# Some problems for physics

1. Dec 4, 2003

### stumillardkm

I have a couple problems for my physics class that I am needing help with: (and not sure which formulas to use)
1. A 1050 kg car has a maximum power output of 150 hp. How steep a hill can it climb at a constant speed of 70 km/h if the frictional forces add up to 700N?

2. A vertical spring (ignore its mass) with a spring constant 1000 N/m is attached to a table and is compressed 0.250 m (vertically).
a. This spring is used to launch a .200 kg ball. What will teh speed of the ball be when it leaves the spring?
b. How hgh above it's original position (spring compressed) wil the ball fly?

3. Designers of todays cars have built "5 mph (8 km/hr) bumpers" that are designed to elsatically compress and reboudn withoug any physical damage at seeds below 5 mph/8km/hr. IF the material of the bumpers permanently deforms after a copression of 1.5 cm, but remains like an elastic spring up to that point, what must het effective spring constant of the bumper material be? Assume the car has a mass of 1600 kg and is tested to ramming into a solid wall.

For #3, I'm thinking that i have to use the PE= 1/2 kx^2 formula for the elastic potential, but the mass is throwing me off.

For #2, I am just lost - want to use the same formula as above, but don't know what to plug in or if I'm using the right one at all!

For #1, I'm not sure where to start - the power eq: work/time makes sense, but then the height??? AUGH

Any help would be great!
Thanks

2. Dec 4, 2003

### Staff: Mentor

You are on the right track. Use conservation of energy. Do it right, and the mass will drop out.
Once again, conservation of energy. You have spring PE, gravitational PE, and KE.
The car's energy goes into two things: raising the car (gravitational PE) and overcoming friction (work done against friction). And yes, power = energy/time.