# Help with physics

• Nov 7th 2012, 12:00 PM
Louisana1
Help with physics
A blue sports car (m=1200-kg) traveling north at 14 m/s is rear-ended by a 2000-kg truck traveling north at 25 m/s. If the truck and car locks bumpers and stick together, what is their speed immediately after the collision?

a. What is the momentum of the car BEFORE the collison
b. What is the momentum of the truck BEFORE the collision?
c. What is the total momentum BEFORE the collision?
d. What is the total momentum AFTER the collison?
e. What is the velocity of the cars after the collison?

Need help with the equations used in this
• Nov 7th 2012, 12:31 PM
fkf
Re: Help with physics
Quote:

Originally Posted by Louisana1
A blue sports car (m=1200-kg) traveling north at 14 m/s is rear-ended by a 2000-kg truck traveling north at 25 m/s. If the truck and car locks bumpers and stick together, what is their speed immediately after the collision?

a. What is the momentum of the car BEFORE the collison
b. What is the momentum of the truck BEFORE the collision?
c. What is the total momentum BEFORE the collision?
d. What is the total momentum AFTER the collison?
e. What is the velocity of the cars after the collison?

Need help with the equations used in this

In the law of conservation of momentum we believe! Therefore the momentum in our system is the before and after the crash. Define an x-axis that is positive in the northern direction. The momentum of the car before the crash is
p = mv => p = 1200*14 = 16800 kg*m/s due north

And the momentum for the truck before the crash is
p = mv => p = 2000*25 = 50000 kg*m/s due north

The total momentum in the system before the crash is therefore
p_tot = p_car+p_truck => p_tot = 16800+50000 = 66800 kg*m/s due north

Due the law of conservation of momentum we have this momentum after the crash as well. And we also know that the speed of the car and the truck is the same. Hence the momentum afterwards can be described as
p_tot = (m_car+m_truck)v => 66800 = 3200*v <=> v ≈ 21 m/s due north
• Nov 7th 2012, 12:46 PM
Louisana1
Re: Help with physics
how did you get 3200 in the last one
• Nov 7th 2012, 12:48 PM
fkf
Re: Help with physics
Quote:

Originally Posted by Louisana1
how did you get 3200 in the last one

Add the mass of the truck to the mass of the car, to get the total mass of the car/truck after the crash, and hence the mass of the moving object with a velocity v which we were searching.
• Nov 7th 2012, 12:49 PM
Louisana1
Re: Help with physics
ok for the E
do i divide 66800/16800
• Nov 7th 2012, 12:50 PM
fkf
Re: Help with physics
Quote:

Originally Posted by Louisana1
ok for the E
do i divide 66800/16800

?