i read this everywhere and obviously it's correct.

BUT,

does that mean a very heavy truck travelling at a constant speed has no force? i.e. this truck has no acceleration, therefore, zero force.

i also think people often think of 2nd derivative of position (acceleration) and velocity-squared as the same think....... but they aren't, are they?

this is a serious question i've wondered about for awhile.

thx in advance

If it hits something, it will decelerate and exert a force on it. A stationary object it hits will accelerate and exert a force on it.

Which people?

Been a long time since I've done any physics, but I think I remember this stuff.

An "object" doesn't have a force. A force acts on the object. If a truck is moving at a constant speed, the net forces acting on the truck are zero. So the forces coming from the engine spinning the tires minus the frictional forces from the road and air equal zero.

A heavy truck traveling at a constant speed has kinetic energy, however. An object in motion can "possess" kinetic energy, which is different than force.

Nope. Derivatives and exponentiation are different operations.

Acceleration is by definition a change in velocity and so if velocity is constant acceleration is zero.

The difficulty is that language, in some manner, sees "force" as any incitement to crisis . For example the "force" of his punch, the force of that constant velocity 100 mph train hitting the sitting car, "he was forced to obey the rules".....

In the vernacular "force" has a broader reach and is more creatively malleable than the scientific term which limits it to a change from a constant state.

Objects don't "have" forces. Forces are applied to objects and not properties (or quantities?) of objects. You might say that the truck is not accelerating, so that there is no net force being applied to it.

The units of acceleration: m/s^2
The units of velocity-squared: m^2/s^2

These cannot represent the same quantity.

What everyone said above is correct. There is no acceleration so the net force on the truck is 0. It would make more sense to talk about its momentum or kinetic energy instead.

True enough, the truck has no "net" force at the moment, but it does possess a s_ _t- ton of momentum and you could use F=MA to figure out the force that was imparted on you when he runs into you and you go from standing still to flying 60 miles an hour in a split second.

Newton's third law, though. Good question.

The truck will NEVER "have" a net force. The truck never "has" a force, period. It can "impart" a force on another object. But the truck never actually "has" that force.

Interestingly, you probably couldn't use F=ma to determine the force imparted from the truck as you wouldn't remain in one piece .

Kinetic energy is the relevant concept here. The amount of "damage" done to an object in a collision is proportional to v^2, not v.

F=ma=m*dp/dt is actually an instantaneous acceleration; so to work backwards to find the force you would need to know the final velocity of the object and how long it was accelerating.

If you really wanted to know the force imparted on the object by the truck when it hits something, the object would have to remain in one piece (edit: I guess technically it wouldn't need to remain in one piece, you could sum the momentum vectors of all the parts but it gets more complicated), and you'd need to measure the velocity of the object(s) after collision and know how long the truck/object remained in contact. Then integrate the force over that time interval. Then you could find the average force imparted.

You would also need to know whether the collision was elastic or inelastic. My intuition says that it's probably inelastic (you're stuck to the grill) and that when we think of people flying when they're hit by a vehicle it's because the vehicle is attempting to stop. But I could be wrong.

Integration seems unnecessary if you're trying to find the average force because you should just be able to use Delta v/Delta t.