Forces

1. Newton's Laws of Motion & Forces

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

3. 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 FORCE “a push or pull” that may cause an object to speed up, slow down or change directions.

4. “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. • * OR * • The velocity of an object remains constant unless acted on by an unbalanced force.

5. Identifying a force: • Source, agent exerting the force • Type: “contact” or “field” • “Contact”- the 2 objects HAVE to be touching • EX. push, normal, pull, thrust, tension, friction, etc. • “Field”- the 2 objects DO NOT have to be touching • EX. gravitational, electric, magnetic

6. The force of friction • The push things give to each other when they rub together. • 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.

7. The net force acting on an object is the vector sum of all the forces acting on it. Examples: 8 N 6 N 9 N 8 N 4 N 3 N 8 N ? 7 N 7 N 5 N 4 N 8 N 12 N 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. 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.

9. Newton’s 1st Law- Inertia • In the absence of “a net force,”an objectmoves with constant velocity. • In the absence of “the thing that will cause a “changein velocity,”an objectmoves with constant velocity. • The causeof constant velocityis NOTHING that willchangethe velocity. • The cause of motion is NOTHING! • Motion Happens!

10. “Law of Interaction” Short Version “For every action there is an equal and opposite reaction.” 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.

11. 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.

12. Newton’s 3rd 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.

13. 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. http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l4b.html

14. Identify at least six pairs of action-reaction force pairs in the following diagram. http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l4b.html

15. Newton’s Third Law deals with acting on objects. two forces two different Newton’s Third Law pairs of forces always, sometimes, never cancel each other out. The only way for two forces to cancel each other out is for them to be equal and opposite and act on the same object!

16. “Law of Acceleration” A net force causes an object to accelerate in the direction of the net force. The acceleration is directly proportional to thenet force andinversely proportional to the object’smass.

17. Fnet a Fnet = m a = m 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/s2.

18. WEIGHT a measure of the gravitational force that a massive object, such as a star or planet, puts on another mass F=ma weight=massxacceleration of gravity Fg=mag An object’s weight on planet Earth innewtons is equal to itsmass in kilogramstimes9.8 m/s2.

19. As a baseball is being caught it’s velocity goes from 30.0 m/s to 0.0 m/s in about 0.0050 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?

20. a = ? m/s2 a = 3.0 m/s2 m = 65 kg m = 45 kg 2. 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/s2, then find the magnitude of and direction of the acceleration of the boy toward the girl. • FBD • Newton’s 3rd Law • F g on b = -F b on g F b on g = -F g on b

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