What are the things that only mechanical engineers know and others don't?

* Why do you see one wheel of the car spinning when in dirt?

Ans :- Because of the Differential mechanism.The automotive differential is designed to drive a pair of wheels while allowing them to rotate at different speeds.

* Why is it difficult to start rotating/ move a wheel/car but easier to maintain the speed ?
Ans :-Static friction > Dynamic friction.Also momentum conservation law.

* To throw a ball as long as possible at what angle should one throw the ball?
Ans :-45° as per projectile motion .

* Why cant you turn a bolt with hand but with help of a spanner with the same hand?

Ans :-Spanner gives the leverage, hence the mechanical advantage of distance vs force.

* Why does vehicle keep on wobbling even after hitting a bump for sometime ?
Ans :-Depends on the damping factor of the suspension system and can be infinite if not damped.

* Ask a mechanical engineer what is the device that stores energy?
Ans :-He might say flywheel instead of battery! Flywheel stores rotational energy.

* Why doesn't diesel engines have spark plugs compared to petrol engines?
Ans :-Because diesel engines works on compression ignition rather than combustion. Hence it has injectors rather than spark plugs.

* Why cant you reverse pedal your bicycle ? or why doesn't the pedal drive you back when you go in a down hill ?
Ans :-It has a freewheel mechanism or over running clutch which can only transfer load from pedal and not reverse .

* How does the small force from your leg stop a fast moving car?
Ans :-Pascals law, the force is multiplied using hydraulic pistons setup.

* Why cant the centrifugal pump without water in it ? (Water priming )
Ans :-Because it works on the principal of vortex and creates suction only when water is available.

* Why is one pulled to the outward of the car when it hits a curve?
Ans :-Whenever a body enters a curve centrifugal force hits all the bodies in the car. All the loosely held mass will be subjected to force and hence motion.

* Why is air cooler when you blow with lips almost closed but hot when lips wide open ?
Ans :-The faster air moves, the lower pressure it has (Bernoulli's principle). So when you blow faster, your stream of air is lower pressure than the surrounding air. Thus the surrounding air fills in the stream. The surrounding air is obviously cooler than the air in your lungs.

* How are you able to lift a car with your hand with a small jack ?
Ans :-Mechanical advantage created by the hydraulic/ screw mechanism converts distance into force. Hence we only have to give less force to lift the car.

* How do one make a more impact resistant material from same metal ?
Ans :-Using various Heat treatment methods.

* What makes bearings help save energy ?
Ans :-It reduces the area of contact hence reducing the energy loss due to friction.Ball bearings for examples have only point contact rather than a bush with line/area contact.

* What are types of springs other than helical?
Ans :-Leaf, Torsional,Belleville Spring etc

* Why does disc brakes have holes in it ?
Ans :-To ventilate the heat generated while braking.

* Why does hot air dry you hair faster than ambient air at same speed ?
Ans :-As air heats up relative humidity decreases, which increases the capacity of air to absorb more moisture.

* Why is it easier to pull a heavy object than to push it on a level ground?
Ans :-When you push there is one component of force that adds to the weight of the body and hence there is more friction.

* Define a fluid?
Ans :-Anything that flows??
Mechanical Version: A substance that continually deforms under an applied shear stress regardless of the magnitude of the applied stress. A fluid is almost in-compressible.

* What does it mean when you car gives out black smoke?
Ans :-The oxygen supply to the engine is less, there might be any block in the air intake system.

* Now! what if the smoke is white?
Ans :-You are burning oil in the engine. Maybe due to some engine leak.

And many more technically.