Understanding Forces

• Force is a push or pull.
• Balanced forces acting on an object do not change its motion.
• Unbalanced forces are unequal and cause a change in an object's motion.
• Friction opposes the motion of one object moving past another.
• Air resistance is the force of air pushing against a moving object.
• Gravity is a force of attraction that pulls all matter together.
• All masses attract each other, with the strength depending on mass and distance.
• Larger masses exert stronger gravitational forces.
• Smaller masses exert weaker gravitational forces.
• Mass is the amount of matter in an object and remains constant regardless of gravity.
• Weight is the force of gravity acting on an object.
• Weight changes with gravity; weighing less where gravity is lower.

Newton's First Law of Motion

• An object at rest stays at rest, and a moving object continues moving until acted upon by a force.
• Examples: A ball remains still until hit, a pencil stays on a desk until moved, feeling pushed back in a seat during airplane takeoff, flying forward when a car brakes suddenly, and the body shifting sideways when a car turns.

Newton's Second Law of Motion

• The greater the mass of an object, the more force is needed to move it.
• Examples: More force to move a bowling ball than a tennis ball, less force to push an empty wagon than a full one, less force to push a child on a swing than an adult, more force to push a full grocery cart than an empty one, and greater speed when multiple dogs pull a sled versus one dog.

Newton's Third Law of Motion

• For every action, there is an equal and opposite reaction.
• Examples: A fireman is knocked backward when turning on a hose, a balloon flies around when air is released, a swimmer moves forward by pushing back on the water, a bird moves forward by pushing down on the air, and a cannon is pushed backward when a cannonball is shot forward.