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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 )

Answer:

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.

Motion under gravity and time of fall

A body of mass m1 and another body of mass m2 are dropped from wqual heights.If time taken by the balls are t1 and t2 respectively then t1=t2. How ?

Answer:

The acceleration due to gravity is independent of mass of body. When two different masses  are dropped from the  same height, they will take same time  to reach the surface of earth.

Motion under gravity

Two stones of mass 5 kg and 10 kg are dropped from the same height above the ground level. Which one will hit the ground first? Why?

Answer: Both the stones will reach the ground simultaneously (provided we neglect the air resistance (the viscosity) and buoyant force.

This is because acceleration due to gravity is independent of mass of the freely falling body.

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)

Answer:

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.

Kinematics Numerical

  1. A particle travels 20 m in 7th second and 24m in 9th sec. find initial velocity?
  2. in a projectile motion , a body thrown from the ground ,at what angle both the vertical height and range will be equal?
  3. The velocity v(cm/s) of a particle is given in terms of time t(in seconds) by the equation v = at+b/t+c. Dimension of a,b, and c are ?

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?

Answer:

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)

Acceleration of a body on a rough surface

A black of mass 1.5kg placed on a rough horizontal surface is pulled by a constant horizontal force of 1.2kg wt. the cofficient of friction between the block and the surface is 0.3.  Find acceleration produced in terms of g.

(Posted by Ankur Srivastava)

Answer:

Image915.gif (242×177)

Applied force, F=1.2 kg wt = 1.2g N (where g is the acceleration due to gravity)

Friction f=µmg=0.3 x 1.5 x g=0.45g N

The net force = ma

Therefore;

ma= F-f

=> 1.5 x a = 1.2 g -0.45g=0.8 g

or

a = (0.8/1.5)g

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