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Physics Ch. 4, FORCES

Physics Ch. 4, FORCES. *Force: An action exerted on an object that may change the object’s state of rest or motion. Forces can cause accelerations. *The SI unit of force is the Newton (N) A Newton is (kg * m/s 2 ). 1 Kg cart. *Forces can act through contact or at a distance.

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Physics Ch. 4, FORCES

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  1. Physics Ch. 4, FORCES

  2. *Force: An action exerted on an object that may change the object’s state of rest or motion.

  3. Forces can cause accelerations. • *The SI unit of force is the Newton (N) • A Newton is (kg * m/s2)

  4. 1 Kg cart

  5. *Forces can act through contact or at a distance.

  6. Force Diagrams: Show forces as vectors • Free Body Diagrams: A type of force diagram that shows only an object and the forces acting ON the object.

  7. Sled: 40 N at 30 degrees. Sled Weighs: 37 N. • What is the Normal Force on the Sled?

  8. Two football players weigh 150 kg each. The players plow into each other with a force of 80 N. Gravitational force on each player is 1,000 N. Draw a free body diagram for each player.

  9. Isaac Newton’s First Law: • An Object at rest remains at rest. An object in motion remains in motion with a constant velocity unless the object experiences an external force.

  10. Matchbox Car

  11. *Inertia: The tendency of an object to resist being moved.

  12. The sum of forces acting on an object is the net force.

  13. Mass is a measure of inertia. • Exp: it’s harder to get a big elephant moving than a basketball.

  14. Normal Force • *The force exerted from the ground (or another object) that Exactly balances gravity in the y direction MUST BE PERPENDICULAR TO THE SURFACE!!!!!!

  15. A man is pulling on his pet penguin with a force of 80 N directed at an angle of 40 degrees to the horizontal. Find the x and y components of the force. • The penguin weighs 70 N. What is the normal force on the penguin if the penguin remains on the ground?

  16. The two forces on a helicopter are 200 N downward (gravity) and 100 N to the right (air resistance). Find the magnitude and direction of the net force.

  17. 35⁰ ramp • Find the net Force. Given: F1 = 22N F2 = 18N F3 = 11 N 2N in X direction

  18. How to find angle of gravity in a ramp problem: 90 – angle of ramp

  19. In a ramp problem: • Fg,x Horizontal component of Fgravity: cos(90 – angle of ramp) * Fgravity,down • Fg,y Vertical component of Fgravity: sin(90 – angle of ramp) * Fgravity,down

  20. A 200 N man stands on a scale, what will it read? He stands on a scale that is tilted 20 degrees. What will the scale read?

  21. *Objects at rest or moving at a constant velocity are at EQUILIBRIUM. The net forces on them are zero.

  22. If an object experiences a net force in some direction, it will accelerate in that direction.

  23. Newton’s Second Law • The acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to the object’s mass.

  24. *Newton’s second law summarized: • Fnet = m*a

  25. You throw a pineapple with a force of 2 N. The pineapple accelerates at 4 m/s2. What is the mass of the pineapple?

  26. Newton’s Third Law: • *Forces always exist in pairs. Or, “for every action, there is an equal and opposite reaction.” • Example: you push on the wall and the wall pushes back.

  27. Action and reaction pairs always act on different objects. • Consider a hammer and nail.

  28. Field forces like gravity also exist in pairs. • Exp: while a skydiver accelerates towards earth, the earth also accelerates towards the skydiver.

  29. *Weight • The force that Earth’s gravity exerts on an object (Fg)is called weight. • Fw = mg

  30. *Normal Force • Opposes gravity, and is perpendicular to the contact surface.

  31. The Force of Friction (Ff) • Friction opposes the applied force.

  32. Static Friction: The friction that keeps a motionless thing from moving.

  33. Kinetic Friction: Friction opposing an object in motion. It is LESS than static friction.

  34. The force of friction is proportional to the normal force (Fn).

  35. Coefficient of Friction: Tells how much friction a certain surface provides. (µ)

  36. Friction Equations: • µk=Fk / Fn • µs=Fs,max / Fn • Ff = µ*Fn

  37. A 30 kg box needs a 80 N force to get it moving. Find the coefficient of static friction.

  38. A 30 kg box needs a 50 N force to keep it moving. Find the coefficient of kinetic friction.

  39. Find the force of friction against a sliding baseball player. The player is 90 kg, and the kinetic coefficient of friction of the dirt on the player is .40.

  40. A person attaches a rope to a 20 kg crate of Snickers bars. She pulls with a force of 90 N at an angle of 30 degrees to the horizontal. µk= .5 • Find the acceleration of the box. • .12 m/s2 to the right

  41. Dragging an object on a flat surface. • 1. Define the X and Y axes, and find the X and Y components of all forces. • 2. Find the normal force by noting that in the Y direction, forces must balance. • 3. Calculate the force of kinetic friction on the box. • 4. Find the net force; then calculate the acceleration.

  42. There are four fundamental forces: Gravity, Electromagnetic, Strong and Weak Nuclear

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