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Buoyant force

What is buoyant force? I’m unable to understand practically from book. please give vast and practical explanation.. (Raj asked)


Whenever a body is immersed in a fluid, say water, it experiences an upward force acting on it. The upward force exerted by a fluid on an object immersed in it is called Buoyant force.

When an object is immersed, it displaces the fluid already present there. The upward force (buoyant force) is the result of the fluid’s attempt to reoccupy the space.

If the object placed in a fluid is denser than the fluid, it will sink. Though it experiences an upthrust, its weight will be more than the upthrust.

The upthrust is equal to the weight of liquid displaced by the immersed part of the body.

If the object is of less density than the fluid, it will come up and float.

For a floating body, the net weight is equal to zero, as the upthrust balances the actual weight of the body.

The Physics of “Touching toes with back and heels against a wall”

If you are standing with your heels against a wall, you cannot touch your toes.Why?

(Asked Camille )


When you are standing against a wall or otherwise, the centre of gravity (the point through which the weight of the body acts; vertically downwards) lies within the area covered by feet, the base.

Normally when we lean forward, the body is so adjusted that the centre of gravity lies within the base.

But when we are standing against a wall, the body cannot adjust itself to adjust the centre of gravity within the base as the backward motion is restricted by the wall. Therefore when we try to lean forward, the CG is shifted out of the base and we tend to fall.


The pressure exerted by a given force can be decreased by increasing the area of contact.


The equation shows that pressure is inversely proportional to area.


The foundation off building is made more broader than the rest of the building so that the pressure exerted by the weight on earth can be decreased.



The shoulder straps of school bags are made wider and soft so that the pressure exerted by the heavy bags on shoulders can be reduced. You can feel the paining pressure if you try to replace the broad strap with a narrow one.




The Military tanks have a broad chain so that the pressure exerted by the heavy vehicle on ground is a minimum and it can travel even through marshy places

  • Heavy Vehicles have more tyres
  • When vehicles get struck in marshy places, wooden planks are used for the vehicle to come up.
Try to add more via comments

A few numerical problems from laws of motion

  1. A worker pulls a 200. N packing crate with an applied force of 55.0 N.  The crate accelerates at a rate of 0.250  m/s(squared).  What is the coefficient of kinetic friction between the crate and the factory floor?
  2. A student pulls a 150. N sled up a 28 degree slope at a constant speed by applying a force of 100. N.  Near the top of the hill he releases the sled.  With what acceleration does the sled go down the hill?
  3. A 200. N crate rests on a ramp; the maximum angle just before it slips is 25 degrees with the horizontal.  What is the coefficient of static friction between the crate and the surface of the ramp?
  4. A man pulls a sled with a weight of 200. N with a constant velocity across a horizontal snow surface.  If a force of 80N is being applied to the sled rope at an angle of 53 degrees to the ground, what is the coefficient of friction between the sled and the ground?
  5. A jet plane is flying with  a constant speed along a straight line at an angle of 30degree  above the horizontal. The weight of plane is 86 500N. Its engine provides a forward thrust T of 103 000N. The lift of force L(directed perpendicular to the wings)and the force R of air resistance (directed opposite to the motion)act on the plane. Find L & R

It is correct to say the weight of the body is 10 kg?

It is correct to say the weight of the body is 10 kg? (Khalid asked)


As such it is not correct. But, kg. wt (kilogram weight) is the unit of weight and therefore someone simply says kg instead og kgwt hoping that it is understood that what they mean is kgwt.

Acceleration due to gravity problem

A body weighs 30 kg on the surface of the earth how much would it weigh on the surface of a planet whose mass is 1/9 the mass of earth and radius is half of that of earth?


weighWeight on earth = 30 kg wt = 30 x g N

g on new planet, g’ = GM’/R’2=(4/9)g

Therefor weight on the new planet will be equal to 4/9 times its weight on earth

(Whereas the mass will be the same)

A question from practical life situation

“I was hit in the head with a 10 lb 10 inch circular disc a truck ran over my ques is? If it launched from ground level traveled 18.3 feet and hit my head i’md im 6’3. At what speed was it traveling? if it weighs 10 lbs what was its weight really like when it hit me? iM lucky to be alive i was just wondering thank you” (Rodney Ellefson  asked)

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