# Homework Help: Plase any one crrect my answer

1. Jun 17, 2007

### freezx

Hi
Can any body correct my answers ?

1- when the speed of a moving object is doubled, its
a- Acceleration is doubled
b- Weight is doubled
c- Kinetic energy is double
d- Kinetic energy is increase four times

2- an iron sphere of mass 40 kg has the same diameter as an aluminium sphere whose mass is 10 kg. the sphere are dropped at the same time from a tower. When they are 115 m from the ground, they have the same:
a- Kinetic energy
b- Potential energy
c- Momentum
d- Impulse
e- Acceleration

3- how much work is done by a force of 10N in moving an object through a distance of 2 m in the direction of the force?
Work= force* distance
= 10 * 2 = 20 J

4- A bag of rice weighs 150 kg. to what height should it be raised so that its potential energy is 10 J ?
PE= mgh
10 = 150 * 9.8 * h
h = 10/(150 * 9.8) = 0.0068 m

5- calculates the work done in lifting 250 kg of water through a vertical height of 8 meters.

PE = mgh
= 250 * 9.8 * 8 = 19600 J
Work = PE
So work = 19600 J

6- A truck weighing 10000N and traveling at 35m/s stops at a distance of 40 m decelerating uniformly. What is the force exerted on it by the brakes? What is the work done by the brakes?
V2 = u2 + 2aS
u2 = -2 aS because the final speed is zero
1225 = - 2 * 40 * a
a = 1225/-80 = -15.31 m/s2
for the time
v = u + at
0 = 35 + (-15.31)* t
- 35 = ( -15.31 ) * t
t = -35/-15.31 = 2.28 s
force = mass * acceleration
10000 = mass * 9.8
mass = 10000/9.8 = 1020.4 kg

force exerted = mass * acceleration
= 1020.4 * - 15.31 = - 15622.32N
work done by the brakes:
work = force * distance
= 10000 * 40 = 400000 Joules

7- A man drops a 15 kg rock from a top of 20 m building. What is its speed just before it hits the ground? What is its kinetic energy when it reaches the ground?
PE= 15 * 9.8 * 20 = 2940 Joules
KE = PE
KE = ½ mv2
2940 = ½ * 15 * v2
2940 = 7.5 * v2
v2 = 2940/7.5
v = square root of ( 2940/7.5)
v = 19.79 m/s
KE= PE
So KE =2940 Joules

2. Jun 17, 2007

### Weave

2) I don't think so, Kinetic E.=(1/2)mv^2, don't they different masses? What forcce acts on both of they equally?

3. Jun 17, 2007

### say_physics04

ok,, i think the answer in number 2 should be E,, because the 2 spheres have same diameter,, so same effect of air resistance,, they were dropped at the same time and have same effect of g,, so acceleration is the same...

i got the same answers on everything but my velocity in number 7 is 19.80m/s
anyway,, the KE i got is the same...:grumpy:

that's all!

4. Jun 17, 2007

### PhanthomJay

Problem 2 should state to ignore air resistance, since none of the answers are correct if it is included.