Forces and Motion
Balanced and Unbalanced Forces Net Force= the overall force acting on an object when all of the individual forces are added together.
Balanced Force= When all forces acting on an object added up equals zero. An object that is being acted upon by a balanced force is undergoing no change in motion. Examples: 1) A ball sitting still on the floor. 2) A ball rolling in a straight line at a constant speed.
Unbalanced force= when all the forces acting on an object are added up and the sum is something other than zero. An object experiencing an unbalanced force will be changing motion, so it would either be changing speed, direction or both. Example 1: An orange is sitting on a table. Balanced or Unbalanced?
Example 2: A car has a net force that equals 0. Balanced or Unbalanced? Example 3: A box has the following forces acting on it: 5 N to the right, 5 N to the left, 8 N straight down, and 8 N straight up. Balanced or Unbalanced? A car is going around a curve at a constant 55 Km/h. Balanced or Unbalanced?
Normal force= the force exerted by an object to balance an outside force acting on it. Example: When you sit on a chair, the chair exerts a normal force on your body that is equal to the force of gravity pulling your mass toward the Earth.
Contact and Noncontact Forces Contact force= must touch an object to change its speed or direction. Friction is an example of a contact force. Noncontact forces= can act on objects from a distance. Gravity and Magnetism are the two most common noncontact forces.
Gravity= a force that pulls two objects with mass toward each other. The size of the gravitational force depends on two factors: 1) the masses of the objects and 2) the distance between the objects. Weight is the force that is applied to an object due to gravity.
Mass or Inertia and Newton's First Law
Force, Velocity and Acceleration, Vector Quantities in Newton's Laws
- Newton's First Law is the law of inertia. An object with no net forces acting on it which is initially at rest will remain at rest. If it is moving, it will continue to move in a straight line with constant velocity.
- Forces are "pushes" or "pulls" on the object, and forces, like velocity and acceleration are vector quantities.
- Vectors have magnitude and direction.
- Adding two forces gives a resultant or net force represented by a third vector. The rule for adding two vectors is to put the tip of the first vector on the tail of the second vector to form the third vector which extends from the tail of the first vector to the tip of the second. (See Figure 2-8 and animation of vector addition in Explore Science.) Click on Adding Vectors. Adding Vectors
- Now we come to the case when the net force on an object is not zero. Newton's Second Law states: F = ma. F is the net force acting on an object. m is its mass. a is its acceleration.
- What is the relationship between mass and weight based on Newton's Second Law? Mass is a property of an object. It depends on the amount of "stuff" in it. But weight is a force that depends on the strength of gravity. That is why objects weigh about 1/6 as much on the moon as they do on earth. On earth, the weight of an object is given by W = Mg, where M is its mass and g is the acceleration due to earth's gravity. Newton' second law says that object of mass M experiences a force W (its weight) and a downward acceleration g.
A Closer Look at Newton's Second Law
- Newton's Second Law, F = ma is a vector equation. It says that the net force (a vector) acting on a mass m (a number) causes an acceleration (a vector) of the object in the same direction as the net force.
- The net force is the combined force of all individual forces acting on an object.
- Newton's First Law can be seen to be the special case in the Second Law when F, the net force, is zero. When that happens, the acceleration a must also be zero. Since acceleration is given by the change of velocity divided by elapsed time, the velocity doesn't change.
Summary: Newton's First and Second Laws of Motion
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Newton's First Law:
If no net force acts on an object it remains at rest, if initially at rest, or it maintains its velocity if initially in motion. -
Newton's Second Law:
F = ma:
The net force F acting on an object with mass m and acceleration a is given by this expression. - Newton's first law is a special case of Newton's second law when F = 0. Since m is not zero, acceleration must be zero. Hence, the velocity must remain contant. An object at rest is one with zero velocity.
Mass and Weight
- Weight of object W is the force exerted ON the object by earth's gravitational field.
- Apply Newton's Second Law to Free Falling Object (neglect air resistance): W = mg, where W = F and g = a.
- Force and acceleration are vectors, so W and a are vectors and they must point in the same direction.
R.S. Panvini
12/27/2003