Speed, force, velocity, (falling object)

[Shane-O]

Homework Statement

(A) An egg (57g) is dropped from a balcony 4m high,calculate the speed with which the egg, when dropped from the first floor balcony will hit the ground. ( use Newtons laws). ( note the time taken for the egg to hit the ground is 0.1s)

(B) What force from the ground will be applied to the egg causing it to crack. The speed before the egg hits the ground will become the initial velocity before the egg stops.

Homework Equations

i know that i am definantly wrong but the only formula i think that i know and can truly use is V=S/T and maybe F=MA.(try make the answer simpler for me)

The Attempt at a Solution

(A)
V=?
S=4m
T=0.1s

V=S/t
V=4/0.1
V=40m/s?

( i know i didnt use Newtons laws but this is the only thing i know)

(B)
F=?
M=57grams
A=40m/s/s?


F=MA
F=40X57
F=2280N?
(im not sure if its right)

 

[tahayassen]

The answer to question A is correct. The average speed the egg took to hit the ground is 40 m/s. The average speed is the change in distance traveled (4m) over the time taken (0.1s).

The answer to question B is also correct depending on what your teacher is expecting. The egg has a gravitational vector pulling the egg down towards the earth. As soon as the egg hits the earth, a normal force is applied to the egg to prevent the egg from going through the ground. I believe you need to know momentum to accurately solve the problem, but if you assume the normal force and the gravitational force are equal, then yes, your answer is correct.

I would wait for a mentor if I were you.

Actually, I have a question for the mentor that comes to solve your problem, if you assume the gravity vector and normal force vector are equal, then doesn't that only prevent further acceleration? How does that cause the object to stop?

 

[Shane-O]

Thank you but I believe I needed to use Newtons laws for the first question and I'm not sure that I did.

 

[tahayassen]

Thank you but I believe I needed to use Newtons laws for the first question and I'm not sure that I did.

I don't seem to see how you could use Newton's laws to solve the question. Maybe I misunderstood the question? Is it asking for the velocity of the egg when it hit the ground? If so, you can use the kinematics equation:

v2 = v1 +a*t
= 0 + 9.81*0.1
= 0.981 m/s

 

[PeterO]

Homework Statement

(A) An egg (57g) is dropped from a balcony 4m high,calculate the speed with which the egg, when dropped from the first floor balcony will hit the ground. ( use Newtons laws). ( note the time taken for the egg to hit the ground is 0.1s)

(B) What force from the ground will be applied to the egg causing it to crack. The speed before the egg hits the ground will become the initial velocity before the egg stops.

Homework Equations

i know that i am definantly wrong but the only formula i think that i know and can truly use is V=S/T and maybe F=MA.(try make the answer simpler for me)

The Attempt at a Solution

(A)
V=?
S=4m
T=0.1s

V=S/t
V=4/0.1
V=40m/s?

( i know i didnt use Newtons laws but this is the only thing i know)

(B)
F=?
M=57grams
A=40m/s/s?


F=MA
F=40X57
F=2280N?
(im not sure if its right)

When an object is dropped, it usually means it starts with zero velocity. If the balcony was 4m high and the egg was dropped, it will take more than 0.1 seconds to reach the ground

I think the 0.1 seconds refers to the time taken for the egg to STOP having hit the ground.

Use the usual formulas to calculate the speed just before hitting the ground.

If you have done enough vertical motion / falling object questions you would know that if an object falls 4.9 m , it takes 1 second, and reaches 9.8 m/s.
Since this egg falls 4 m it will take a little less than a second, and reach a slightly slower speed.

 

[Shane-O]

thanks for the tips but i haven't learned anything about that so far on kinematics equations ect.

 

[Shane-O]

When an object is dropped, it usually means it starts with zero velocity. If the balcony was 4m high and the egg was dropped, it will take more than 0.1 seconds to reach the ground
I think the 0.1 seconds refers to the time taken for the egg to STOP having hit the ground.

Well its a hypothetical question and the egg does take 0.1s to complete the fall but you make a good point

 

[PeterO]

When an object is dropped, it usually means it starts with zero velocity. If the balcony was 4m high and the egg was dropped, it will take more than 0.1 seconds to reach the ground
I think the 0.1 seconds refers to the time taken for the egg to STOP having hit the ground.

Well its a hypothetical question and the egg does take 0.1s to complete the fall but you make a good point

I strongly disagree.

Clap you hands twice as quickly as you can.
The time between claps will be approx 0.1 seconds.
You can't even throw an egg fast enough to get to the ground in 0.1 seconds.

For simplicity: let's adjust this to a 4.9m balcony.

The egg will reach a speed of 9.8 m/s, having accelerated for a whole second under the influence of gravity - the force is the weight of the egg. (0.057 x 9.8) N

When the egg hits the ground it stops in just 0.1 seconds. That requires 10 times the acceleration, so 10 times the force*, so the force that cracks the egg is 10 x (0.057 x 9.8) N
* 10 times the force to stop it in 1/10 th the time.


Your balcony was only 4 m high, so the speed is a little less, and the "crcking Force" will also be a little less.

The equations you need to use are listed here

http://en.wikipedia.org/wiki/Equations_of_motion

They are also explained there.

From the "Classic Form" of the equations, you use No 5 for Part A, then No 1 for Part B.

 

[Shane-O]

Everyone knows that an egg would not really take 0.1 seconds to fall four meters. This question has been just simply used to test my knowledge about Newtons laws, force, velocity.(which isn't very much).

 

[PeterO]

Everyone knows that an egg would not really take 0.1 seconds to fall four meters. This question has been just simply used to test my knowledge about Newtons laws, force, velocity.(which isn't very much).

If you will finally get your head around the idea that the egg fell from 4m and then stopped in 0.1 seconds when it struck the ground, you may be able to increase your knowledge.

 

[Shane-O]

Here i will state the question directly:
Assume the egg is not encased in an egg holder:(Note the time taken by the egg to hit the ground is 0.1s and the hight of the balcony is 4m.)
a). calculate the speed with which the egg, when dropped from the first floor balcony will hit the ground.(Use Newtons laws.)

This is exactly what the question asks and i do agree with you when you say it would take around 1.3 seconds or so to reach ground and would take 0.1 seconds for the egg to stop moving. so yes you are right even though we were discussing about kind of different things