I was wondering if my answers are correct.
1. A 50kg person is taking a ride on an elevator travelling up at a steady speed of 2.5m/s. Find the time fo the elevator trip if the elevator does 4900 J of work on the person.
My answer: First I used the formula f=mg to find the force and got -490N. Then, I used the firmula w=fd and found the distance which is 10m. Lastly, I used v=d/t to find the time and got -4.0 seconds. Time shouldnt be negative but I got a negative number from the force equation.
2. A car slows from 108km/h to 90km/h over a course of 4.0s. THe force of the brakes was 1000N. What is the work done by the brakes on the car?
My answer: 110 000N
3. A pair of sleds weighs 25 kg and experience frictional force of 25N. If a dog team applies a 175 N force pulling it 12m find:
a) work done by friction
b) work done by dog team
c) net work done (using net force). how does the net work compare to answers from a and b
d) what form of energy does the frictional force represent? what form of energy does the net force produce?
My answers:
a) 300 N•m
b) 2100 N•m
c) net force=200N and net work=2400N•m. I'm not sure how it would compare to the previous answers.
d) frictional force represents thermal energy and net force produces kinetic energy?
5 answers
My answer: First I used the formula f=mg to find the force and got -490N.
************
- 490 N is gravity force DOWN on person.
+ 490 N is force elevator exerts UP on person
*************************
Then, I used the firmula w=fd and found the distance which is 10m.
***********
YES
***********
Lastly, I used v=d/t to find the time and got -4.0 seconds. Time shouldnt be negative but I got a negative number from the force equation.
+++++++++++++++++++++
Well my force is POSITIVE :)
+++++++++++++++++++++++ So I get +4 sec
My answer: 110 000N
+++++++++++++++++++++++++++++++++++++++
How far?
In constant acceleration, average velocity is (final +initial)/2
v = 99 *1000/3600 = 27.5 m/s
goes 4 s * 27.5 m/s = 110 meters
1000N * 110 m = 110,000 Joules
So YES
b) 2100 N•m YES (or Joules
c) net force=200N and net work=2400N•m.
***************
I get + for pull and - for friction so
net = 175 - 25 = 150N
Net work = 150 * 12 = 1800 Joules
and
1800 = 2100 -300 indeed
*********************
I'm not sure how it would compare to the previous answers.
******************
useful work = work done - heat generated
*********************
d) frictional force represents thermal energy and net force produces kinetic energy? yes
we need to find time but we cant find time if we don't have the distance. So first we will find distance.So apply a=Fnet/m ,
a=0(speed is constant).
0*50=Fnet=0N
then as we have the mass the other force applying here is the gravitational force
so Fnet=Fapp-Fg(gravitational force is acting in the opposite direction)
We will find Fg=mg,
Fg=50*9.8=490N
so Fnet=Fapp-Fg
0N=Fapp-490
4900N=Fapp
W=Fapp*d(work is the product of force applied and distance covered)
4900=490d
4900/490=d
10m=d
No we have distance and velocity we can easily find time
v=d/t
t=d/v
t=10/2.5
t=4seconds