2 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?
3 What is force? Think about cars - in your jotter try to: 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
4 Newton’s First Law 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. An object will remain stationary or move at a constant speed in one direction unless an unbalanced force acts upon it.
Newton’s 1 st Law: An object will remain stationary or move at a constant speed in one direction unless an unbalanced force acts upon it. Having EQUAL AND OPPOSITE forces acting on a car is effectively the same as having NO force acting. drive drag The car moves at constant speed. What happens to a car if forces are balanced?
What happens when there are unbalanced forces acting on an object? Speed increases until drag grows to match drive. Speed decreases until drag matches drive again. For example when a driver who is travelling at constant speed accelerates or puts on the brakes. Accelerate: Brake: Back arrow twice to see again… drivedrag What happens to a car if forces aren’t balanced?
7 Find the UNBALANCED FORCE In each of the following calculate the unbalanced force and its direction. 30N50N (a) 10N60N (b) 50N45N (c) 10N50N (d) 60N 10N 50N (e) 20N 60N110N (f) 90N10N
8 Find the UNBALANCED FORCE - answers 30N (a)(d) (e) (f) 20N (b) 50N (c) 5N 20N
9 Unbalanced force = zero (how do you know?) and the car is stationary. Weight Reaction (push back from ground) Is the car… Stationary Moving with constant speed Accelerating Decelerating No Drive Force No Drag Force If it was moving there would be a drag force. FORCES acting on a CAR - 1
10 Unbalanced force = zero The car remains at the same speed, in the same direction Is the car… Stationary Moving with constant speed Accelerating Decelerating FORCES acting on a CAR - 2
11 Unbalanced force is NOT zero, so… The car accelerates in the direction of the unbalanced force (forwards) Is the car… Stationary Moving with constant speed Accelerating Decelerating FORCES acting on a CAR - 3
12 Unbalanced force is not zero, The car slows down (decelerates). Is the car… Stationary Moving with constant speed Accelerating Decelerating FORCES acting on a CAR - 4
13 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 FORCES acting on a CAR - 5 Drag Drive force Road force Weight
1. Draw four diagrams of a car ( same car in each case – nothing too elaborate ). Car one is at rest. Car two is moving at constant speed forwards. Car three is accelerating. Car four is decelerating. Each diagram should show all of the labelled forces acting on the cars. 2. Explain why all cars have a top speed. Extra if you have time: A car which was moving at constant speed drives off a cliff. 3. Explain – in detail - what happens next in terms of the motion of the car and all of the forces involved. Include diagrams. FORCES acting on a CAR - POSTER
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? – let’s see…..
Both cars are moving at constant velocity. No acceleration. Forces are BALANCED Watch what happens when the drivers brake…. 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? The car with the BIGGEST braking force stops in a SHORTER time. BIG force SMALL stopping time SMALL force BIG stopping time Stopping cars safely:
19 Force and time. 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
20 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?
New car with crumple zone. Old car without crumple zone. CRUMPLE ZONES Is there any difference between the two collisions?
Watch how long each crash lasts Compare the impacts in slow motion… The car with the crumple zone takes longer to stop
Let’s now think about forces… This car takes longer to stop. This car stops quickly Assuming the cars have the same mass, Which car is stopped by the biggest force? LONG TIME SMALL FORCE SHORT TIME LARGE FORCE Which car is safest to travel in? Smaller forces break less bones! Bigger forces – DANGER! Crumple zones save lives!! Bitesize Crumple Zones in Cars Bitesize Crumple Zones in Cars
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.
26 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
27 Crumple Zones 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.
You have probably seen TV shows where an ordinary car is fitted with an amazing body kit and a fancy stereo before being given a stunning paint job. Your group’s task is to take an ordinary physics trolley and fit it with a bumper or crumple zone to make it safer in a collision. You can make it stylish, but 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.
29 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?
30 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 (20min) Car Safety info Let’s examine in detail how forces play a huge part in safely stopping a driver:
The car stops – but the dummy continues to move as there is no unbalanced force to stop it. The car is about to brake suddenly. The dummy has NO seatbelt. The dummy hits the dashboard and stops VERY QUICKLY. SHORT time – LARGE FORCE! NOT SAFE!! LARGE FORCE Forces on a passenger during a crash
With an airbag, the time to stop the dummy is BIGGER. LONG time – SMALL force SAFER!! Now let’s see the difference when an AIRBAG is used SMALL FORCE Forces on a passenger during a crash
How do seatbelts protect the passenger? When the car stops, the seatbelt provides an unbalanced force to stop the dummy SAFELY! SEATBELT FORCE Forces on a passenger during a crash
34 Stopping passengers safely- seat belts and air bags. 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
35 Stopping passengers safely- seat belts and air bags. 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.
36 Crash test dummy investigation Set up the ramp and low wall at the bottom of the ramp for the car to crash into. 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. clay person dynamics trolley ramp on stand low wall
38 Crash test dummy investigation – Analysis 1.Describe Newton’s 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?
39 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.
40 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?
41 Guess the car stopping distances at each of these speeds: 20 mph - ? 30 mph - ? 40 mph - ? 50 mph - ? 60 mph - ? 70 mph - ? Safe stopping distance 12 m 23 m 36 m 53 m 73 m 96 m 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.
42 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
Stopping distances Thinking distance is the distance a car travels before the brakes are applied. Braking distance is the distance a car travels whilst the brakes are being applied. Stopping distance = thinking distance + braking distance Stopping distance Braking distance Thinking distance How long does it take a moving vehicle to stop? The stopping distance is the sum of the thinking distance and the braking distance. Sees cat Presses brake Stops FACTORS AFFECTING STOPPING DISTANCE STOPPING DISTANCES ANIMATION ROSPA CRASH SIMULATOR
Do you know your speed limits? ?
45 Match up the words with their definitions. Stopping distance Friction Thinking distance Braking distance One of forces the road exerts on the tyres as the car is stopping. The distance a car travels whilst it is braking. The distance a car travels before the brakes are applied. The sum of thinking distance and the braking distance.
46 What factors affect braking and thinking distance? Thinking distanceBraking distance Speed of car Road conditions Drugs and alcohol Tiredness Medication Condition of tyres Condition of brakes
47 Stopping a car depends on… Braking distance Too much alcohol Thinking distance Tiredness Too many drugs Wet roads Driving too fast Tyres/brakes worn out Icy roads Poor visibility
48 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
49 Why do bald tyres 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, especially in wet conditions..…..so there is hardly any unbalanced force to slow the car – so it goes further before stopping
50 a)What do we call the distance the car travels before the driver puts their foot on the brakes? b)Name one factor that could increase the distance the car travels in this time. 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 Medication, drugs/alcohol, speed of car, tiredness Thinking distance = Stopping distance – braking distance = 50m – 35m = 15m Question : A car is moving along an open road. Suddenly, a sheep walks into the road.
51 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.
Measuring your reaction time results You could use a table like this to record reaction (thinking) time: Record these at different speeds (thinking time won’t change – you will just adjust the speed dial each time). Stopping distance = thinking distance + braking distance You could use a table like this for the road conditions task. Graphs and bar charts are good ways of presenting information like this. Speed (mph) Thinking time (milliseconds) Thinking distance (metres) Braking distance (metres) Stopping distance (metres) Road conditions Thinking time (milliseconds) Thinking distance (metres) Braking distance (metres) Stopping distance (metres) dry wet icy
53 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
54 Investigating stopping distance Cars and carpet o Measure the mass of the car o 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. stopping distance light gate toy car mask fitted to car The idea of this experiment is to enable you to understand the connection between speed and stopping distance of a toy car.
55 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.
56 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,
57 What can increase stopping distances? 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