6th Grade Science-Chapter 9 Notes

Chapter 9, Lesson 1 Notes

*A force is a push or pull.  When one object pushes or pulls another object, the first object exerts a force on the second object.

*You exert a force on a chair when you pull it away from a table.  Like velocity and acceleration, a force is described by its strength and by the direction in which it acts.

*Pushing left is a different force from pushing right.  The direction and strength of a force can be represented by an arrow.  The arrow points in the direction of the force.  The length of the arrow tells you the strength of the force.  The strength of a force is measured in an SI unit called the newton (N).

*Every day you are using forces to act on objects.  The two main types of forces are contact forces and forces that act at a distance.  Contact forces include applied forces, the normal force, and friction.  Electrical force, magnetic force, and gravitational force act at a distance.

*To feel the effects of contact forces, an object must touch another object.  An applied force is a force that is put on an object by another object.  The normal force is the force that acts between objects when they are in contact with each other.  Normal here means perpendicular.  This force acts perpendicular to the surface of contact such as the wall that pushes back and supports you when you lean against it.

*The force that two surfaces exert on each other when they rub against each other is called friction.  Friction acts in a direction opposite to the direction of the object’s motion.  Two factors that affect the force of friction are the types of surfaces involved and how hard the surfaces are pushed together.

*Some forces push or pull an object without touching the object.  The force between two charged objects is called electrical force.  Atoms contain charged particles called electrons and protons.  If a proton, which carries a positive charge, is near an electron, which carries a negative charge, they attract one another.  The attraction or repulsion between magnetic poles is magnetic force.  Gravitational force, or gravity, is a force that pulls objects toward each other.

*The law of universal gravitation states that the force of gravity acts between all objects in the universe that have mass.  So, any two objects in the universe that have mass attract each other.  For example, you and your pencil are attracted to each other.  However, you do not notice the attraction between such small objects as you and your pencil because these forces are extremely small compared to the force of Earth’s attraction.  You observe only the effects of the strongest gravitational forces.

*Two forces affect the gravitational attraction between objects: mass and distance.  The more mass an object has, the greater its gravitational force.  The shorter the distance is between two objects, the stronger the gravitational force between the objects.

*Weight is a measure of the force of gravity on an object.  Mass is a measure of the amount of matter in an object.  Mass is the same on Earth as it would be on any other planet.  But weight would vary on each planet, since each planet’s gravitational force is different.

Chapter 9, Lesson 2 Notes

*Often more than one force acts on an object at the same time.  The combination of all the forces on an object is called the net force.  It determines if and how an object will accelerate.  If all the forces acting on an object are balanced, the object’s motion will not change.  When the sum of all forces acting on an object is unbalanced, the object’s motion will change.

*You can find the net force on an object by adding together the strengths of all the individual forces on the object.  When the total is a nonzero number, the forces are said to be unbalanced.  When the total is 0, the forces are balanced.  Balanced forces do not change the motion of an object.

*When the forces on an object act in opposite directions, you find the strength of the net force by subtracting the strength of the smaller force from the strength of the larger force.  This net force is in the same direction as the larger force.

Chapter 9, Lesson 3 Notes

*If an object is not moving, it will not start moving until a force acts on it.  If an object is moving, it will continue at a constant velocity until a force acts to change its speed or direction.

*Newton’s first law of motion states that an object at rest will remain at rest unless acted upon by a nonzero net force.  An object moving at a constant velocity will continue moving at a constant velocity unless acted upon by a nonzero net force.

*All objects resist changes in motion.  Resistance to change in motion is called inertia.  The greater the mass of an object, the greater its inertia, and the greater the force required to change its motion.

*Newton’s second law of motion states that an object’s acceleration depends on its mass and on the net force acting on it.  This relationship can be written as acceleration= net force/mass.

*The formula can be rearranged to show how much force must be applied to an object to get it to accelerate at a certain rate.  Net force= mass x acceleration.

*Acceleration is measured in meters per second per second (m/s2).  Mass is measured in kilograms (kg).

*Newton’s second law shows that force is measured in kilograms times meters per second per second (kg.m/ss).  This unit is also called the newton (N), which is the SI unit of force.

*Newton’s third law of motion states that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object.

*Another way to state Newton’s third law is that for every action there is an equal but opposite reaction.  Action and reaction forces do not necessarily cancel out because they may act on different objects.