# Physics Help

You're driving at 39km/h , when the traffic light 25m away turns yellow.

I need to find the constant acceleration required to stop at the light, the stopping time, and weather the acceleration is reasonable.

I know how to find the average and instantaneous acceleration, but how to obtain the constant, I have no clue.

1. 👍 0
2. 👎 0
3. 👁 203
1. 39,000 m/3600 s = 10.83 m/s

average speed during braking = 10.83/2 = 5.42

25 = 5.42 t
t = 25/5.42 = 4.62 seconds to stop

v = Vi + a t
0 = 10.83 + a (4.62)
a = -2.34 m/s^2

1/4 g is not an unreasonable deacceleration

1. 👍 0
2. 👎 0
posted by Damon
2. 39 km/hr = 10.83 m/s

s = Vi*t + 1/2 at^2
25 = 10.83t + .5 at^2

v = Vi + at
10.83 + at = 0
so,

t = 2.35s
a = -4.46 m/s^2

1. 👍 0
2. 👎 0
posted by Steve
3. Go with Damon - I switched the a and the t.

1. 👍 0
2. 👎 0
posted by Steve

## Similar Questions

1. ### Physics

A person driving her car at 54 km/h approaches an intersection just as the traffic light turns yellow. She knows that the yellow light lasts only 2.0 s before turning to red, and she is 30 m away from the near side of the

asked by Sydney on September 11, 2011
2. ### physics

A person driving her car at 56km/hr approaches an intersection just as the traffic light turns yellow. She knows that the yellow light lasts only 20.s before turning to red, and she is 30m away from the near side of the

asked by george on February 4, 2013
3. ### Physics

A person driving her car at 45km/h approaches an intersection just as the traffic light turns yellow. She knew that the yellow light lasts only 2s before turning red,and shi is 28m away from the near side of the intersection.

asked by Lee on September 17, 2016
4. ### science

When the front of a 5.0 m long bus traveling at a constant speed of 20 m/s is 55 m away from an intersection, the traffic light turns yellow. It will take 3.0 s before the traffic light turns red. The driverâ€™s reaction time is

asked by Nica on July 27, 2014
5. ### phys

A motorist is driving at 20m/s{\rm m/s} when she sees that a traffic light 200m{\rm m} ahead has just turned red. She knows that this light stays red for 15s{\rm s} , and she wants to reach the light just as it turns green again.

asked by Av on September 16, 2014
6. ### applied mechanics

a motorist driving his car at a speed of 72km/hr observes that the traffic lights ahead of him turns red if he comes to a halt at the traffic light 5seconds starting from first find the distance of the car from the light when the

asked by toly on January 24, 2017
7. ### physics

a car is approaching a traffic light at 25m/s when it turns red. The driver takes 1 second to react and put his foot on the brake pedal, after which the car decelerates at 2m/s^2. find the distance needed for the car to come to

asked by Keith on January 25, 2011
8. ### physics

A motorist is driving at 20m/s{\rm m/s} when she sees that a traffic light 200m{\rm m} ahead has just turned red. She knows that this light stays red for 15s{\rm s} , and she wants to reach the light just as it turns green again.

asked by ehiuof on September 17, 2014
9. ### Science Help

1.) A rail gun uses electromagnetic energy to accelerate objects quickly over a short distance. In an experiment, a 2.00 kg projectile remains on the rails of the gun for only 2.10x10-2 s, but in that time it goes from rest to a

asked by Kenny on October 18, 2014
10. ### Physics

A car is going 20m/s when they see a traffic light 200m ahead turn red. The traffic light is known to stay red for 15s. The driver wants to reach the light as it turns green. It takes them 1s to press the breaks and the car slows

asked by Confused on October 7, 2008

More Similar Questions