Acceleration when time is not given

[PhysicsAdvice]

1. A cyclist traveling 21km/hr stops over a distance of 15m. His total mass is 73kg. What is his acceleration?



2. T=D/V, A=V/t



3. Since time is displacement over velocity, I can use the displacement and velocity to find time and then solve acceleration by a=v/t. Am I on the right track or should the mass have come into play there?

 

[cdotter]

$$V_f^2=V_i^2+2ax$$ would work?

 

[fss]

3. Since time is displacement over velocity, I can use the displacement and velocity to find time and then solve acceleration by a=v/t. Am I on the right track or should the mass have come into play there?

No-

x = vt

...is only valid when v is constant, which it is not.

 

[PhysicsAdvice]

I tried $$V_f^2=V_i^2+2ax$$ which comes to
0=441+2a (225),
-441/225=2a,
-1.96=2a,
-0.98=a

but the answer is -1.1, error?

 

[cdotter]

I tried $$V_f^2=V_i^2+2ax$$ which comes to
0=441+2a (225),
-441/225=2a,
-1.96=2a,
-0.98=a

but the answer is -1.1, error?

I'm not sure what units you're using? Convert everything to SI units (velocity in m/s, distance in m) and it turns out fine.

 

[PhysicsAdvice]

vi=21km/hr vf=0km/hr distance=15m, so do i need to convert the 21km/hr to m/s?

 

[PhysicsAdvice]

nevermind I tried it again and found the right answer, thanks!