# Homework Help: Hw Help(2)

1. Sep 25, 2006

### hatingphysics

Hw Help(2)!!!

I have no idea how to answer these problems....can someone show e how to work them out...and the answers?!

1) A set of crash tests consists of running a test car moving at a speed of 12.4 m/s (27.28 m/h) into a solid wall. Strapped securely in an advanced seat belt system, a 61 kg (134.2 lbs) dummy is found to move a distance of 0.87 m from the moment the car touches the wall to the time the car is stopped. Calculate the size of the average force which acts on the dummy during that time.

2) Three forces in the x-y plane act on a 5.90 kg mass: 15.50 N directed at 11o, 12.60 N directed at 128o, and 4.70 N directed at 209o. All angles are measured from the positive x-axis, with positive angles in the Counter-Clockwise direction. Calculate the magnitude of the acceleration. Calculate the direction of the resultant force using the same sign convention as above (in degrees).

3) A 38.3 kg girl and a 55.8 kg boy are on the surface of a frozen lake, 10.5 m apart. Using a rope, the girl exerts a horizontal 5.15 N force on the boy, pulling him toward her. Calculate the magnitude of the girl's acceleration.

4) A 4.1 kg penguin runs onto a huge sheet of frictionless Antarctic ice. At t=0 it is at x=0 and y=0 with an initial velocity of 0.63 m/s along the positive x-axis. It slides while being pushed by the wind with a force of 0.55 N directed along the positive y-axis. Calculate the magnitude of the penguin's velocity at t = 11.09 s. Calculate the direction of his velocity at that time, i.e., give the angle with respect to the x-axis.

2. Sep 25, 2006

### Mindscrape

The first one is an easy problem that utilizes the basic idea that force is the change in momentum over change in time.

The second one you ought to start by drawing out all your vectors, and resolving them into their respective components.

The third one you will want to consider both Newton's second law (force proportional to mass and acceleration) and Newton's third law (equal opposite).

For the last one you can get by with kinematics.

Post your ideas and where you are stuck if you want more than that.