The best overall definition for force is the one you learned in elementary school a force is a push or pull. Formally, a force is a physical quantity that can affect the motion of an object.

Forces can be conservative (a force is conservative if the work it does on an object moving between two points is independent of the path the objects takes between the points). The work done by a conservative force depends only on the initial and final positions of the object, not how it gets between them. Gravity is a conservative force. Any potential energy function is associated with a conservative force.

Forces can also be nonconservative. A force is nonconservative if the work it does on an object depends on the path taken by the object between its final and starting points. These include kinetic friction, air resistance, and propulsive forces. Realistically, in almost any situation, both conservative and nonconservative forces act on an object at the same time. To calculate energy in real situations, you must be able to separate the work done by the conservative forces from the work done by the nonconservative forces.

Although a force or forces (because they don’t actually exist singly) can change the motion of an object, they don’t necessarily.

Balanced forces are those forces, acting on an object, whose sum is zero. Take any number of forces acting on an object, take their vector sum. If that sum = 0, there is no change in motion.

This is the essence of Newton’s 1st law of motion. It does not state that no forces act on an object; it simply states that when the sum of the forces acting on an object is zero, there is no change in motion (acceleration).

Note that because the angle of application is important in forces, they are vector quantities. Therefore, the sum of the forces acting on an object is the resultant of the vectors (forces).

### Question 1

A baseball of mass m is thrown upward with some initial speed. If air resistance is neglected, the force acting on the ball when it reaches its peak is
A. mg and upward
B. mg and downward
C. zero
D. none of these

### Question 2

You are talking by interplanetary phone to a friend who lives on the Moon. She tells you she has just won 1 N of gold in a contest. Excitedly, you tell her you won 1 N of gold in the Earth-version of the contest. Who is richer?
A. you
C. it’s a tie

Answer (B – On the moon, with its smaller gravitational force, it would take more mass to equal 1 N, therefore the person on the moon would have more gold and be richer, 6 times more!)

### Question 3

Picture a child being pulled on a sled by a rope. The sled is moving horizontally. The person pulling has the roped pulled at an upward angle. Is the magnitude of the normal force exerted by the ground on the sled…?
A. equal to the total weight of the child plus the sled
B. greater than the total weight
C. less than the total weight
D. possibly greater or less than the total weight depending on the size of the weight relative to the tension in the rope