Force Fundamentals
This kind of quantity has both size and direction.
A vector
This sketch shows only the forces acting on one chosen object.
A free-body diagram
This is an object’s resistance to a change in motion.
Inertia
This law says acceleration depends on net force and mass.
Newton’s second law
This is the gravitational force on an object.
Weight
Gravity is this type of force, because it can act without touching.
A field force
This is the vector sum of all forces on an object.
Net force
This quantity measures inertia.
Mass
If A pushes B, B pushes A with a force that is equal and this.
Opposite
“Normal” in normal force means this relation to the surface.
Perpendicular
Friction and tension are this type of force, because the objects must touch.
A contact force
An object is in this state when the net force on it is zero.
Equilibrium
A coffee cup on a tray keeps sliding when you jerk the tray backward. The cup does that because of this.
Inertia
You push a wall and the wall pushes you. Those two pushes never cancel on one diagram because each push acts on this.
Different objects
This kind of friction acts on an object that is not yet sliding.
Static friction
The newton is this combination of kilograms, meters, and seconds.
1 kg·m/s²
A hockey puck glides in a straight line at steady speed on ice. The vector sum of all forces on it is this.
Zero
An object can keep a constant velocity when the net force has this value.
Zero
Earth pulls the Moon, and the Moon pulls Earth; that pair shows the third law also applies to these forces.
Field forces
After sliding starts, this kind of friction is usually smaller than the maximum static friction.
Kinetic friction
Everyday pushes such as friction and the normal force are, at the deepest level, due to this fundamental force.
The electromagnetic force
The force a table exerts straight up on a book resting on it.
The normal force
Taking an object to the Moon changes its weight but not this.
Mass
The partner of “Earth pulls down on a book” is not the table’s normal force; it is this.
The book pulling up on Earth
If you stack a second identical box on the first, this force roughly doubles, so friction is harder to overcome.
The normal force