Energy Loss in Head on Collision of Two Cars w/Same Speed and Mass

AI Thread Summary
In a head-on collision of two railroad cars, each weighing 7,000 kg and traveling at 90 km/h, the mechanical energy lost is calculated using kinetic energy formulas. Initially, the kinetic energy was incorrectly calculated due to the use of km/h instead of m/s. After converting the speed to 25 m/s, the correct kinetic energy is found to be 2.1875 million joules for one car, leading to a total energy loss of approximately 4 million joules. The discrepancy in the initial calculation was clarified by addressing unit conversion. The final answer aligns with the book's solution, confirming the energy lost in the collision.
mrsteve
Messages
9
Reaction score
0

Homework Statement



Two railroad cars, each with a mass of 7,000kg and traveling at 90 km/h collide head on and come to rest. How much mechanical energy is lost in this collision?

Homework Equations



KE=\frac{1}{2}mv^2

The Attempt at a Solution



KE=\frac{1}{2}(7000)(90)^2=2.835*10^7

Mechanical energy = KE+PE. PE = 0.

Energy lost = 2KE or 5.67*10^7

Book answer is 4*10^6

Where am I going wrong? Thanks..
 
Physics news on Phys.org
If your result is in joules, the velocity units of km/h is not helping.
 
Convert the speed to standard units of m/s.
 
Ahh... So 90 km/h = 25 m/s.

\frac{1}{2}(7000)(25)^2=2.1875*10^6 => 2.1875*10^6(2) = 4.375*10^6 => 4*10^6

Thanks!
 
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
View full post »

Thread 'Egg Drop Challenge! Need ideas for winning designs'

I was thinking using 2 purple mattress samples, and taping them together, I do want other ideas though, the main guidelines are; Must have a volume LESS than 1600 cubic centimeters, and CAN'T exceed 25 cm in ANY direction. Must be LESS than 1 kg. NO parachutes. NO glue or Tape can touch the egg. MUST be able to take egg out in less than 1 minute. Grade A large eggs will be used.
View full post »

Similar threads

Pan suspended by a spring (Energy + SHM)
Replies
20
Views
3K
Calculate the speed and angle of the center of mass before a collision
Replies
3
Views
907
Speed of charged balls after collision
Replies
13
Views
1K
Find elastic energy in the compressed spring.
Replies
12
Views
3K
What is the maximum attainable speed for the car?
Replies
4
Views
2K
Help with question on motion: Avoiding a rear-end collision
Replies
6
Views
3K
Super Basic Collision Problem - Preparation for SR
Replies
10
Views
1K
Momentum and percent kinetic energy loss
Replies
3
Views
5K
Time period of SHM & Energy conservation in pulley-spring-block system
Replies
24
Views
3K
Inelastic Collision Energy loss
Replies
1
Views
1K

Hot Threads

Recent Insights

Back
Top