Physical Science
Chapter 3 Notes
Section 1: Forces
Force – A push or a pull that one body exerts on another. A force is required to start or change
motion.
• Force is measured in newtons.
• One newton is the force required to move to a 1 kg mass to accelerate 1 meter per
second each second.
Net force – the sum of all of the forces acting on an object
Balanced forces - forces on an object that are equal in size and opposite in direction
Friction - the force that opposes motion between surfaces. The amount of friction between two
surfaces depends on how hard the surfaces are forced together, the materials that make up the
surfaces, and the smoothness of the surfaces.
• Static friction - the friction between two surfaces that are not moving past each other
• Sliding friction - the force that opposes the motion of two surfaces sliding past each other
• Rolling friction - the friction between a rolling object and the surface it rolls on
Friction exists with liquids and gases as well as with solids. Friction between a solid surface and a
liquid or gas is called drag.
Gravity – an attractive force between any two objects that depends on the masses of the objects
and the distance between them
• The force we gravity is described by the universal law of gravitation. This law states that
the strength of an object’s gravitational force depends on its mass; the more mass an object
has the greater the gravitational force it exerts on other objects.
• The force of gravity between two objects depends on two things:
• The mass of both objects—the bigger the mass, the bigger the force
• The distance between the objects—the greater the distance the weaker the gravitational
force that pulls them together.
Weight – The gravitational force exerted on an object.
• Because weight is a force it is measured in newtons.
• Equation for weight:
• IMPORTANT: On Earth g = 9.8m/s2, and is referred to as the acceleration due to gravity
Weight = mass x gravity, or: W = mg
Section 2: Newton's Laws of Motion
Newton’s First Law of Motion – an object moves at a constant velocity unless an unbalanced
force acts on the object
• Inertia – the tendency of an object to resist any change in its motion
Newton's Second Law of Motion – describes how force, mass, and acceleration are connected.
• The net force acting on an object causes the object to accelerate in the direction of the net
force.
• The equation for Newton’s 2nd Law:
acceleration =
•
net force
F
, or: α =
mass
m
Force is measured in newtons, mass is measured in kilograms, and acceleration in m/s2
Newton's Third Law of Motion - when an object exerts a force on a second object, t he second
object exerts a force on the first that is equal in size and opposite in direction.
Section 3: Using Newton’s Laws
Air resistance – friction-like force that opposes the motion of objects that move through the air
Terminal velocity – the maximum speed an object will reach when falling through a substance
such as air.
• When the air resistance of a falling object and the force of gravity are balanced and the object
no longer accelerates.
Free fall – occurs when gravity is the only force acting on an object
• When in space astronauts are not weightless in that there they are still gravitationally bound
to Earth. The astronauts appear weightless because they are in a state of free fall
Centripetal force - an unbalanced force causing an object to accelerate toward the center of a
curved path
Momentum – a property of a moving object due to its mass and velocity.
• The momentum of an object is the product of its mass and velocity.
• The equation for momentum:
momentum= mass x velocity, or: ρ=mv
•
•
Where: p = momentum, m = mass, and v = velocity
The Law of Conservation of Momentum: If no external forces act on a group of objects,
their total momentum does not change