# What a question

1. Jan 3, 2005

### rcgerrity

so my teacher likes to confuse me on purpose, here's the first question:

the kineric energy of a moving object is definted as 1/2MV^2 where M is the mass of the object and V is its velocity. Show that if I exert a force F on an object while it moves through a distance D, its kinetic energy will be F*D.

2. Jan 3, 2005

### dextercioby

1.Are u familiar with the concepts of "work","kinetic energy","potential energy"??
The theorem of kinetic energy variation states that the variation of the kinetic energy of a body is equal to the work done on it by the external exerting forces.
Since the variation of kinetic energy is is equal to the energy in the final state,and the work is defined as the product between the force and the distance,the theorem states:
$$\Delta KE=W \Rightarrow KE=F\cdot D$$

2.If not,use the definition of force (F=ma) and Galilei's formula
$$v_{fin}^{2}=v_{init}^{2}+ 2aD$$.
Multiply by "m",take the initial velocity 0 (as before) and divide by 2,to get the same answer.

Daniel.
Daniel.

Last edited: Jan 3, 2005
3. Jan 3, 2005

### rcgerrity

i have another one for ya, thanks so much by the way

a crane is raising a steel beam(mass 350 kg) the maximum tension the supporting cable can take without breaking is 7000 N
a) is it safe to life the beam at a constant speed of 3 m/s? prove answer
b) the crane operator becomes impatient and turns up the power so that the beam is accelerating upward at 6 m/s^2. is the cable able to carry the load?
c) what is the heaviest beam the crane can safely lift with the acceleration of 6 m/s^2?

thanks so much!

4. Jan 3, 2005

### dextercioby

This should be in the homework forum and i'm sure one of the mods will move it.The problems are not for me,are for you to solve.
What are your ideas to solving the problem??You're not expecting me to solve it for you,without a part of the work from you as well. :grumpy:

Daniel.

5. Jan 3, 2005

### rcgerrity

physics

i was just hoping for a little help, i have ideas.

for a, i was attempting to use f=ma, but i get stuck b/c i don't exactly know what to do with the acceleration then b/c its speed not acceleration that i am testing

6. Jan 3, 2005

### dextercioby

For 'a)',what is the tension in the string??
For 'b)',what is the tension in the string.??
For 'c)',what is the tension in the string,what is the maximum mass it can lift??

Daniel.