Help solving for average force please

  • Thread starter xxphysics
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  • #1
xxphysics
38
3

Homework Statement


When it crashes into a bridge support that does not move, a car goes from 85 km/h to 0 in 1.23 m.
A) What is the impulse delivered to the 70-kg driver by the seat belt, assuming the belt makes the driver's motion identical to the car's motion? Assume that the initial direction of motion of the car is the positive direction.
B) What is the average force exerted by the belt on the driver?
C) If the driver were not wearing the seat belt and flew forward until he hit the steering wheel, which stopped him in 0.0145 s, what would be the average force exerted by the steering wheel on him?

Homework Equations


F = m*a
J = delta p
p = m*v

The Attempt at a Solution


So I got part A, -1650 N*s, by just converting 85 km/h to 23.61 m/s and multiplying that by the mass (70 kg). Then for part B I found what I thought was the acceleration by dividing the 23.61 m/s velocity by 1.23 m (73.8 s). I got 0.319992 m/s^2 and multiplied that by the 70 kg mass to get the force. This gave me an answer of -22 N, which was wrong. I then checked to see if I interpreted the "1.23 m" wrong, checking to see if I should've used 83 s instead of 73.8 s, and got - 20 N but this was still wrong.

What am I doing wrong?
 

Answers and Replies

  • #2
22,225
5,125
Do you really think that it takes 73 seconds for a seat belt to stop you in a sudden collision?
 
  • #3
xxphysics
38
3
Do you really think that it takes 73 seconds for a seat belt to stop you in a sudden collision?
I mean no, but could you help me think of another way to approach this problem? What would be another way to find the acceleration so I can use F = m * a ?
 
  • #4
22,225
5,125
You determined the acceleration incorrectly. Are you familiar with the suvat equations? If so, which one of these equation involves velocities, acceleration, and distance only?
 
  • #5
xxphysics
38
3
You determined the acceleration incorrectly. Are you familiar with the suvat equations? If so, which one of these equation involves velocities, acceleration, and distance only?
ohhhh meters instead of minutes; dumb mistake, thank you.
 
  • #7
SteamKing
Staff Emeritus
Science Advisor
Homework Helper
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Homework Statement


When it crashes into a bridge support that does not move, a car goes from 85 km/h to 0 in 1.23 m.
A) What is the impulse delivered to the 70-kg driver by the seat belt, assuming the belt makes the driver's motion identical to the car's motion? Assume that the initial direction of motion of the car is the positive direction.
B) What is the average force exerted by the belt on the driver?
C) If the driver were not wearing the seat belt and flew forward until he hit the steering wheel, which stopped him in 0.0145 s, what would be the average force exerted by the steering wheel on him?

Homework Equations


F = m*a
J = delta p
p = m*v

The Attempt at a Solution


So I got part A, -1650 N*s, by just converting 85 km/h to 23.61 m/s and multiplying that by the mass (70 kg). Then for part B I found what I thought was the acceleration by dividing the 23.61 m/s velocity by 1.23 m (73.8 s). I got 0.319992 m/s^2 and multiplied that by the 70 kg mass to get the force. This gave me an answer of -22 N, which was wrong. I then checked to see if I interpreted the "1.23 m" wrong, checking to see if I should've used 83 s instead of 73.8 s, and got - 20 N but this was still wrong.

What am I doing wrong?
Where does the 73 seconds come from?
 
  • #8
xxphysics
38
3
  • #10
jbriggs444
Science Advisor
Homework Helper
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Do you really think that it takes 73 seconds for a seat belt to stop you in a sudden collision?
Oh, I see it. 1.23 m times 60 seconds per minute = 73 seconds.

However, the 1.23 m actually means 1.23 meters.

Obtaining a average (over time) force is difficult without assuming constant deceleration. Obtaining an average (over distance) force is easier.
 
  • #11
xxphysics
38
3
Excellent. So now what do you get for a?
- 226.598 m/s^2 and -15862 N. Thanks again!
 
  • #12
THE BRAINIAC
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0
How do we find the force that the belt exerts on the driver?
 
  • #13
berkeman
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How do we find the force that the belt exerts on the driver?
Welcome to PF.

What is the definition of "Impulse"? What is the Relevant Equation?
 
  • #14
haruspex
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Welcome to PF.

What is the definition of "Impulse"? What is the Relevant Equation?
Unfortunately the time for which the belt exerts a force is not given, so the equation for impulse is not going to help.
Further, the equation xxphysics used to find the acceleration assumes it is constant. There is, as @jbriggs444 noted, no way to answer the question as posed. Instead of asking for average force, the question should have asked what the force is assuming it is constant.

@THE BRAINIAC, what SUVAT equation did xxphysics use and what variable was it used to find? How does that quantity relate to the force required to stop the driver?
 
  • #15
berkeman
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Unfortunately the time for which the belt exerts a force is not given, so the equation for impulse is not going to help.
Oops, you're right. I only skimmed the problem and thought I saw time mentioned, but re-reading I see now it's the distances that are given. Thanks.
 

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