Question

The Hyundai Blue-Will Plug-In hybrid is a compact 4-door, 4-seat sedan.It is a test bed of new ideas, including panoramic glass roof with solar
cells for recharging batteries, a thermal generator that converts hot exhaust gases into electricity, drive-by-wire steering, lithium polymer
batteries, and touch screen controls. Blue-Will promises an EV driving range up to 40 miles on a charge and an economy rating up to 100 mpg.
Helping to achieve those numbers is a direct injection 1.6 liter 152-hp four, a CVT and a 100 kw electric motor..

1. A Hyundai Blue-Will hybrid is capable of an acceleration of 4.3 m/s. It has a mass of 770 kilograms of steel, 180 kilograms of iron, 110 kilograms of plastics, 80 kilograms of aluminum, 60 kilograms of rubber, 20 kilograms of glass, and 8 kilograms of other materials. The driver has a mass of 85 kg. a Sketch the system of the Blue Will b.Sketch the system of the driver c. What is the force on the driver from the acceleration? d.Compare the force of the car on the driver to the driver’s weight, using a ratio. 2. The Hyundai Blue Will hybrid is driving at a velocity of 125 kph, and crashes into another car traveling at a velocity of 35 kph. Assuming the drivers are of comparable mass, and the car bumpers lock on impact: a. What is the final velocity of the cars? b. If the famous Blue Will crashes into a tree, instead, in 2.6 seconds, what is the impulse? c. What is the total force exerted by the tree in stopping the Blue Will? 3. The Famous Blue Will hybrid is being driven at a velocity of 75 kph when the driver sees a tree ahead. a. The driver locks his brakes, causing the car to skid. The coefficient of friction of rubber on dry concrete is 1.0 (static) and 0.7 (kinetic). How much force will the brakes apply to the car? What difference would it make if the driver pumped the brakes? b. After 1.0 second of braking, what will be the car’s velocity? c. How far in advance will the driver need to start braking to avoid the tree? 4. The Hyundai has about the same drag as a Camry (about 0.36) and air has a density of about 1.21 kg/m3. The cross section of the Hyundai is about 1.5 m × 1.5 m. Take the torque provided by the engine at speed as about 250 lbf. a. What is the force applied by air resistance at 75 kph? b. Would this help the driver avoid hitting the tree in the previous problem? Why or why not? c. What is the maximum velocity of the car? Is this answer reasonable? Why or why not?

Answer 1

1. c. The force on the driver from the acceleration is:

d. The total mass of the Blue-Will hybrid is:

The weight of the car is then:

The ratio between the car'r weight and the force on the driver due to it's acceleration is:

This means that the weight of the car is nearly 33 times the force felt by the driver due to its acceleration.

2. a.The bumper system absorbs the energy of the impact, causing the two cars to stick together after the collision, and the conservation of momentum for the drivers gives:

b. The impulse is equal to the change in momentum:

c. The force on the car is given by:

3. If the car skids (slides) the force done by friction is:

If the driver pumped the brakes:

b. The desceleration caused by the sudden brake is:

The initial speed (before applying the brakes) is 75 km/h = 20.83m/s, after 1 second the speed will be:

(This is equal to 50.304 km/h)

The time required to stop the car is:

So the driver will need to start braking 3.037 s in advance to avoid the crash.  

4. From the definition of air drag:

where ρ is the air density, A the cross-sectional area, and C is a numerical drag coefficient

b. The force applied by the brakes is over than 41 times the force of the air drag, so it doesn't mark a difference with the stopping time

c. The maximum speed of the car is given by: