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Thrust and Pressure: Increasing pressure by decreasing Area

Thrust is defined as the force acting normal to a surface. Pressure is the thrust acting per unit area.

P=F/A

The equation suggests that the pressure is inversely proportional to area. We can increase the pressure for a given force by decreasing the area. Here are some examples.

Cutting Tools

Cutting tools have sharp edges with very low area so that high pressure

can be applied by a small force. You can realise this if you try to cut a tomato with a blunt knife and try again after sharpening it.

Nails and Drawing Pins

Nails and pins have sharp ends so that it can pierce into wooden boards and walls easily by applying a small force.

In a Karate Punch

 

In martial arts, the normal punch is executed in such a way that only two knuckles are hitting the target so that maximum pressure can be imparted.

If the entire flat portion of the fist is used, the pressure will be very much less and the opponent will not be hurt much. Punch is only just one of the many attacks in Karate. You can analyse the various attacks and blocks with a view to find the application of pressure in it.

TRY to find out more examples where we decrease area to increase pressure.

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.

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

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

The kinematics of a Dolphin Jump

A dolphin jumps from the surface of water and return to the surface of water after reaching the height of  1.8m.Find the initial velocity from 85km/h to 100km/h?

What happens when we suck in water from a drinking straw that has a hole in it?And why?

Let’s first see what happens when we “suck” in water using a straw. We are actually reducing the pressure inside the  mouth. When we do not suck, the pressure on the surface of water and inside our mouth will be the same.

The Physics of Drinking with strawBut when we start to suck, the pressure inside the mouth is reduced and water is pushed into our mouth due to the greater pressure on the surface of water caused by the atmosphere.

If there is a hole, we are not able to produce a low pressure area inside our mouth by sucking in.

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