I was kind of bothered by this. Newtons first law states that Force is equaLs to mass multiplied with acceleration. Tomove an object you need to exert force. Assume that we have a car moving constantly at the speed of 60mph. Since I said constant, then it means there would be no acceleration right? Since there's no acceleration, 0 m/s2 multiplied by the mass of the car would mean that there's no force or F = 0, right? Then, why would a car going at constant 60mph would hit a man and would send the man flying? The speed was constant, and there was no acceleration, which means no force, but why did the car move the man when in fact you need force to move something. Does this mean that I proved that Newton's law is quite wrong?
2007-09-18 21:48:36 · 11 answers · asked by love_doctor 4 in Science & Mathematics ➔ Physics
No, it means you don't understand Newton's 2nd law.
2007-09-18 21:55:13 · answer #1 · answered by bestonnet_00 7 · 7⤊ 0⤋
Force equals mass time acceleration is the Newton's second law.
Once again read Newton's first law.
It states that any object will be at rest unless a force acts on it.
Or you can say till a force acts on it will be at rest.
Similarly Newton's First law states also about objects in uniform velocity.
An object which some how starts to move with some velocity then it will be in uniform motion till a force acts on it.
If no force acts on it will continue to move with the same velocity.
Hence for constant velocity as you have stated the force which is mass time acceleration is zero. If an object moves with constant speed then again its acceleration is zero and hence the force is zero.
If you a push a plate on a dining table it will move through some distance and will stop.
The distance it moves depends upon the smoothness of the table. Or in other words if friction is less then it will move through a longer distance.
If one can make a perfectly friction less table one you push the plate it will move with constant speed it will never stop unless a force acts on it. But in every day life we cannot see such a material and friction is always present.
If we apply a force which is equal but opposite tothe frictional force then the object will move with constant speed and the net force acting on it zero.
Hence we can read Newton's law as, "an object will move with constant speed or velocity if the net force acting on it is zero.'
2007-09-19 02:32:31 · answer #2 · answered by Pearlsawme 7 · 0⤊ 0⤋
Not at all. The car travelling with constant velocity has no acceleration but it does have MOMENTUM given by mass * velocity.
In order to stop the car a Force must be applied to it, i.e. the brakes or it hits another object.
In the example of a car, if you took your foot off the accelerator, the car would slow down - why? this is due to friction from the road on the tyres, air resistance and other anomalies that are associated with energy transfer. Therefore with a car, you are always applying a force from tyhe engine to maintain your constant speed to overcome these outside forces.
Newton is correct with his F = ma equation for classical mechanics, it is only when we deal with very fast moving objects and extremely small objects that these laws do not work, then we must turn to Einsteinian equations of motion along with reletivity and Quantum mechanics.
2007-09-18 21:58:46 · answer #3 · answered by Doctor Q 6 · 3⤊ 0⤋
The car traveling at a constant 60 mph has no acceleration, and therefore not NET force acting on it.
That doesn't mean that no force will EVER act on it.
When the car hits the pedestrian, the car briefly exerts a force on the pedestrian and sends him flying. At the same time, because of Newton's Third Law, the pedestrian briefly exerts an equal and opposite force on the car, causing it to slow down slightly.
2007-09-19 00:46:08 · answer #4 · answered by ZikZak 6 · 1⤊ 1⤋
no. and by the way, that was Newton's second law. Newton's first law is about inertia. anyway, although a car running at a constant speed of 60mph can have no force according to Newtons second law, there is what we call conservation of mechanical energy and conservation of momentum. you see, the car has a momentum. imagine this, a billiard ball hit with a constant velocity would hit another billiard ball at rest. it's because of momentum. check the links below. or you may consult your physics book for further explanations.
2007-09-18 21:59:15 · answer #5 · answered by maan 2 · 2⤊ 0⤋
And I hope you know there is a third law too.
These three laws hold to a good approximation for macroscopic objects under everyday conditions. However, Newton's laws (combined with Universal Gravitation and Classical Electrodynamics) are inappropriate for use in certain circumstances, most notably at very small scales, very high speeds (in special relativity, the Lorentz factor must be included in the expression for momentum along with rest mass and velocity) or very strong gravitational fields.
2007-09-18 21:59:32 · answer #6 · answered by Silver Surfer 2 · 2⤊ 0⤋
First of all this is the second law of motion.
a constant force is required to keep the vehicle moving at 60 kmph, because if the the force is removed then the force of friction will overcome and the car will stop at last.so,the force is there but to balance the force of friction.If there is no frictional force,the object will have a force of zero to move the body,and it will make a body move because it has some momemtum due to its velocity.
2007-09-19 00:00:42 · answer #7 · answered by King HVJ 2 · 0⤊ 0⤋
the F is the total of the forces, including the frication force between the ground and the car, the pull of the enginerring. and the result is that the frication force is equal to pull and reverse direction. so the sum of the force is zero.
the F is a vector
2007-09-18 22:34:09 · answer #8 · answered by yelena 1 · 0⤊ 0⤋
lol not a pro at this.. but i believe like this
there is no such thing as a constantly moving object for this theory.. F=ma .. Acceleration will be 60.. then ... because the car couldnt have been moving from the start 60mph.... I know what your thinking.. it's kinda wierd if you think it that way.. But if you see all the moving things move(accelerate) because of something else that makes it accelerate meaning that there was time when they did not move.. so when they move it becomes acceleration
2007-09-18 21:56:25 · answer #9 · answered by Anonymous · 0⤊ 5⤋
it is an empirical equation, derived from and examined by employing experimental documents basically, no mathematical derivation ought to ever instruct it. As such the only evidence i'm going to offer is the orbits of the planets, the action of your automobile once you hit the gas, airplanes flying, wind blowing, birds chirping etc.
2016-10-09 11:01:25 · answer #10 · answered by zaheer 4 · 0⤊ 0⤋