# SG Transport - Lesmahagow High School

S3 FORCES THE PHYSICS OF CAR SAFETY C. Rossi, Dumfries High School S3 Forces: The Physics of Car Safety Lesson 1,2 Forces on cars 1. What do we already know about the effects of a force? 2. How are these effects important in ensuring car users can travel safely? 3. Can you recall from S2 what is meant by balanced and unbalanced forces? 4. How is the speed of a car is controlled by the sizes of the drive force and the drag forces? 5. How does the time taken to stop a car affect the size of the braking force? 6. Why are crumple zones used in car design? 2 What is force?

Think about cars - in your jotter try to: 3 give 5 examples of where you might see a force. E.g. Opening a door, pressing the accelerator or brakes, turning a spanner, using a jack, turning the steering Write a 3 things a force can do to a car. Change the speed, direction or shape of the car Newtons First Law An object will remain stationary or move at a constant speed in one direction unless an unbalanced force acts upon it. Balanced forces When there are two or more forces acting on an object and they add up to a total force of

zero, we say the forces are balanced. Unbalanced forces When the total force is NOT zero we say there is an unbalanced force. This is equal to the force left over and will cause the object accelerate or change direction or shape. 4 What happens to a car if forces are balanced? Having EQUAL AND OPPOSITE forces acting on a car is effectively the same as having NO force acting. drag drive The car moves at constant speed. Newtons 1st Law: An object will remain stationary or move at a constant speed in one direction unless an

unbalanced force acts upon it. What happens to a car if forces arent balanced? For example when a What happens when driver whounbalanced is travelling at there are constant speed forces acting on an accelerates or puts on the object? brakes.

Accelerate: drive Brake: drag Back arrow twice Speed increases until drag Speed decreases until drag to see again grows to match drive. matches drive again. Find the UNBALANCED FORCE In each of the following calculate the unbalanced force and its direction. (a)

30N (d) 60N 50N (b) 10N 10N (e) 60N 50N 20N 10N 50N (c)

50N 7 (f) 10N 45N 60N 90N 110 N Find the UNBALANCED FORCE - answers (a) (d) 20N (b)

20N (e) 50N (c) 5N 8 (f) 30N 20N FORCES acting on a CAR - 1 Reaction (push back from ground) Is the car Stationary No Drive Force

No Drag Force Moving with constant speed Accelerating Weight Decelerating Unbalanced force = zero (how do you know?) and the car is stationary. If it was moving there would be a drag force. 9 FORCES acting on a CAR - 2 Is the car Stationary

Moving with constant speed Accelerating Decelerating Unbalanced force = zero The car remains at the same speed, in the same direction 10 FORCES acting on a CAR - 3 Is the car Stationary Moving with constant speed Accelerating Decelerating Unbalanced force is NOT zero, so

The car accelerates in the direction of the unbalanced force (forwards) 11 FORCES acting on a CAR - 4 Is the car Stationary Moving with constant speed Accelerating Decelerating Unbalanced force is not zero, The car slows down (decelerates). 12 FORCES acting on a CAR - 5 Road force Drive force

Drag Weight Road force and weight are always equal If the drive force equals the drag steady speed If the drive force is bigger than drag car accelerates If drag is bigger than the drive force car decelerates 13 FORCES acting on a CAR - POSTER Draw four diagrams of a car (same car in each case nothing too elabora ar ar ar ar one is at rest. two is moving at constant speed forwards. three is accelerating.

four is decelerating. ch diagram should show all of the labelled forces acting on the cars. Explain why all cars have a top speed. tra if you have time: ar which was moving at constant speed drives off a cliff. Explain in detail - what happens next in terms of the motion of the car of the forces involved. Include diagrams. Stopping cars safely: Let us now try to figure out how stop a car safely. What do we need to do to stop a car? Apply the brakes!! The FORCE of the brakes SLOWS the car. Does the SIZE of the force matter? lets see.. Stopping cars safely: oth cars are moving at constant velocity.

o acceleration. Watch what happens when the drivers brake. orces are BALANCED Stopping cars safely: The drivers now apply the BRAKES. But the far away driver brakes harder! But which car stops QUICKEST? Watch again---- Stopping cars safely: Which car stops QUICKEST? BIG force SMALL stopping time

SMALL force BIG stopping time The car with the BIGGEST braking force stops in a SHORTER time. Force and time. 19 When a _____is used to slow down a car it is useful to remember that A BIG force will slow the car in a ______time, and

A SMALL force will slow the car in a ______time Missing words: LONG, force, SHORT Force and time. When a force is used to slow down a car it is useful to remember that A BIG force will slow the car in a SHORT time, and A SMALL force will slow the car in a LONG time

How are these ideas useful in other car situations? What if the car COLLIDES to a stop? 20 CRUMPLE ZONES New car with crumple zone. Is there any difference between the two collisions? Old car without crumple zone. C. Rossi, Dumfries High School Compare the impacts in slow motion

The carhow withlong theeach crumple Watch crashzone lasts takes longer to stop C. Rossi, Dumfries High School Lets now think about forces Smaller This car LONG TIME SMALL takes forces FORCE longerless

break to stop. bones! Assuming the cars have the same mass, Which car iszones safest tosave travel in? Crumple lives!! Which car is stopped by the biggest force? Bitesize Crumple Zones in Cars Bigger This car forces stops quickly DANGER ! C. Rossi, Dumfries High School

SHORT TIME LARGE FORCE Which car is the safest? How should a car be designed to cope safely with collisions? The Road Transport Laboratory suggests the most important safety features are The driver and passenger should be able to experience the car slowing down comfortably. The inside of the car should not be squashed. Most of the energy of the collision should be absorbed by the car body and not the driver and passengers. Crumple Zones

When a c_____ z____ is part of the bodywork of the car that is designed to crumple during a c_____. This ensures that the car and occupants are stopped in a relatively L____ time. That then also means the FORCE used to slow the car is relatively S______. Small forces will not tend to h____ the occupants of the car as m_____. Missing words: crash, much, LONG, harm, crumple zone, SMALL 26 Crumple Zones

27 When a crumple zone is part of the bodywork of the car that is designed to crumple during a crash. This ensures that the car and occupants are stopped in a relatively LONG time. That then also means the FORCE used to slow the car is relatively SMALL. Small forces will not tend to harm the occupants of the car as much. Yourhave You groups probably task isseen to take TVan

shows ordinary where antrolley physics ordinary and car fit itiswith fitted a bumper with an amazing or crumple body zone kit and to make a fancy it safer stereo in a before being

collision. Yougiven can make a stunning it stylish, paintbut job. safety is more important! You will be given a choice of materials plus card and sticky tape. Your design must be able to be fixed to the front of the test trolley with Blu-Tac. It should not add more than 2 cm to the length of your trolley. It must not have a large effect on

the performance of the trolley. Your design will be tested. The test will measure the force acting on the vehicle during a head-on collision. S3 Forces: The Physics of Car Safety Lesson 3,4 Seat belts and air bags What can we say about the speed of a car if the forces acting on it are balanced? Why are seat belts are used in cars? If the time taken to bring the driver or passenger to a stop is increased, how will this affect the force used to slow them? Why are airbags are used in cars? 29 Crash test dummies The Physics of Car Safety Car designers safety test their designs using real cars and life

size models known as crash test dummies. Crash test dummy video Crash test dummies cartoon (20 min) Car Safety info 30 Lets examine in detail how forces play a huge part in safely stopping a driver: Forces on a passenger during a crash The car is about to brake suddenly. The dummy has NO seatbelt. LARGE FORCE The car stops but the dummy continues to move as Thetodummy

there is no unbalanced force stop it.hits the dashboard and stops VERY QUICKLY. SHORT time LARGE FORCE! NOT SAFE!! Forces on a passenger during a crash Now lets see the difference when an AIRBAG is used SMALL FORCE With an airbag, the time to stop the dummy is BIGGER. LONG time SMALL force SAFER!! Forces on a passenger during a

crash How do seatbelts protect the passenger? SEATBELT FORCE When the car stops, the seatbelt provides an unbalanced force to stop the dummy SAFELY! Stopping passengers safelyseat belts and air bags. 34 When the car stops, the seatbelt provides an ________ force that prevents the driver or passenger from hitting the ___________

With an airbag, the time to stop the driver or passenger is __________. A __________ force is needed so there is less chance of ________ the driver or passenger. Missing words: smaller, dashboard, hurting, unbalanced, bigger Stopping passengers safelyseat belts and air bags. 35 When the car stops, the seatbelt provides an unbalanced force that prevents the driver or passenger from hitting the dashboard.

With an airbag, the time to stop the driver or passenger is bigger. A smaller force is needed so there is less chance of hurting the driver or passenger. Crash test dummy investigation Set up the ramp and low wall at the bottom of the ramp for the car to crash into. clay person ramp on stand

36 dynamics trolley low wall Form three clay people of varying masses to represent an adult, a teenager and a baby. Place one of the people in the car and let it roll down the ramp and crash into the wall. Measure the distance the person flies out of the car over the wall and record the distance in a data table. Complete three trials with each clay person released from the

same height on the slope. Record your results. Finally repeat the experiment using an elastic to represent a seatbelt that restrains the person from flying off the trolley. Crash test dummy investigation Results Trial Baby Teen Adult 37 Distance clay person flew (cm 1 2 3 1 2

3 1 2 3 Observations Crash test dummy investigation Analysis 1. Describe Newtons first law of motion. 2. What factors, besides mass, could have affected how far each passenger flew out of the car? 3. Why did the people fly out of the car? 4. What variables were held constant in this investigation? 5. What could be done to the car to make the people fly out farther? 6. When the seat belts were put on the passengers what happened to the people when the car hit the low wall and stopped? 38

Crash test dummy investigation Answers 1. An object will remain stationary or move at a constant speed in one direction unless an unbalanced force acts upon it. 2. Speed of the car, friction between the person and ground, air resistance 3. They were unrestrained so did not have an unbalanced force acting on them to stop them with the trolley. 4. Speed of the car release height on ramp, friction 5. Release the car from higher on the ramp, so making the crash speed higher. 6. Because there was an unbalanced force from the seatbelt to person, the person did not fly off the trolley. 39 S3 Forces: The Physics of Car Safety Lesson 5,6 Safe stopping distance 1. What are braking distance, thinking distance and stopping distance of a car? 2. What can affect the braking distance, thinking

distance and stopping distance of a car? 3. How does stopping distance depends on driver reaction time and car speed? 4. How does braking distance depend on vehicle speed, vehicle mass, and road surface? 40 Safe stopping distance Stopping distance depends on the speed of the car That is why we have speed limits the more likely it is for a driver to have to stop, the lower the speed limit. 41 Guess the car

stopping distances at each of these speeds: 20 mph -12 ?m 30 mph -23 ?m 40 mph -36 ?m 50 mph -53 ?m 60 mph -73 ?m 70 mph -96 ?m Safe stopping distance Stopping distance is the distance travelled by the car from when the driver notices the hazard to when the car stops.

Stopping distances are spilt into two sections: thinking distance: the distance gone as reacts to whatever is making him/her stop braking distance: the distance travelled once the brakes are pressed Stopping distance = thinking distance + braking distance 42 Stopping distances How long does it take a moving vehicle to stop? STOPPING DISTANCES ANIMATION

Stops Thinking TheBraking stopping distance distance distance is is thethe is the distance sum distance of the a car thinking a car travels travels distance before andwhilst

thethe braking the brakes brakes distance. areare applied. being applied. Presses brake Braking distance Sees cat Thinking distance Stopping distance

Stopping distance = thinking distance + braking distance FACTORS AFFECTING STOPPING DISTANCE 43 ROSPA CRASH SIMULATOR Do you know your speed limits? ? Match up the words with their definitions. 45 Stopping distance One of forces the road exerts on the tyres as the car is stopping. Friction

The distance a car travels whilst it is braking. Thinking distance The distance a car travels before the brakes are applied. Braking distance The sum of thinking distance and the braking distance. What factors affect braking and thinking distance? Thinking distance Braking distance Speed of car

Speed of car Drugs and alcohol Road conditions Medication Tiredness Condition of tyres Medication Condition of brakes Tiredness Drugs and alcohol Speed of car Condition of tyres

46 Condition of brakes Road conditions Stopping a car depends on Tirednes s Too many drugs Thinking distance 47 Poor visibility Wet roads

Icy roads Tyres/ brakes worn out Too much alcohol Braking distance Driving too fast Why can an icy road increase the stopping distance of a car? When you brake the car goes into a skid as the friction between the road and tyres is very

small.. ..so there is hardly any unbalanced force to slow the car so it goes further before stopping 48 Why do bald tyres increase the stopping distance of a car? 49 When you brake the car goes into a skid as the friction between the road and tyres is very small, especially in wet conditions..

..so there is hardly any unbalanced force to slow the car so it goes further before stopping Question : A car is moving along an open road. Suddenly, a sheep walks into the road. a) What do we call the distance the car travels before the driver puts their foot on the brakes? Thinking distance b) Name one factor that could increase the distance the car travels in this time. Medication, drugs/alcohol, speed of car, tiredness c) The braking distance is 35m for the car. If the stopping distance is 50m, how far did the car travel before the driver put their foot on the brakes? Thinking distance = Stopping distance braking distance = 50m 35m

= 15m 50 Measuring your reaction time. Do this: Find your reaction time for a dry road at 30 mph. What are the thinking and braking distances at this speed? Record your results. Without changing any other controls, turn the road conditions dial to wet. What are the thinking and braking distances for these conditions? Repeat for icy conditions. Can you explain these results? Try to use the correct science words. 51 Measuring your reaction time results You could use a table like this to record reaction (thinking) time:

Record these at different speeds (thinking time wont change you will just adjust the speed dial each time). Speed (mph) Thinking time (milliseconds) Thinking distance (metres) Braking distance (metres) Stopping distance (metres) 20 30

Stopping distance = thinking distance + braking distance You Road could useThinking a table like this for the road conditions task. time Thinking Braking Stopping conditions (milliseconds) distance (metres) distance (metres)

distance (metres) dry wet icy 52 Graphs and bar charts are good ways of presenting information like this. Measuring your reaction time. Do this: Find your reaction time for a dry road at 70 mph. What are the thinking and braking distances at this speed? Record your results. There is a gap beneath the reaction timer that you can fit a CD into.

Try to remove a CD from the gap and replace it with another whilst measuring your reaction time. Record your reaction time, thinking and braking distances. Think of other distractions when driving. Find their effects on your reaction time. REACTION TIME JAVA APPLET 53 Investigating stopping distance Cars and carpet The idea of this experiment is to enable you to understand the connection between speed and stopping distance of a toy car. mask fitted to car light gate toy car

stopping distance o Measure o the mass of the car Allow the car to run down the ramp and record: o (a) the speed (mask length / time o (b) the distance that the car travels on the carpet before stopping o Repeat this twice so that you have three pairs of values of speed and distance. o Record your results in a table. 54 Investigating stopping distance Cars and carpet RESULTS mass

of car (g) mask length (cm) time to pass light gate (s) speed = mask length /time (cm/s) stopping distance (cm) 1. What effect did increasing the speed have on the stopping distance of the car? 2. What does this result lead you to think about real cars

and car safety? 3. Can you think of another experiment you could do that would discover what else might affect the stopping distance of the car? If you have time try the experiment . 55 What can increase stopping distances? Braking distance is affected by: The c___ . .(worn brakes, extra weight, bald tyres etc) The r____ . (poor surface, spilt oil etc) The w_______ ..(wet, icy - anything that reduces friction) Thinking distance is affected by: The driver's r________time. Your reaction time can be affected by: d__________in the car, distractions outside the car (not just distractions s_______reflected into eyes

can affect the time interval between the braking occurrence happening and the driver reacting to it), a___________, m______________, a___ - your thinking processes slow as you approach old age Missing words: medication, distractions, road, age, reaction, car, sunlight weather, alcohol, 56 What can increase stopping distances? 57 Braking distance is affected by: The car . .(worn brakes, extra weight, bald tyres etc) The road . (poor surface, spilt oil etc) The weather ..(wet, icy - anything that reduces friction)

Thinking distance is affected by: The driver's reaction time. Your reaction time can be affected by: distractions in the car, distractions outside the car (not just distractions - sunlight reflected into eyes can affect the time interval between the braking occurrence happening and the driver reacting to it), alcohol, medication, age - your thinking processes slow as you approach old age

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