• Support PF! Buy your school textbooks, materials and every day products Here!

Final Kinetic Energy and speed of three projectiles fired from a building's roof

  • #1

Homework Statement



Three different mass projectiles, mA > mB > mC, are launched from the top of a building at different angles with respect to the horizontal. Each particle has the same initial kinetic energy.

Which particle has the greatest kinetic energy just as it hits the ground? Why?
Which particle has the greatest speed just as it hits the ground? Why?

Homework Equations



(KE + PE)i = (KE + PE)f
KE = .5 mv^2
PE = mgh

The Attempt at a Solution



I don't know if it is the particle that has the highest mass m or attains the highest height h. Obviously, the one that attains the highest point will have the highest final velocity. vf = sqrt(2gh)

My teacher said that's wrong and now I don't know what to put.
 

Answers and Replies

  • #2
1,198
5
Hint: What can you say about the kinetic energy of each projectile as it reaches the same level as when it was launched? At that point, think about potential energy.
 
  • #3
Hmm that is very helpful. At the point where they were launched, the energy of each projectile would equal its initial kinetic energy. So the initial kinetic energy + mgh at that point would equal the total energy, so mA has the highest?
 
  • #4
1,198
5
Yes, you are correct.

Each projectile will have its speed increased by (2gh)^.5 when it hits the ground. That implies something about which one is going the fastest when it hits.

This is all the assistance I can give. I'll be away from a computer for the next few days.
 

Related Threads on Final Kinetic Energy and speed of three projectiles fired from a building's roof

Replies
5
Views
1K
Replies
3
Views
7K
Replies
1
Views
2K
Replies
4
Views
16K
Replies
3
Views
3K
Replies
10
Views
8K
Replies
9
Views
4K
  • Last Post
Replies
1
Views
620
  • Last Post
Replies
6
Views
2K
Top