Nosing Car?

Question

When a car accelerates forward, it tends to rotate about its center of mass. The car will nose forward:

a) when the driving force is imposed by the rear wheels (for front-wheel drive the car would nose downward).

b) whether the driving force is imposed by the rear or the front wheels.

The answer (sorry Alexander) is b. If the car is accelerated forward, the tires push backwards on the road, which in turn pushes forward on the tires. This force of the raod o the tires not only accelerates the car forward, but produces a torque about the center of mass of the car. Whether this force is exerted on the rear or front or both sets of wheels, the line of action of the force is along the roadway surface and produces a rotation of the front of the car upward and the back of the car downward (which increases traction for rear-wheel drive vehicles).

If you sketch the problem, you shoud, be able to see that in all cases the force of friction that accelerates the car tends to rotate the car counterclockwise about its center of mass.

It's easy to see that when the brakes are applied and the force and consequently the torque is oppositely directed, the car noses downward.

This tipping effect is particularly evident in a power boat. If you sketch the forces acting upon a power boat, you should note that when the boat accelerates the net force is forward and the torque tips the boat counter-clockwise, but when the boat decelerates and the force of water resistance is predominant, the net force is backward and the torque tips the boat clockwise.

Now a question for your teachers (if you're still in school)! The car noses up only while it is accelerating and goes back to level once it has achieved a constant speed. But the motor boat stays nose up. How come? When the boat's speed is constant the friction drag on the hull bottom is equal and opposite to the force on the prop, but the prop is deeper in the drink and is farther than the hull bottom from the boat's center of mass. So the bottom friction and prop act together to produce a turning couple or torque.

Answer 1

B. In either case, the accelerating force is forward and applied where the tires of the driven wheels contact the ground, so the line of action of the force is below the CM of the car. This results in a positive pitch torque, nosing the car up and reducing the weight on the front wheels. As a result, all else being equal, it takes less acceleration to slip and lose forward traction in front drive cars. And this is why you can see a greater nose-up in rear drive cars.
EDIT: Re linlyons's edit, I tend to shy away from talking about weight distribution change since superficially it seems to imply mass shifting, but of course it really only means change of the down-forces at the front and rear wheels. Note that the CM doesn't move relative to the car, except for the quite minor changes due to the wheel masses moving in opposite directions vertically. The weight distribution always shifts rearward in acceleration and forward in braking. You can duplicate this effect statically by parking on a hill. The amount of shift depends on the height of the CM. The reason FWD cars seldom burn rubber is that their static weight distribution is heavily biased forward; FWD 60/40 vs. RWD 50/50 is typical. An additional effect causing nose up is the torque at the drive axle. This can be large or small (if acceleration is constant the torque is proportional to the diameter of the drive wheels), but the reaction torque on the differential housing (and thus on the car) is always in the pitch-up direction.
Alexander's answer seems to be correctly pointing out that the question refers to "nose forward". If Dr. H intended that, it's hard to know what he's asking, which is why linlyons's and my answers assumed he meant "nose up".

Answer 2

As it often happens with Rev. H questions, the answer is

c) None of the above

Kirchwey is right, of course.
In both cases the car nose goes bacward und upward.