Thrust and Pressure | Gravitation | Assessment | English | Grade 9

What is the SI unit of density and relative density?

• Both have the same unit, Kilogram per metre cube

• Density-Kilogram per metre cube Relative Density-no unit

• Density-Kilogram per metre cube Relative Density-Kilogram per metre

• None of these

Answer explanation

SI unit of density is kilogram per metre cube. Relative density has no unit as it is a ratio of similar quantities.

A block of brick was placed on a table, what is the pressure exerted by the brick?

• Weight of the brick per unit area

• Mass of the brick per unit area

• Weight of the brick per unit surface area

• Mass of the brick per unit surface area

Answer explanation

Thrust exerted by the brick is the weight of the brick per unit area.

A iron rod of mass 5 kg and a rubber ball of mass 0.5 kg fell into the water and displaced 1 liter of water each. What is the amount of buoyant force exerted on both the bodies?

• Buoyant force (Iron rod)= 49 N Buoyant force (rubber ball)= 4.9 N

• Buoyant force (Iron rod)= 48 N Buoyant force (rubber ball)= 3.9 N

• Buoyant force (Iron rod)= 9.8 N Buoyant force (rubber ball)= 9.8 N

• Buoyant force (Iron rod)= 1 N Buoyant force (rubber ball)= 1 N

Answer explanation

According to Archimedes' principle amount of liquid displaced by a body is the amount of buoyant force acting on it. Therefore, buoyant force acting on both the bodies is the amount of liquid displaced, which is equal in this case. Weight of displaced liquid = 1 liter(1lit= 1kg) =1 × 9.8m/s² = 9.8 N

In a birthday party, 5 kg biriyani was placed in a dish of dimension 5m × 5m × 5m and mass 5kg. After the party only 20⁒ of the biryani was left due to which it was shifted to a dish whose dimensions and mass were reduced by 60⁒.Calculate the pressure exerted by the dish before and after the party?

• 4 Pa and 5 Pa

• 2 Pa and 7.5 Pa

• 4 Pa and 7.5 Pa

• 2 Pa and 5 Pa

Answer explanation

Before the party, mass of biryani= 5kg, mass of vessel = 5kg Total mass = 10 kg weight = 10 × 10 = 100 N Area= 5 × 5 = 25m² P1= 100/25 = 4 Pa After the party, mass of biryani= 1 kg, mass of vessel = 2kg Total mass = 3 kg weight = 3 × 10 = 30 N Area= 2 × 2 = 4m² P1= 30/4 = 7.5 Pa

A wooden cubical box of mass 2 kg with dimensions of 40 cm × 10 cm × 20 cm was placed on the floor. What will the minimum and maximum pressure exerted by the box?

• Minimum = 245 N/m² Maximum= 980 N/ m²

• Minimum = 245 N/m² Maximum= 490 N/ m²

• Minimum = 490 N/m² Maximum= 980 N/ m²

• Equal pressure is exerted at all times.

Answer explanation

Thrust= Force = 2 × 9.8m/s² = 19.6 N Pressure will be minimum when the box is placed in 40cm × 20cm dimension Minimum Pressure = 19.6N/ 800 × 10⁻⁴ m² = 19.6/ 0.08 = 245 N/ m² Pressure will be maximum when the box is placed in 20 cm × 10 cm dimension Maximum Pressure = 19.6N/ 200 × 10⁻⁴ m² = 19.6/ 0.02 = 980 N/ m²

An man of mass 50 kg on earth. He went to moon and exerted a pressure twice to that in earth. What is the pressure exerted on the moon. Assume area is same in both the cases.

• 25 Pa

• 40 Pa

• 245 Pa

• None of these

Answer explanation

Here, the area of contact is same on both the cases.We know weight on moon is 1/6 to that on earth. Hence pressure on moon would never be twice to that on earth. So, let's assume area is 2m² Weight of the man on earth= 50× 9.8m = 490 N Weight of the man on moon = 490/6 =81.6 N Pressure on earth = 490/2 = 245 Pa Pressure on moon = 81.6/ 2 = 40.8 Pa