Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Newton's First Law and objects in motion

  1. Nov 22, 2006 #1
    Im wondering what situations involving objects in motion , you could take advantage of Newton's first law of motion?

    I have one situtaion but im not sure of another one that could be considered taking advantage of Newton's law...
  2. jcsd
  3. Nov 22, 2006 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    What does Newton's 1st Law say about objects in motion? What situation did you come up with?
  4. Nov 22, 2006 #3
    Well off the top of my head Newton's first law of motion is
    " An Object in a constant state of motion, will remain in a constant state of motion unless acted upon by an external net force"

    I can easily think of example just not one that would be considered taking advantage of it...my example was cutting a log with an axe. You first hit the log lightly so it is stuck on the blade of the axe then you swing it down and when you hit a hard surface (A stump) the wood stops but the axe keeps moving through the log cutting it in half....
  5. Nov 22, 2006 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You're taking advantage of his 2nd law in your example, not his first. You are overcomplicating the 1st law, which you stated pretty well, which I have modified slightly in red above. Supposing a car is traveling on a level road at a steady speed of 60mph. It will forever stay at that speed in a straight line unless an unbalanced force acts on it. So when its moving steadily at a constant speed of 60mph, there must be no net force acting on it, that is, with your foot on the pedal, the driving force between the tires and the road is exactly counterbalanced by friction and air drag resistance forces. Now suppose you start up a hill without applying any more pressure to the pedal. Now there is an unbalanced force acting...gravity.. which will slow you down unless you give the car more gas by depressing the pedal further. If you do this, the car will then increase its speed again and will remain at constant speed as long as the now stronger driving force is exactly counterbalnced by the friction, air drag, and gravity forces. Now, supposing the car starts going downhill. Here's where you take advantage of Newton 1. Because gravity is acting downhill in your favor, the unbalanced gravity force will increase your speed, so now you can take your foot off the pedal and save on fuel by letting the gravity force, exactly counteracted by friction and air drag, keep you moving at constant speed, and further, you won't get a ticket for speeding!
  6. Nov 23, 2006 #5
    Newton's First law implicitly defines what a force is by describing what happens in the absence of forces.

    In the absence of forces and when viewed from an inertial reference frame an object at rest will remain at rest and an object in motion will maintain its motion at a constant velocity (constant speed along a straight line).

    Also, the first law implies the concept of inertial reference frames:

    If an object does not interact with any other objects, it is possible to identify a reference frame in which the object has zero acceleration (thus a constant velocity which may or may not be zero).

    Newton's second law tells us what happens when the net force is zero:

    When viewed from an inertial reference frame, the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.

    Examining the first and second laws closely, we see there is a slight difference. Although it is correct to say that in the absence of forces an object has a net force of zero, the converse is not true. It is highly possible for an object to have several forces acting on it and the net force be zero. Case in point: an object at rest on the earth's surface. Gravity and the normal force are acting on the object, but the net force is zero.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook