Ch 2 Unit A Flashcards
(42 cards)
Define work.
Work is said to be done only when the force applied on the body makes the body move (that is there is a displacement of the body)
Factors of work are:
1) the magnitude of the force applied
2) the displacement produced by the force
3) cosine of the angle theta between the directions of force f and displacement s
Work is a ____ product of force and displacement.
Dot
Work is scalar or vector?
Scalar
Two expressions for work?
1) W = FS
2) W = FScos thetta
a) W = F* S cos thetta
b) W = F cos thetta * S
Why is work a scalar quantity?
Since force F and displacement S are vector quantities and work W is a scalar quantity so work is expressed as a dot product (or scalar product) of force and displacement vectors .the dot product of two vectors is scalar.
What are the three special cases?
Case 1: if the displacement is in the direction of force that is theta = 0° then cos 0° is equal to 1°
Case 2: when the displacement is normal to the direction of force that is theta = 90 degrees then cos 90 degrees = zero, that is, work is equal to zero
Case 3: if the displacement is in a direction opposite to the force that is theta is equal to 180 degrees then cos 180 degrees is equal to -1 therefore the work done on the body is negative.
Give three examples for case 1
1) in free fall of a body of mass M under gravity through a height h from a to b
2) when a cool he does work on the load when he raises it up against the force of gravity in an upward direction
3) when a body revolves in a circular path under the influence of the centripetal force ,on completing one round, the displacement becomes zero so the work done by the centripetal force is zero
2 examples of case 2
1) when a cooli walks on a horizontal ground while carrying a load on his head no work is done against the force of gravity because the displacement of load is normal to the direction of force of gravity which is vertically downwards.
2) when a body moves in a circular path in horizontal plane no work is done since the centripetal force on the body at any instant is directed towards the centre of the circular path and the displacement at that instant is along the tangent of to the circular path that is normal to the direction of force on the body that is theta is equal to 90°. Thus the work done is zero
2 examples of case 3
1) when a body moves on a surface rolling ball the force of friction between the body and the surface is in a direction opposite to the motion of the body that a ϴ is equal to 180 degrees thus the work then on the body by the force of friction is negative
2) when A ball of mass M is thrown upwards from A to B to a height h the displacement h is opposite to the direction of force of gravity MG so the work done on the body by the force of gravity MG in displacement h is negative
Conditions for the work done by a force to be zero:
1) when there is no displacement
2) when the displacement is normal to the direction of force
In a force-displacement graph which quantity is x and y axis?
Y axis- force
X axis - displacement
Which formula do we use in a force displacement graph if force is variable?
1/2FS
Which formula do we use in a force displacement graph if the force is constant?
F*S
W=??
Mgh
What is the si and cgs unit of work?
SI unit- joule (J) = 1N1m
Cgs unit- erg = 1dyne1cm
Bigger units of work?
Kilojoule=10^3
Megajoule=10^6
Gigajoule=10^9
Define joule
1 joule of work is said to be done when a force of 1 Newton displaces a body through a distance of 1 metre in its on direction
Define erg
One erg of work is said to be done when a force of one dyne displaces a body through a distance of 1 cm in its own direction
Relationship between joule and erg?
1 joule= 10^7 erg
Define power.
The rate of doing work is called power
Formula of power?
P = Work / time
Factors of power
1) the amount of work done by the source
2) the time taken by the source to do the side work
