# Car deceleration with object attached to roof

## Homework Statement

A friend of mine gave me this problem to solve and I have no idea where to begin:
A car weighing 5000lbs traveling at 160km/hr with a payload of 45 kg on the roof hits a wall and stops after crumpling 3 ft. What is the minimum strength of the bond to hold the payload on the car.

## Homework Equations

I have no idea - I converted the weight of the car to 2268 kg but I don't know what use that information is in the broader equation.

I think I can use s=vt-(1/2)at^2 and s=ut-(1/2)at^2 to figure out the time it takes for the car to come to a stop and then use the same equation to determine the acceleration of payload - i.e. I think I have to figure out the deceleration of the car and then use that as the acceleration on the payload.

Again, I'm not sure what to do with the weight of the payload. I figure I can find out the downward force in Newton meters (441.3) but don't know how to figure out the shear stress applied to the objects to determine the minimum strength of the bond.

PhanthomJay
Homework Helper
Gold Member

## Homework Statement

A friend of mine gave me this problem to solve and I have no idea where to begin:
A car weighing 5000lbs traveling at 160km/hr with a payload of 45 kg on the roof hits a wall and stops after crumpling 3 ft. What is the minimum strength of the bond to hold the payload on the car.

## Homework Equations

I have no idea - I converted the weight of the car to 2268 kg but I don't know what use that information is in the broader equation.

I think I can use s=vt-(1/2)at^2 and s=ut-(1/2)at^2 to figure out the time it takes for the car to come to a stop and then use the same equation to determine the acceleration of payload - i.e. I think I have to figure out the deceleration of the car and then use that as the acceleration on the payload.

Again, I'm not sure what to do with the weight of the payload. I figure I can find out the downward force in Newton meters (441.3) but don't know how to figure out the shear stress applied to the objects to determine the minimum strength of the bond.
Don't forget also to convert the 3 feet to meters. You can calculate the acceleration directly from one of the kinematic equations; then you are correct that the acceleration of the payload must be the same as the acceleration of the car. So use Newton 2 on the payload: the bond strength is just the net force acting on the payload, in the horizontal direction. You don't have to get into the shear stress on the bond.