# Uniform acceleration problems

I'm having trouble with two problems:

1. A driver is travelling at 25 m/s when she spots a sign that reads "BRIDGE OUT AHEAD". It takes her 1.0 s to react and begin braking. The car slows down at a rate of 3.0 m/s^2.
Luckily, she stops 5.0 m short of the washed out bridge.

a) How much time was required to stop the car once the brakes were applied?

My attempt:

t= 0 m/s - 25 m/s (divided by) 3.0 m/s^2
t= -8.3
t =(8.3 s)

b) How far was the driver from the bridge when she first noticed the sign?

My attempt:

d = 0.5(V1 + V2)t
d = 0.5(0 + 25) 8.3
d = 104 + 5.0 (she fell short)
d = 109 m

2. A box accidently falls from the back of a truck and hits the ground with a speed of
15 m/s. it slides along the ground for a distance of 45 m before coming to rest.

Determine:

a) the length of time the box slides before stopping

My attempt:

d = 0.5(V1 + V2)t
45 = 0.5(15 + 0) t
solve for t and get
t = 6

c) the time it takes to slide the last 10 m

I don't know this one.

Can someone just quickly check if Im doing them right.

2c)Figure out the acceleration with the information you have and apply $$s=vt-\frac{1}{2}at^2$$ with s=10.

Thanks :) I am I doing the other ones correctly?

Doc Al
Mentor
1. A driver is travelling at 25 m/s when she spots a sign that reads "BRIDGE OUT AHEAD". It takes her 1.0 s to react and begin braking. The car slows down at a rate of 3.0 m/s^2.
Luckily, she stops 5.0 m short of the washed out bridge.

a) How much time was required to stop the car once the brakes were applied?

My attempt:

t= 0 m/s - 25 m/s (divided by) 3.0 m/s^2
t= -8.3
t =(8.3 s)

b) How far was the driver from the bridge when she first noticed the sign?

My attempt:

d = 0.5(V1 + V2)t
d = 0.5(0 + 25) 8.3
d = 104 + 5.0 (she fell short)
d = 109 m
Don't forget that it takes her 1.0 second to react.
2. A box accidently falls from the back of a truck and hits the ground with a speed of
15 m/s. it slides along the ground for a distance of 45 m before coming to rest.

Determine:

a) the length of time the box slides before stopping

My attempt:

d = 0.5(V1 + V2)t
45 = 0.5(15 + 0) t
solve for t and get
t = 6

c) the time it takes to slide the last 10 m
Start by finding the acceleration.

learningphysics
Homework Helper
I'm having trouble with two problems:

1. A driver is travelling at 25 m/s when she spots a sign that reads "BRIDGE OUT AHEAD". It takes her 1.0 s to react and begin braking. The car slows down at a rate of 3.0 m/s^2.
Luckily, she stops 5.0 m short of the washed out bridge.

a) How much time was required to stop the car once the brakes were applied?

My attempt:

t= 0 m/s - 25 m/s (divided by) 3.0 m/s^2
t= -8.3
t =(8.3 s)

I'd do it like this (0-25)/(-3)... since it is slowing down acceleration is -3...

b) How far was the driver from the bridge when she first noticed the sign?

My attempt:

d = 0.5(V1 + V2)t
d = 0.5(0 + 25) 8.3
d = 104 + 5.0 (she fell short)
d = 109 m

You also have the 1.0s before she reacted... so you need to add 25m/s*1.0s = 25m

2. A box accidently falls from the back of a truck and hits the ground with a speed of
15 m/s. it slides along the ground for a distance of 45 m before coming to rest.

Determine:

a) the length of time the box slides before stopping

My attempt:

d = 0.5(V1 + V2)t
45 = 0.5(15 + 0) t
solve for t and get
t = 6
Looks good.

c) the time it takes to slide the last 10 m

I don't know this one.

find the acceleration of the block...

Don't forget that it takes her 1.0 second to react.

Start by finding the acceleration.

So I would subtract 1.0 s from 8.3?

I'd do it like this (0-25)/(-3)... since it is slowing down acceleration is -3...

You also have the 1.0s before she reacted... so you need to add 25m/s*1.0s = 25m

Looks good.

find the acceleration of the block...
Thanks :) That helped a lot!

Doc Al
Mentor
So I would subtract 1.0 s from 8.3?
No. The 8.3s is for the accelerated portion, which you've already figured out. How far does she go before she even hits the brakes?

No. The 8.3s is for the accelerated portion, which you've already figured out. How far does she go before she even hits the brakes?
Doesn't say :S

Doc Al
Mentor
Doesn't say :S
You know the speed and the time!