New to physics. need help with some solutions

1. May 19, 2014

ford f100

Hello. I am trying to learn physics on my own. I have always thought it was a neat thing.

I am wondering if you could show me how to do these solutions. I cant figure it out.

#1 A cyclist (total mass of 68kg including the bike) travelling initially at 6.5m/s slams on the brakes and skids to a stop in 9.4m. The coefficient of kinetic friction between the tires and the trail is 0.93

a) Calculate the magnitude of the force of kinetic friction
b) Calculate the work done by the force of friction during the skid

#2 A roast in a refrigerators freezer compartment has a potential energy of 35J relative to the floor if the roast is 1.7m above the ground what is the mass of the roast?

Thanks

2. May 19, 2014

Hardik Batra

You can use this equation.

U = mgh

U=35J
g=10m/s2
h=1.7m
From this you can get m = ..................

3. May 19, 2014

ford f100

Ok thanks so the answer would be 5.95kg?

4. May 19, 2014

You can use $F=ma$ and the kinematic equations here.(The equation involving initial speed, final speed, distance and acceleration. Find the acceleration and then find the force acted.)

No. Show the working.

5. May 19, 2014

dauto

Problem #1 is overdetermined - that means it has more data than needed and the given data is not self consistent so it has no solution.

6. May 19, 2014

Regtic

Nice catch. We can still pretend it's on, say planet x, and has a different gravitational acceleration.

Try and find the acceleration first using kinematic equations like

$(V_f)^2 =(V_i)^2 + 2a (Δx)$

$\Delta x =V_i \Delta t + \frac {1}{2} a (\Delta t)^2$

$V_i = V_f +a \Delta t$

Last edited: May 19, 2014
7. May 19, 2014

AlephZero

That depends what you assume. The question doesn't say both wheels were locked in the skid. If the bike only has brakes on one wheel, you can calculate the fraction of the bike's weight that is acting on that wheel.

8. May 19, 2014

Redbelly98

Staff Emeritus

Please stop providing equations until after the OP has posted an attempt, including work, toward solving the problem.

Learning physics by self-study is to be encouraged and even admired, but please remember our usual guidelines for "homework" help apply to self-study as well.

9. May 19, 2014

Regtic

Oh sorry I thought it would be ok considering he seemed to have made an attempt.

Noted for the future

10. May 19, 2014

Redbelly98

Staff Emeritus
No worries. There was an attempt on question #2, but not on #1.