Motion with Constant Acceleration

[Fiorella]

Please I need help with these two problems...the answer is in the back of the book but as much as I've tried I can't find the procedure to do them

I really appreciate your help...thank you so much beforehand!

1) The operation manual of a passenger automobile states that the stopping distance is 50 m when the brakes are fully applied at 96 km/h. What is the magnitud of decelaration? What is the stopping time?

Answer: -7.1 m/s^2

2) The front end of an automobile has been designed so that upon impact it progressively crumples by as much as 0.70 m. Suppose that the automobile crashes into a solid brick wall at 80 km/h. During the collision the passenger compartment deccelerates over a distance of 0.70 m. Assume that the decceleration is constant. What is the magnitud of the decceleration?

Answer: -350 m/s^2

 

[MadPhysics08]

hey! I could help u...for problem no.1
tried this equation it may help you! let:
V^2=v^2+2ad

where:
V=final velocity
v=initial velocity
a=acceleration
d= displacement (in unit of meter)

but; first you must
convert km/hr to m/s
and v=initial velocity will be zero because
it start at rest... (automobile)

 

[homeworkboy]

this is easy...
ok now it says stopping distance is 50m
Since brakes are applied, final velocity is 0. Initial Velocity is 96 km/h.

so S=50m, u= 96km/h = 26.67m/s, v = 0.

so v^2 = u^2 + 2aS

so a = v^2 - u^2/ 2s = 0 - (26.67)^2/2X50 = -711.28/100 = - 7.112 m/s2

 

[homeworkboy]

Oh sorry forgot about time...since v = u + at...t = v-u/a

so t = 0 - 26.67/-7.1 = -26.67/-7.1 = 3.76 sec

 

[MadPhysics08]

great...

 

[Fiorella]

Thank you SO much you guys !

Homeworkboy I have a question...what does "S" and "u" stand for?

thank you again lol