Section+3+chap+3

=__//**Section 3- Energy and Work: Why air bags?**//__= toc

What Do You See?
page 276 2/18/11

Cars crashing with one dummie hitting a airbag the other hitting the windshield.

What Do You Think?
page 276 2/18/11

slows down your head

Investigate
page 276 2/18/11

2a. nothing 3a. 4cm crack 4a. broke completely at 20 cm 5a. it didn't crack and a lot of the rice flew out 6a. 2 cm 7a. the first one broke and the second one was fine. the rice helped it slow down instead of hitting a hard surface.

=Notes= 2/28/11

What is the force exerted on the egg if it drops 95 cm onto a bowl of rice- compressing the rice 0.5cm. mass of egg= 55g we need to reduce the speed of the egg to 0m/s -we need to get rid of the KE -it takes __work__ to change the KE of an object W=deltaKE fd=KEf-KEi Fd=0-1/2mVi^2 F=-mvi^2/2d

First determine the velocity of the egg when it arrives at the surface of the rice Vi=0 Vf=? a= -9.8m/s^2 d= .95m t= ? Vf^2=Vi^2+2ad Vf^2=0+2(-9.8)(-.95) Vf=4.3m/s

F=-mvi^2/2d F=-(.055)(4.3)^2/ (2)(.05) F= -10N

Physics Talk
page 279 2/28/11

To stop an automobile safely, braking system must decrease speed of vehicle eliminating kinetic energy They need to create a system that transfers the energy of the automobile safely with a small force and a large distance or a large distance and a small force
 * kinetic energy**:the energy possessed by a moving body is called kinetic energy. KE=1/2mv^2
 * W=deltaKE**

Checking Up
page 287 2/28/11

1. A body's kinetic energy is determined by the mass and velocity of the object 2. When work is done, the kinetic energy can either increase or decrease depending on direction of force 3. there would be more force depending on the distance 4. the unit is Joules for work and KE

Physics To Go
page 290 3/1/11

1a. If the stopping distance is increased there will be more force with the hard ball and it will hurt 1b. bending your knees absorbs a lot of the energy 1c. the cord is stretched out which lets off energy 1d. it takes pressure away when the wrestler hits the mat 2. the KE triples 3. the egg thrown faster will travel twice as far 4. 30 feet and 2 N, 10 feet 6 N, 60 feet, 1 N 5. 6a. 60kg/18=3.3 J 6b. (60kg)(3J)=180 6c 7. b 8. 1920/ 60 = 32 c

What Do You Think Now?
page 288 3/1/11 The airbag slows you down gradually so there is less pressure