# Acceleration when time is not given

1. Oct 25, 2010

### PhysicsAdvice

1. A cyclist travelling 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?

2. Oct 25, 2010

### cdotter

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

3. Oct 25, 2010

### fss

No-

x = vt

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

4. Oct 25, 2010

### 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?

5. Oct 25, 2010

### cdotter

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.

6. Oct 25, 2010

### PhysicsAdvice

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

7. Oct 25, 2010

### PhysicsAdvice

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

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