*Note: Type-setting limitations do not allow for arrows on the vector labels, so we have used boldface only.Concepts and Skills to Review:
Scientific notation and significant figures (Section 1.4)
Units (Section 1.5)
Problem-solving techniques (Section 1.7)
Summary
Newton's laws of motion:
If no forces act on an object, then its state of motion does not change. If at rest, it remains at rest; if moving, it continues to move in a straight line with constant speed.
A non-zero net force acting on an object causes its state of motion to change.
In an interaction between two objects, the forces that each exerts on the other are equal in magnitude and opposite in direction.
If the net external force acting on a body is zero, the body is in equilibrium. A body in equilibrium moves at a constant speed along a straight line. (The speed can be zero, in which case the body is at rest.)
At the fundamental level, there are four interactions: gravitation, the strong and weak nuclear interactions, and the electromagnetic interaction. Contact forces are large-scale manifestations of many microscopic electromagnetic interactions.
Gravity and electromagnetic forces are the only long-range forces. All other forces (in a macroscopic view) are contact forces.
The SI unit of force is the newton. 1.00 N = 0.2248 lb. 1 N = 1 kg·m/s2
Force is a vector quantity—it has a direction as well as a magnitude. Vectors have their own rules for addition.
The net force is the vector sum of all the forces acting on a body. Since all the internal forces come in third-law pairs, we need only sum the external forces.
A free-body diagram (FBD) includes vector arrows representing every force acting on the chosen object, but no forces acting on other objects.
The magnitude of the gravitational force between two objects is
The weight of an object near Earth's surface is the gravitational force acting on the object due to the Earth. An object's weight is proportional to its mass: W = mg. Near Earth's surface, g = 9.8 N/kg downward.
The magnitude of the electric force between two charged particles is
The normal force is a contact force perpendicular to the contact surfaces that pushes each object away from the other.
Friction is a contact force parallel to the contact surfaces. In a simplified model, the kinetic frictional force and the maximum static frictional force are proportional to the normal force acting between the same contact surfaces.
fs≤ μsN fk = μkN.
The static frictional force acts in the direction that tends to keep the surfaces from beginning to slide. The direction of the kinetic frictional force is in the direction that would tend to make the sliding stop.
Many springs obey Hooke's law as long as they are not stretched or compressed too far:
F = kx
Cords and pulleys can be used to change the direction of a force without changing its magnitude.
