18 votes

Where does "excess" energy go when a vehicle uses a lower gear to move at a given speed?

the amount of energy used per engine revolution is fixed (not precisely true I know) This is the error. The work per engine revolution is the torque at the engine. If the energy dissipation per wheel ...
Dale's user avatar
  • 100k
10 votes

Where does "excess" energy go when a vehicle uses a lower gear to move at a given speed?

Ignoring all losses from the gearbox on to the wheels, let’s just look at the engine. Internal combustion engines are about 30% efficient. About 65% of the energy is lost as heat, 5% as friction. ...
Rich's user avatar
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10 votes

Where does "excess" energy go when a vehicle uses a lower gear to move at a given speed?

If you are in a situation where the energy used to keep the car moving is unchanged, but you are putting more energy into the engine, then the difference must be losses in the engine. Whether you are ...
Cort Ammon's user avatar
8 votes

Why cars don't accelerate according to $F = ma$?

The car experiences not only the force made by the engine. There are also other forces: friction and air resistance. The air resistance force is proportional to $v^2$, hence it becomes more and more ...
Thomas Fritsch's user avatar
6 votes

Force applied to sphere on a smooth surface

lets write the equations $$ M\dot v=F\\ I_C\dot\omega=F\,\rho$$ where $~\rho~$ the distance from the center of mass to the force $~F~$ the velocity at the contact point $~v_c~$ is $$ v_c=\omega\,r-v \...
Eli's user avatar
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5 votes

Why do objects with more mass fall at a faster rate (factoring in air resistance)?

When falling, the force on an object is its weight $mg$ (mass $m$, gravitational acceleration $g$) and air resistance $F_\mathrm{drag}$, if any. The former acts downward, and the latter, a function of ...
Chemomechanics's user avatar
5 votes

Energy loss due to friction in Rolling Motion

Rather, the answers everywhere seem to indicate that NO energy is lost due to friction when rolling without slipping. Energy can be lost in the form of rolling resistance, sometimes called rolling ...
Bob D's user avatar
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4 votes

Can't Newton's Third Law be proved from the other two?

In obtaining your first equation, you have used Newton's 3rd law implicitly, so your argument is circular. Newton's 2nd law is a statement about the motion of individual particles. On its own, it has ...
d_b's user avatar
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3 votes

Why cars don't accelerate according to $F = ma$?

The motive force produced by the engine is not constant because of limitations in the mechanical design of engines. Each time a car goes up a gear, it trades some of the motive force for allowable ...
John Alexiou's user avatar
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3 votes

Energy loss due to friction in Rolling Motion

Consider the energy of a rolling body, $$KE(t) = KE(t_0) - Energy\ lost\ to\ friction$$ If the energy lost to friction is zero, the kinetic energy is constant. In a freely rolling body the rotational ...
Rich's user avatar
  • 829
3 votes

Can't Newton's Third Law be proved from the other two?

Your argument works through one case where you can demonstrate that the forces must be equal. Newton's law states that all cases must satisfy this property. Consider a case where the objects do not ...
Cort Ammon's user avatar
2 votes
Accepted

How this equation arises in free body diagram of rolling motion?

You can move a force to another point parallel to its original line of action as long as you add a force couple to account for the torque it applied at its original point of application. That gives ...
Bob D's user avatar
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2 votes

Where does "excess" energy go when a vehicle uses a lower gear to move at a given speed?

No matter what causes them, energy "losses" inside an engine go directly to heat. This makes the engine warmer, making to coolant warmer trying to not overheat the engine.
StanS's user avatar
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2 votes

Where does "excess" energy go when a vehicle uses a lower gear to move at a given speed?

At the same vehicle speed, a lower gear requires more revolutions but less force — you don't need to depress the gas pedal all the way in the lower gear but may have to in a higher gear. But in the ...
Peter - Reinstate Monica's user avatar
2 votes

Wrong concept in rolling question

There is one horizontal force, $F$, acting on the sphere. Applying two equal magnitude, $F$, and opposite direction to each other forces, $F_1$ and $F_2$, at the centre of mass $C$ results in a force ...
Farcher's user avatar
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2 votes
Accepted

Understanding conservation of angular momentum in relation with rotating objects

There is most certainly an internal centripetal force within the material keeping it in circular motion. In real objects, this is provided by the intermolecular/interatomic forces, which are never ...
Vincent Thacker's user avatar
2 votes

Average Speed in one half of an elliptical orbit

If you have Mathematica software available, you can get the circumference of an ellipse easily in closed form via the 'Complete Elliptic Integral of the Second Kind' (EllipticE in Mathematica) . I ...
Thomas's user avatar
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1 vote

Physics Principle of Balancing Board

Whilst doing exercise, I found that having my two feet as close together as possible actually made me more stable. The opposite for me as I do not want the line of action of my weight from my centre ...
Farcher's user avatar
  • 96.2k
1 vote

What happens if an object hits stationary objects on a pulley?

In a collision, if two objects stick together kinetic energy cannot be the same before and after the collision. This is why it is called a perfectly inelastic collision. There is some force that does ...
BioPhysicist's user avatar
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1 vote
Accepted

Why need the extra definitions for rotational motion?

The fundamental reason is that momentum is not simply a vector (a quantity with direction and magnitude) but also has a line of action. A force, which is the rate of change of momentum, likewise. So ...
Nullius in Verba's user avatar
1 vote

System of rigid bodies: is the net work of some kinds of internal forces always 0?

I'm trying to understand under what circumstances we can say that the internal forces do no net work on the system. Internal forces of a system are those between parts of a system. These forces do ...
Bob D's user avatar
  • 71.8k
1 vote

Understanding conservation of angular momentum in relation with rotating objects

There is no centripetal force acting on the ball that keeps it in circular motion This is not correct. Each part of the ball is held in place because it is connected to the surrounding parts of the ...
gandalf61's user avatar
  • 53.2k
1 vote

Volume change of a deformable cylinder with a uniform spinning angular velocity

Materials are deformable volumetrically, so that, even if the cylinder is infinitely long (so that the axial strain is zero), it can still deform radially, and its cross sectional area and volume can ...
Chet Miller's user avatar
  • 33.6k
1 vote

Volume change of a deformable cylinder with a uniform spinning angular velocity

In theory this cylinder shouldn't change its cross sectional area or volume as it spins with a uniform angular velocity. That's not correct. The material of the cylinder is in tension along its ...
Bob D's user avatar
  • 71.8k
1 vote
Accepted

Could two planets trapped between two stars orbit each other?

What you are asking is essentially whether a double planet can have its barycenter at a Lagrange point of a binary star system. This is definitely true. As long as the pair can be well approximated as ...
Anders Sandberg's user avatar

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