mass of rider only. 

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