If the Force Continues to Be Applied What is the Velocity of the Box at 3 00

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Basic Newton's Laws and Applied Force: 3 boxes

  • Thread starter cassie123
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Homework Statement


Three boxes are in contact with each other on a frictionless horizontal surface as shown. The masses of the boxes are m1 = 10 kg, m2 = 20 kg, and m3 = 30 kg. A horizontal force F = 90 N is applied to m1.
Calculate:
a. the acceleration of the three boxes.
b. the net force on each box.
c. the contact forces between the boxes.
Screen Shot 2015-07-16 at 3.45.49 PM.png

Homework Equations


Newtons second law
F=ma

The Attempt at a Solution


total mass = 60 kg
F = ma

1) a = F/m = 90/60 = 1.5 m/s^2

2) net force on each box
F1 = m1a = 10*1.5 = 15N
F2 = m2a = 20*1.5 = 30 N
F3 = m3*a = 30 *1.5 = 45 N

Am I missing forces in calculating the net force?

c) contact force
90-15 = 75 N btwn 1st and 2nd box
75- 30 = 45 N btwn 2nd and 3rd box

The above is my best guess although I am not confident that this is the way to calculate contact forces. (What are contact forces?) Should I be using something like f=(F*(m2+3))/(m1+(m2+3) or f=(F*m2)/(m1+m2) to find the contact force between boxes 1 and 2?

Answers and Replies

Theres a two force which affects first two boxes (m1 and m2).But theres only one force effects third box which is contact between m2 and m3.Contant force means If you push m1 the force will affect the system and in the system m1 will push m2.But every force has a opposite force so m2 applies force on m1.
For this problem,If you want to answer question quick,start with the last box.Theres one force act it.Find that force.

Your find the acceleration right.
After find that force think this way.
Theres one box in the middle which its m2.Now theres two forces acting on it the contant force between m2 and m3 and contant force between m1 and m2.And their net force will be m2a=F which its 30N.

To this solution step by step.Youll find the answer

The above is my best guess although I am not confident that this is the way to calculate contact forces.

You should be more confident as you've got the solution precisely correct.
I did not notice you were right.What a shame for me
You should be more confident as you've got the solution precisely correct.

So it's right as shown? Never mind the other guesses for contact force? Thanks so much!!
Theres a two force which affects first two boxes (m1 and m2).But theres only one force effects third box which is contact between m2 and m3.Contant force means If you push m1 the force will affect the system and in the system m1 will push m2.But every force has a opposite force so m2 applies force on m1.
For this problem,If you want to answer question quick,start with the last box.Theres one force act it.Find that force.

Your find the acceleration right.
After find that force think this way.
Theres one box in the middle which its m2.Now theres two forces acting on it the contant force between m2 and m3 and contant force between m1 and m2.And their net force will be m2a=F which its 30N.

To this solution step by step.Youll find the answer


Thanks for your explanation! :)

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