Work-energy theorem, no equations

warmfire540
quick question:
I have to write about the work-energy theorem, and I've already defined it, but i need to mention a couple of cases where this theorem is applied in daily life around us..
I dont' quite understand where we would use this theorem in everyday life..
just a few simple ideas to get me started would rock!

Homework Helper
Hi warmfire540! … rock'n'roll!

Going upstairs? Dragging things? Pulling people's legs?

warmfire540
Hi warmfire540! … rock'n'roll!

Going upstairs? Dragging things? Pulling people's legs?

Well...here is the wikipedia definition:
“According to the work-energy theorem if an external force acts upon an object, causing its kinetic energy to change from Ek1 to Ek2, then the mechanical work (W) is given by:
E=Kf-Ki=1/2mv^2

What exactly does this mean? How is going upstairs an example of of htis theorem applied in daily life around us?

Staff Emeritus
Gold Member
Yeah. A simpler way of stating that would be:

"The net work done on an object is always equal to its change in kinetic energy."

From this, one of the first conclusions you can draw is that if no work is done on an object, then it's kinetic energy won't change.

As for daily life examples, I won't give you full answers, but I'll hint at them.

Hint 1: You're pushing an object initially at rest, you know its mass, you know the constant force with which you're pushing it, and you know over what distance your hand remains in contact with it. Can you see that you can use the work-energy theorem to determine the object's final velocity after you've stopped pushing it, *without* resorting to first calculating the acceleration of the object due to that force?

Hint 2: Say you have a system in which all the forces are conservative* (e.g. a ball thrown straight upwards in a gravitational field). Since the force is conservative, all of the ball's kinetic energy will eventually be converted into gravitational potential energy. Based on this knowledge, can you see that the work-energy theorem (applied to the gravitational force) can determine how high the ball will go, based on its initial velocity?

*What it means to say that the gravitational force is conservative is that that even though the force opposes the ball's motion and therefore does negative work, reducing the ball's kinetic energy, the kinetic energy the ball loses is not permanently "lost" (as it would be if the force were friction, which would waste the ball's energy of motion by converting it to heat). Instead, the kinetic energy is converted into gravitational potential energy and is recoverable.

warmfire540
thanks!

SIMON@CHUEU
Elasticity

HEY DO KNOW HOW TO SOLVE ELASTICITY QUESTIONS?
IF YES PLEASE TRYE THIS ONE i.e. WHEN WATER FREEZES, IT EXPANDS BY ABOUT 9%. WHAT PRESSURE INCREASE WOULD OCCUR INSIDE UR AUTOMBILE ENGINE BLOCK IF THE WATER IN IT FROZE? BULK MODULUS OF ICE IS 2x10^9 N/M^2

SIMON@CHUEU