& ForcesForces

Isaac Newton Isaac Newton (1642–1727) Isaac Newton proposed that the tendency of an object was to maintain in its current state of motion.

“Law of Inertia” An object at rest remains at rest, and an object in motion continues in motion at a constant speed in a straight line, unless acted on by a nonzero net force (unbalanced force). * OR * The velocity of an object remains constant unless acted on by an unbalanced force.

Objects tend to stay at rest OR object tend to stay in motion at a constant speed.

Zero Net Force the condition of an object that is not accelerating If an object is at rest or moving at constant speed, there will be no net force acting on the object. All forces will be balanced.

INERTIA the tendency of an object to resist any change in its motion Inertia is a property of matter and does not depend on the position or location of the object. MASS (kg) a quantitative measure of inertia The greater the mass the greater the inertia

Examples of Newton’s 1 st Law a) car suddenly stops and you strain against the seat belt b) when riding a horse, the horse suddenly stops and you fly over its head c) the magician pulls the tablecloth out from under a table full of dishes d) the difficulty of pushing a dead car e) lawn bowling on a cut and rolled lawn verses an uncut lawn f) car turns left and you appear to slide to the right

To sum it up… Newton’s 1 st Law- Inertia In the absence of “a net force,” an object moves with constant velocity. In the absence of “the thing that will cause a “change in velocity,” an object moves with constant velocity. The cause of constant velocity is NOTHING that will change the velocity. The cause of motion is NOTHING! Motion Happens!

“Law of Acceleration” A net force causes an object to accelerate in the direction of the net force. The acceleration is directly proportional to the net force and inversely proportional to the object’s mass.

Second law: The greater the force applied to an object, the more the object will accelerate. It takes more force to accelerate an object with a lot of mass than to accelerate something with very little mass. The player in black had more acceleration thus he hit with a greater amount of force

Second law :  The greater the force, the greater the acceleration  The greater the mass, the greater the force needed for the same acceleration  Calculated by: F = ma  (F = force, m = mass, a = acceleration)

WEIGHT a measure of the gravitational force that a massive object, such as a star or planet, puts on another mass F = ma F = ma weight = mass x acceleration of gravity F g = ma g An object’s weight on planet Earth innewtons An object’s weight on planet Earth in newtons is equal to itsmass in kilogramstimes9.8 m/s 2 is equal to its mass in kilograms times 9.8 m/s 2.

a = F net m =ma The SI unit of force is the newton, named in honor of Isaac Newton. One newton of force is the amount of force needed to cause a one kilogram mass to accelerate at a rate of 1 m/s 2.

The net force acting on an object is the vector sum of all the forces acting on it. Examples: If an object is remaining at rest, it is incorrect to assume that there are no forces acting on the object. We can only conclude that the net force on the object is zero. 8 N 12 N 3 N 8 N 7 N 4 N 6 N 5 N 9 N 4 N8 N 7 N 8 N

Examples of Newton’s 2 nd Law a) hitting a baseball, the harder the hit, the faster the ball goes b) accelerating or decelerating a car c) The positioning of football players - massive players on the line with lighter (faster to accelerate) players in the backfield d) a loaded versus an unloaded truck

Examples of Newton’s 2nd Law The second law states that unbalanced forces cause objects to accelerate with an acceleration which is directly proportional to the net force and inversely proportional to the mass. This one is telling us that big heavy objects don’t move as fast or as easily as smaller lighter objects. It takes more to slow down a charging bull then to slow down a charging mouse.

FORCE = Any push or pull which causes something to change its speed or direction What is a Force?

Forces can be BALANCED or UNBALANCED Balanced forces are equal in size (magnitude) and opposite in direction UNbalanced forces are not equal in size (magnitude) and/or opposite in direction. If the forces on an object are UNBALANCED, we say a NET force results. What is a Force?

Can you think of examples of forces? Balanced Forces? Unbalanced Forces? What is a Force?

Identifying a force : Source, agent exerting the force Type: “contact” or “long-range” –“Contact”- the 2 objects HAVE to be touching EX. air resistance, applied (push or pull), friction, normal, tension, thrust, etc. –“long-range”- the 2 objects DO NOT have to be touching EX. gravitational, electric, magnetic

GRAVITY GRAVITY: An attraction force between all masses force The greater the mass, the greater the force Acceleration due to gravity = 9.8 m/s/s or 9.8 m/s 2 What is Gravity?

Weight (F g ) is a measure of the gravitational force between two objects The greater the mass the greater the force (weight) Measured in units called newtons (N)

Weightlessness – free from the effects of gravity, a myth - true no where

Air resistance : The force of air exerted on a falling object The air pushes up as gravity pulls down Dependent upon the shape and surface area of the object and its speed When the air resistance magnitude equals the force of gravity magnitude, terminal speed is reached Terminal speed is the highest speed that an object will reach as it falls freely in air

The force of friction The push things give to each other when they rub together. This changes motion into heat. Friction is the force that acts in the opposite direction of motion of the object In Newton’s world, if you could eliminate friction, then you would see more things keep on moving in a straight line. Newton’s first law only makes sense when all forces, including friction, are accounted for.

As a baseball is being caught it’s velocity goes from 30.0 m/s to 0.0 m/s in about s. The mass of the baseball is 0.145kg. What is the baseball’s acceleration? a = ΔV / Δt What is the magnitude and direction of the force acting on the ball? What is the magnitude and direction of the force acting on the player who caught the ball?

“Law of Interaction” Short Version “For every action force there is an equal and opposite reactive force.” Longer Version When one object exerts a force on a second object, the second exerts a force on the first that is equal in magnitude, but opposite in direction.

third law : For every action force, there is an equal and opposite reaction force. (Forces are always paired)

Examples of Newton’s 3 rd Law a)rockets leaving earth b)guns being fired c) two cars hit head on d) astronauts in space e) pool or billiards f) jumping out of a boat onto the dock g) sprinklers rotating

Examples of Newton’s 3 rd Law Newton’s third law: "For every action, there is an equal and opposite reaction." When you fire a gun you feel the recoil. Some of the funniest things in cartoons follow physics that have been exaggerated or just plain ignored. Wyle Coyote hangs suspended in space over that canyon for a lot longer than an object would in reality, but it is the anticipation of the drop and Wyle's facial recognition of the upcoming pain that is so classically cartooney. So some laws are stretched for comical effect.

Examples of Newton’s 3 rd Law

Example When you bump into something, the size of the bruise is determined by Newton's 3rd Law. When you hit the object it hits you back with the same force. You only get back what you put into it.

Newton’s 3 rd Law, the Law of Interaction If (object 1) exerts a (type) force in the (direction) direction on (object 2), then (object 2) is exerting a (same type) force directed (opposite direction) on (object 1) simultaneously.

Consider the interaction depicted below between foot A, ball B, and foot C. The three objects interact simultaneously (at the same time). Identify the two pairs of action-reaction forces. Use the notation "foot A", "foot C", and "ball B" in your statements. aws/u2l4b.cfm

Identify at least six pairs of action-reaction force pairs in the following diagram. laws/u2l4b.cfm

A 65 kg boy and a 45 kg girl use an elastic rope while playing tug – of – war. The two are on an icy frictionless surface. If the acceleration of the girl is 3.0 m/s 2, then find the magnitude of and direction of the acceleration of the boy toward the girl. FBD Newton’s 3 rd Law F g on b = -F b on g m = 65 kgm = 45 kg a = 3.0 m/s 2 a = ? m/s 2 F b on g = -F g on b

How do Newton’s Laws of motion apply to these situations? an object rests in your hand an object rests in your hand a ball is tossed upward a ball is tossed upward a car windshield hits a bug a car windshield hits a bug a person sits on a table a person sits on a table a person jumps up from the floor a person jumps up from the floor a baseball bat hits a baseball a baseball bat hits a baseball a truck and car hit head-on a truck and car hit head-on