# What net external force is requiered?

My wife is taking a Physics class and is having some trouble. I've never taken physics but I've tried to help, with no luck.

1. Homework Statement
What net external force is required to accelerate a 1500kg car from rest to 100km/h in 10 seconds?

2. Homework Equations
She believes that the right equation is:

F=ma

3. The Attempt at a Solution

100km/1hour x 1hour/60sec = 100/60 = 1.7km/s

a=(1.7 km/s)/10s
a=0.17s/km

F=1500kg x 0.17s
F=255
-------------------------------
So we got '255N'

The book says the answer is '4167N'
But it does not say how to get this.

She is studying for a test tomorrow.
Any help would be great.

Thanks

Last edited:

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PhanthomJay
Homework Helper
Gold Member
My wife is taking a Physics class and is having some trouble. I've never taken physics but I've tried to help, with no luck.

1. Homework Statement
What net external force is required to accelerate a 1500kg car from rest to 100km/h in 10 seconds?

2. Homework Equations
She believes that the right equation is:

F=ma

3. The Attempt at a Solution

100km/1hour x 1hour/60sec = 100/60 = 1.7km/s

a=(1.7 km/s)/10s
a=0.17s/km

F=1500kg x 0.17s
F=255
-------------------------------
So we got '255N'

The book says the answer is '4167N'
But it does not say how to get this.

She is studying for a test tomorrow.
Any help would be great.

Thank
1. Homework Statement

2. Homework Equations

3. The Attempt at a Solution
You just changed the clock. There are 3600 seconds in an hour! Also, watch your units...your acceleration should be in meters/sec^2, not km/sec^2, to get the proper unit of the external force in newtons.

Thank you

after posting we did convert it to metters but did not catch the min and seconds mixup.

Thanks again