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- The Nature of FORCES I. WHAT IS A FORCE? a. A force is ___________________ a push or a pull b. A force is described by: its strength 1.___________________ the direction 2.___________________ c. The strength of a force is measured in a unit called ____________ NEWTON [SI unit = ______ N ] “You exert about 1 newton of force when you lift a small lemon” 2 I. WHAT IS A FORCE? d. Often, more than a single force acts on an object at a time. The combination of all forces acting on an object is called the ____________. net force e. When forces act on an object from the same “add” the forces together direction, _____________________________. 1N 6N 5N NET FORCE f. The object will always move in the direction ____________________. of the larger force 3 I. WHAT IS A FORCE?: g. When forces act on an object from subtract the forces “opposite” directions ____________________. 5N 10 N 5N h. If the opposing forces are of equal strength ________________________________________ there is “no net force” and no change in the object’s motion (forces cancel out) _______________________________________ 4 II. TIPPING THE BALANCE 1. Unbalanced forces cause an object to: start moving a. _________________ stop moving b._________________ c._________________ change direction 2. Unbalanced forces acting on an object results in a net ___________ force and cause a change in ______________________. the object’s motion 5 II. TIPPING THE BALANCE 3. When forces are exerted on an object, the object’s motion does not always change. 4. Balanced forces: are equal forces acting on one object in a. _________________________________ opposite directions _________________________________ do not change the object’s motion b. _________________________________ 5. Write 3 examples of balanced forces: a cup resting on a table 1. _________________________________ a truck moving at constant speed 2. _________________________________ arm wrestling or “tug of war” 3. _________________________________ 6 II. TIPPING THE BALANCE Practice: Two tug boats are TOWING a submarine into port. Tugboat A exerts a force of 2000 newtons on the submarine. Tugboat B exerts a force of 4000 newtons in the opposite direction. Tugboat A 2000 N • Tugboat B 4000 N Draw arrows showing the individual and combined forces of the tugboats. B) 1. What is the combined force on the barge? 2000 N (tugboat 7 2. Is this is a balanced or unbalanced force? unbalanced III. THE FORCE OF GRAVITY a. The force of GRAVITY 1. Gravity is the force that pulls objects to Earth ________________________ a. Two factors that affect gravity are: mass of object 1. ______________________ distance 2. ______________________ between all objects 2. The force of gravity acts _______________________ increases 3. If mass increases, the force of gravity ____________ 4. If distance increases, the force of gravity Decreases ____________ 8 IV. WEIGHT 1. WEIGHT is A measure of the gravitational force ____________________________________ exerted on the an object. ___________________________________ 2. ________varies with the strength of the WEIGHT gravitational force, but ______ MASS does not. WEIGHT = mass x gravity or [W=mg] 9 V. OTHER FORCES 1. Elastic force: Force exerted by stretching ___________________________ ___________________________ certain materials 2. Buoyancy: Upward force by H20 on an ___________________________ ___________________________ object 10 V. OTHER FORCES 3. Magnetism: ___________________________ Objects exert an attractive or repulsive forces on each ___________________________ other ___________________________ 4. Static Electric: amount of electric charge on ___________________________ ___________________________ the surface of an object 11 *** Let’s Review *** $ What is the difference between mass and weight? $ Explain the difference between a balanced and unbalanced force? 10 N = ? $ 5N $ Will an object move if the net force acting on an object are unbalanced? 12 VI. FIGHTING FRICTION a. Friction: The force that 2 surfaces exert on each other when ____________________________________________ ____________________________________________ they move against each other b. The strength of friction depends on two factors: how hard surfaces push together 1. ___________________________________ the types of surfaces involved 2. ___________________________________ c. Friction acts in a direction Opposite to the direction of object’s motion ___________________________________________ It must overcome friction d. For an object to move __________________________ e. Without friction ____________________________________________ A moving object will not stop until it strikes another ____________________________________________ object 13 VII. TYPES OF FRICTION a. STATIC friction: acts on objects that are ___________________________ not moving ___________________________ b. SLIDING friction: friction between 2 solid ___________________________ ___________________________ surfaces sliding past one ___________________________ another 14 VII. TYPES OF FRICTION c. ROLLING friction: the resistance that occurs 1.____________________________ when a round object such as a ___________________________ ball or tire rolls on a flat surface ___________________________ is easier to overcome than 2.____________________________ __________________________ sliding friction d. FLUID friction: friction between a solid object 1._____________________________ and a fluid ____________________ 2. 3 types of fluid friction include • ____________ Water (H20) • ____________ oil air • ____________ 15 VII. TYPES OF FRICTION PRACTICE: 1. To actually move an object, you must use a force that is greater than the force of _________ STATIC friction. FLUID friction. 2. Air resistance is a type of ________ Name 3 examples of each type of friction below: SLIDING ______________ ______________ ______________ ROLLING ______________ ______________ ______________ FLUID _______________ _______________ _______________ 16 *** Let’s Review *** $ Which kind of friction keeps your shoes from slipping on the floor? $ [T/F] To move an object, you must use a force that is less than the force of static friction? $ What would happen if there was no friction? $ How would you describe the texture of a surface that produces a lot of friction? 17 VIII. NEWTON’S LAWS OF MOTION a. Newton’s FIRST law: (INERTIA) An object at rest will stay at rest a. __________________________ An object in motion will stay in b. __________________________ motion unless acted upon by an __________________________ force __________________________ b. INERTIA a. _____________________________ The tendency of an object to _____________________________ resist a change in motion b. The greater the _____of an mass object the greater the inertia. 18 VIII. NEWTON’S LAWS OF MOTION c. Newton’s SECOND law: (ACCELERATION) a. Is the rate in which an object speeds UP or DOWN b. acceleration depends on the object’s______ mass and on the ______ force acting on the object. m/s2 c. Units for acceleration = ______ Net force Acceleration = -----------------PRACTICE mass A speedboat pulls a 55 kg water 1. Fnet = acceleration x mass skier. The force causes the skier 2) (55 kg) 2. Fnet = (2.0 m/s to accelerate at 2.0 m/s2. Calculate 3. Fnet = 110 kg * m/s2 the net force that causes this 19 acceleration. Fnet = 110 N VIII: NEWTON’S LAWS OF MOTION c. Newton’s THIRD law: equal opposite 1. For every action there is an ______but _________ reaction. Normal Force Force of Gravity WILL NOT MOVE 20 *** Let’s Review *** $ Explain Newton’s First Law of Motion $ [T/F] The greater the mass the greater the inertia $ What is the formula for acceleration? Name the forces acting on the $ Jeep? 21