CGR Lesson 7B Impulse - WordPress.com

Impulse and car safety

Book pg 35 and 38 Syllabus 1.21 and 1.23

Copyright CGrahamphysics.com 2016

Starter - Momentum: true or false?


Today’s lesson

Impulse Aim ¡  Know the definition of

impulse ¡  Know how to reduce the

force in a collision ¡  Use the idea of

momentum to explain safety features


What is an impulse?


Definition – which one is the correct?

¡  a sudden strong and unreflective urge or desire to act.

¡  a driving or motivating force; ¡  a pulse of electrical energy;

a brief current or "nerve impulses“ ¡  a force acting briefly on a body and

producing a finite change of momentum.


Impulse

Ft = mv – mu The quantity Ft is called the impulse, and of course mv – mu is the change in momentum (v = final velocity and u = initial velocity)

Impulse = Change in momentum Copyright CGrahamphysics.com

2016

Units

Impulse is measured in Ns because

Impulse = force x time N s


Impulse

Note; For a ball bouncing off a wall, don’t forget the initial and final velocity are in different directions, so you will have to make one of them negative. In this case mv – mu = +3m – (-5m) = 8m

-5 m/s

+3 m/s


When a force is applied to an object, the object’s velocity changes. This means that its momentum will also change.

Force and change in momentum

l  Time is measured in seconds (s).

l  Momentum is measured in kilogram metres per second (kg m/s).

l  Force is measured in newtons (N).

force = change in momentum time


¡  The force of impact can be reduced by increasing the time two objects are in contact

How can damage in a collision be

reduced?



Car crashes and momentum What happens if two cars travelling very quickly collide?

•  Both cars come to a stop in a short space of time.

•  Cars and their occupants experience a large change of momentum very quickly.

A very large change of momentum in a short space of time means the car occupants will experience a very large force.

This means they are likely to sustain serious injury.

Why could this cause serious injury?


Reducing forces in collisions

force = change in momentum

time

The best way to reduce the forces in a collision is usually to increase the time taken for the person to decelerate.

A longer deceleration means that change in momentum occurs over a longer time. There is therefore a smaller force acting on the person.

Air bags increase the time it takes for a driver’s head to decelerate, bringing them to a more gradual stop.


Increasing collision time calculation A 90 kg driver with no seatbelt crashes at 12.5 m/s, impacting against the steering column in 0.04 seconds. What is the force on the driver?

momentum = mass × velocity = 90 × 12.5 = 1125 kg m/s

= 1125 0.04

= 28 125 N


With a seatbelt, the driver would have stopped in 0.2 seconds.

Can you calculate the force on the driver if they’d worn a seatbelt. Copyright CGrahamphysics.com

2016

A rugby ball of mass 0.5 kg is kicked from stationary to a velocity of 8 m/s. The kicker’s foot is in contact with ball for 0.1 seconds. What force does the kicker use?

Change in momentum question 1

= 40 N


time

= (0.5 × 8) – ( 0.5 × 0)

0.1 = 4

0.1


A tennis ball is rolled at a toy car of mass 0.1 kg. The car is moved with a velocity of 0.5 m/s. If the ball and car are in contact for 0.05 seconds, with what force is the tennis ball rolled?

Change in momentum question 2

= 1 N


time

= (0.1 × 0.5) – ( 0.1 × 0)

0.1 = 0.05

0.05


Change in momentum calculations


Newton’s second law Newton’s second law of motion states that:

force = mass × acceleration

Acceleration is calculated by dividing change in velocity by time. This can be substituted into Newton’s second law:

force = time


time

Compare this equation with the one below. What do you notice?

mass × change in velocity

change in momentum time = mass × acceleration


Multiple-choice quiz


Car crash Web quest ¡  A traffic accident occurred in a 35 km/hr speed limit zone on

Millway Street in which a 3000-kg Cadillac Escalade SUV rear-ended a 2000-kg Subaru Outback Wagon that was stopped at a stop sign.

The entire police investigative division has gone on vacation to Bora Bora to relax, so the mayor has contracted with you and your team of experts to determine what happened and what traffic laws were broken. Copyright CGrahamphysics.com

2016

http://aplusphysics.com/educators/activities/carcrash_home.html

¡  Your team will provide the mayor with a detailed accident report (including all equations and work) that includes mass, velocity, and momentum of both vehicles both prior to and after the collision.

¡  Further, the mayor has requested you create a visual demonstration / re-creation of what happened to assist with the insurance company’s investigation. This may take the form of a PowerPoint presentation, a 3’x4’ poster, a web site, or an annotated digital video.

¡  The mayor has provided you with the following diagram drawn by police officers at the site of the accident:


Each group member will choose one of the following roles. The group’s success will depend upon how well each individual accomplishes their responsibilities

¡  a. Auto Expert: This individual will research the physics of linear motion and determine, based upon accident site analysis, how fast the SUV and wagon were moving immediately following the collision.

¡  b. Collision Expert: This individual will research the physics of basic collisions and determine, based upon data provided by the auto expert, how fast the SUV was initially moving.

¡  c. Investigator: This individual will research elastic and inelastic collisions, then analyze the accident scene as well as reports from the auto expert and collision expert in order to determine what type of collision occurred.


Fulfill your individual roles. ¡  Before you can put together a comprehensive

picture of what happened, each individual member of the group must visit several websites and research their portion of the accident in order to understand what is required and what steps you must take in order to accomplish your mission.

¡  Click on the appropriate link in the website to guide you through your role-specific tasks.


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CGR Lesson 7B Impulse - WordPress.com