# Homework Help: Force Problems (need help !)

1. Sep 29, 2008

### jcumby

1.) A car moving at 50 km/h collides with a truck, and the front of the car is crushed 1.1 m as it comes to a complete stop. The driver is wearing a steabelt, but the pasenger is not. The passenger, obeying Newton's first law, keeps moving and slams into the dashboard after the car has stopped. If the dashboard compresses 5.0 cm on impact, find and compare the forces exerted on the driver by the seabelt and on the passenger by the dashboard. Assume the two have the same 65 kg mass.

I'm really confused about this one...thus far I have the intertia for both the driver and the passenger at 3250 N (65 kg * 50 km/h), but I'm not even sure if I've done that right.

2.)An elevator moves upward at 5.2 m/s. What is the minimum stopping time it can have if the passengers are to remain on the floor?

I know I need to figure out the normal force on a passenger on the verge of not remaining on the floor, and I know that normal force has to be some combination of mass and acceleration (9.8 + 5.2?), but other than that I'm stuck.

3.) Blocks of 1.0, 2.0, and 3.0 kg are lined up on a table (without friction). A 12 N force is applied to the 1.0 kg block. What force does the 2.0 kg block exert on the 3.0 kg block?

This must have something to do with inertia...but I'm not sure how to begin

2. Sep 29, 2008

### minifhncc

Convert units to SI and use equation v2=u2+2aS, after you find acceleration (deceleration in this case) for car (ie. also acceleration for driver, as driver moves with car body), and for the passenger, sub acceleration for driver and passenger into F=ma.

I know I need to figure out the normal force on a passenger on the verge of not remaining on the floor, and I know that normal force has to be some combination of mass and acceleration (9.8 + 5.2?), but other than that I'm stuck.
[/quote]
Use equation a=(v-u)/t -- You know that a=9.8ms-2, v=0 u= -5.2ms-1, considering that down is positive.

With no friction? Sounds like a dodgy question, anyone else agree?

Anyway, I'd start by finding out force due to gravity for each mass.

Hope this helps

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