Human clay

Newton’s 1st and 2nd lawInertia and pressureIn terms of collisions

Purpose of demonstration

• In this demo, you will investigate the forces acting on clay ‘crash test’ dummies when they are involved in collisions

• Discover how the 1st 2 Laws of motion describe what happens in collisions

• Discover what is meant by inertia and pressure

Materials that need to be acquired by each group

• Ramp• Wooden Car• Clay• Scissors• 3-4 straws• 4-6 world history textbooks or bricks (by back

window)• Camera (phone or better)• Later:• Ribbon (pick up from teacher when needed)• Metal wire

Answers to all questions/observations etc… are to

be written in composition book

Photos, drawings , notes may be added, if required or desired

Story

Opening Lab Questions

1) Which way do the passenger’s head and body move when a bus crashes into the car’s rear bumper?

2) What forces are on the passenger’s head and body during collision between bus and rear bumper of passenger’s car?

Task #1

• Car stopped , bus rolling, clay figure in car

Procedure/Construction• Partners (2),

• Construct the crash test dummy and ramp (if needed)

• Note: use bendable straw as neck with:– no more than ½ inch of the non bendable straw showing from

top of neck. – The entire bendable part should be seen between head and

body– Head should be small enough NOT to force straw to bend

immediately

• Put book under end of ramp, attach plastic extension

Crash test dummy

• Only head and body

• Can not be anatomically correct!

Set up/ observations

• Car is at base of ramp, pointing away from ramp

• Bus Rolls down ramp

• Observations at initial point of crash, only

• Do not interfere (crash) with the experiments of others

Take/Draw picture of clay figure’s head, after collision, to be included

in comp book/homework

Use a video camera to get a frame by frame image of the collision

resultsIf possible

Questions on observations

1. Describe the initial motion of the head and body of the crash test dummy. What is the final position of the dummy’s head?

2. During the collision what forces were applied to the body of the clay figure? List name and direction of ALL Forces.

3. During the collision, what forces are applied to the head of the clay figure? List name and direction of all forces

Definition review

• Net Force: sum of all outside forces applied to an object

• Balanced forces: An equal amount of force is applied to an object both directions in any dimension. Net force = 0

• Unbalanced forces: more force is applied to object in one direction than applied in the opposite direction. Net force does not = 0

Making sense Questions

4. What parts of the body were at rest before the collision? What does that indicate about the net force on object?

5. Did all parts of the body accelerate at the instant of impact? What must be true about the net force on the part if it accelerates?

More Questions

6. Did both head and body of figure accelerate at initial impact? Base your answer on the forces applied at impact.

7. Is there a force pushing the head backward upon collision? How would you explain (in terms forces and acceleration) what really occurs upon collision for head and body?

Newton was a pretty smart guy

Came up with an efficient way to describe the relationship between

motion and forces

Newton’s first law

• An object at rest, will remain at rest• An object in motion, will maintain the same

motion• UNLESS…• Acted upon by a unbalanced outside force

• (where the net external force on the object is not zero)

The first law has another name

• The law of Inertia

• Inertia means the resistance to change in motion (acceleration)

• All things want to move at constant velocity and will do so until unbalanced forces are applied to it

Inertia question

8. What factor is the main indicator of the amount of inertia an object has? Explain your response.

Inertia Study: Task 2

• Trial #1: Place bus at bottom of ramp• Let car roll down ramp and impact bus• Measure displacement of bus, resulting from

impact (record in comp book)

• Trial #2: Return bus to bottom of ramp, place 4-6 books on top of it.

• Let car roll down, measure resulting displacement of bus

Other inertia questions

9. What is the fundamental value of an object that determines the amount of inertia it has?

10. If a large vehicle collides with a much smaller vehicle, which will most likely undergo a greater change in motion? Why?

Inertia demo

• Need Cup, coin, index card

Final Questions on tasks 1 &2 (do in class or as homework)

10) What is the common name used for neck injuries sustained in a rear end collision?

11) What feature installed in every normal modern car helps prevent such injuries to you? Why can you still get whiplash from a front end collision?

12) Is inertia a force that causes or stops motion? Explain your answer

13) List values which changed, would affect the acceleration of a person’s head during a collision.

Task #3 (front-end collision)

Task #3

• Clay heads added to rolling bus

Set up/procedure• Set up is similar to task 1, except clay is used for

to only make heads for school bus riders

• Note: make sure heads have the right amount of mass. Too much, head flops before impact. Too little, straw will not act like a neck during collision.

• Start bus with heads at top , place car at bottom of ramp, release bus and observe heads

Take/Draw picture of clay figure, after collision, to be included in

comp book/homework

Use a video camera to get a frame by frame image of the collision

resultsIf possible

Questions

14. What did you observe about the general motion of the heads upon impact?

15. List the forces on the head at moment of impact.

16. Use the 1st law, and/or the terms inertia, forces, acceleration to describe why the heads moved as they did.

Question 17

If Fluffy the ‘Kitty’ was sleeping (without a seatbelt) in the rear window platform …

A.Describe Fluffy’s motion if the car was involved in a head-on collision?

B.How fast does Fluffy initially move?

Fluffy

Task #4

Seat belts and Pressure

Procedure• Create a clay figure as before except no straws are to

be used.

• This clay figure will ride down a ramp and crash into a barrier twice with a ribbon seatbelt and twice with a wire seatbelt. Each seatbelt should have 2 parts: a lap belt and shoulder harness

• Start car at same position for all trials, Books or bricks should be placed at same distance from bottom of ramp for all trials

Questions (always in compbook)18. Why use both lap belt and shoulder harness? Answer in

terms of forces.

19. Predict the effects of the ribbon and wire seat belts will have on the clay figure at impact. Be as detailed as possible

20. Run the trials. Did your results match your predictions? Explain your answer if there is a difference

21. Which seatbelt applied more force to the passenger? Explain your reasoning for the answer.

Newton again

His second law quantified the relationship between 3 values of an object:

(a)cceleration(m)assNet force applied to object

Fnet = ma Fnet = net force

So…

22. What is true about the masses of the objects involved and their accelerations during all 4 runs?

23. What does that mean about the amount of force involved in pressing the seat belt into the clay figure on each run?

24. If the forces are the same, why is there a difference in results?

Pressure

• Pressure is defined as the amount of force applied over a certain area.

• Greater the force, greater the pressure• Greater the area, smaller the pressure

• P = F / A

Observations on pressure

24. Which seat belt exerted more pressure on the clay figure? Why?

25. Use the idea of pressure to explain how a person can break a board using a karate chop but not a punch.

26. If a wider belt is safer in terms of pressure, how comes car seatbelts are not made of material that is a foot wide?