Force A. Force is the push or pull exerted on an object. 1. Contact force: “I can not touch you without you touching me” 2. Field force: ‘How does the bird catch to worm? a. Explains subatomic particles, magnetism and gravity. 3. Force is a vector, net force a. net force is the sum of all forces acting on an object. b. net force = zero, is at rest c. net force zero, is in motion

Force B. Unit: Newton (N) where: 1 N = Kg m/s 2 means the amount of force required to accelerate 1 m/s 2 1 pound = 4.45 N ie: How much N force does a 200 lb man exert? 200(4.45) = 890N What is the weight expressed in Newtons

Force Force diagram: show the direction and magnitude of a force exerted on an object. At rest: In motion: **There is always an unblance in the force applied in order for an object to move!** **Gravity is always a constant!** **Force is a vector with both direction and magnitude!**

Force Diagrams Draw a Force Diagram for the following: 1. A book is setting on the lab bench at rest. 2. A ball rolls off the lab bench then falls to the floor. Ignore air resistance. 3. A rope is used to lower a box to the ground with respect to the box. 4. A box is lifted to the top of the top of the lab bench from the floor with respect to the box.

Force Samplers 1. A force pushes on a box to the left with a force of 100 N. Another set of forces of 25 N and 125 N pushes on the box to the right. Draw a diagram and what is the net force? 2. A force of 75N pushes on a object to the west and a second force pushes with a force of 50 N to the west. Draw a diagram and find the net force.

Newton’s First Law of Motion Also called the ‘Law of Inertia’ Inertia: Resistance to changes in motion. Stated: An object at rest will stay at rest and an object in motion will tend to stay in motion unless acted upon by some ‘net external force’. Net external force: the sum of all the vector forces acting on an object.

Newton Second Law of Motion An object will move in the direction of the force applied and to the magnitude of that force. F = ma N = Kg m/s 2 F = ma use the triangle Weight: F w = mg NOTE: Gravity is m/s 2. Weight is define as the downward force on an object. Gravity is then 9.81 m/s 2

Newton Second Law of Motion Integrate Kinematic Equations into forces via Newton’s Laws. Convert: N  Kg  lbs 1 lbs = 4.45 N = 2.2 kg What is the weight of a 15 kg object? What is the weight of the same object on Jupiter? Jupiter gravity is 2.71 x Earth’s.

Newton Third Law of Motion Action/Reaction: For every force there is an equal and opposite force that reacts. Forces acting on an object at rest: Fn = force normal, acts opposite gravity Fg = force due to gravity, opposite Fn Fa or F װ = applied or parallel force Ff = friction force opposite the motion F ┴ = perpendicular force act at 90 o to an incline

Resolution of Forces

Force Integrated w/ Kinematics We will use the kinematic equation along with F = ma to solve problems. V f = V i + a∆t V f 2 = V i 2 + 2a∆dF = ma ∆d = V i ∆t + 1/2a∆t 2

Sampler What force is needed to stop a 280 Kg mass that has traveled from 3.2 m/s to 5.6 m/s in a distance of 40 m?

Sampler A BB loaded into the BB gun travels 0.90 m down the barrel when the trigger is pulled. If it takes 0.88 s for the BB to reach the end of the barrel, what force has been applied to the BB? The mass of a BB is 0.99g.

Sampler What is the weight of an object that is moved from 2.3 m/s to 5.4 m/s in 3.5 s with a force of 83 N?

Sampler Questions: Newton’s Laws 1. In a tug-of-war, one team pulls to the left with a force of 15 N and the second team pulls with a force of 10 N. Both teams pull only in the horizontal direction. a.Which team will win the tug-of-war? b.Which direction and magnitude will win? c.Which of Newton’s Laws apply? Explain!

2. Which line on the given graph indicates a greater force applied with a constant mass? Explain! A velocity (m/s 2 ) B Time (s) 3. In the graph above, where would you draw line B if the forces acting are equal for an equal mass?

4. Two forces act on a single object. One force is 156 N and the other 126 N a. What is the net force if each are pulled in the same direction, east? b. What is the net force if each is pulled in the opposite direction? c. Which of Newton’s Laws apply? 5. An object setting on a table has a force of inertia equal to 25 N. a. If 25 N is applied east, describe the motion? b. What force would move the object to the east? c. Which of Newton’s Law apply?

6. Three forces are acting on a single object. One force of 150 N pulls east and two forces of 75 N and of 55 N pulls west: a. What is the net force acting on the object? b. Which of Newton’s Laws best applies? 7. Two separate objects are seen moving at the same rate of acceleration. a. What can be concluded about the force causing their acceleration. b. Which of Newton’s Law best applies?

8. How can Newton’s Second Law be used to find the weight of an object? 9. Only horizontal forces apply. a. An object is pulled with a force of 75N accelerates at 1.25 m/s 2. What is the mass of the object? b. How much force must be applied to accelerate a 5 Kg mass at 0.75 m/s 2 ? c. At what rate of acceleration will a 500 Kg mass move with a force of 625N?

10. If you push against the wall with a force equal to 55 N, what is the force that the wall is pushing against you? 11. How does Newton’s Third Law explain why a ball slows down when thrown directly into the air? 12. A rope pulls on an object from rest to a final velocity of 2.5 m/s in a distance of 5 m. If the object has a mass of 45 Kg, what is the force needed to accelerate the object in this distance?

13. You jump into your car with a Solo cup of coke in your lap. You begin to drive off when you remember you forgot your Physic book. You stop suddenly. a. What will most likely happen to the coke in the cup? b. Which of Newton’s Law best applies here? c. What term would best describe what happen with the coke when you stopped suddenly?

Resolution of Forces Net Force: F net 2 = F x 2 + F y 2 Direction: Tan θ = F y / F x A force acts on an object of 6 N W, 4 N E, 5 N N and 2 N S. What is the net force acting on the object and in what direction will the object move?

Resolution of Force Trigonometric Functions: SOH COH TOA 1. A 5.4 Kg box is pulled horizontally along a floor with a force of 86 N by a rope fixed at a 28 o angle to the horizontal. What is the vertical and horizontal force? 2. An object is pulled left with a force of 380 N while upward force of 280 N, downward with 156 N and to the right with a force of 450 N. What is the net force acting of the object and direction. pp121, 1-4

Frictional Forces Friction: a force that acts between two surfaces and in the opposite direction FfFa =Fp

Friction Type of Friction: 1. Static Friction: Ffs, force of friction between two surfaces at rest. Starting friction 2. Sliding Friction: Ffk, force of friction between two surfaces in motion. Kinetic friction. Ffk = Fp = ma ** Ffs > Ffk **

Friction Chstc: 1. Friction acts between objects in contact 2. Friction is independent of surface area. 3. Friction depends on type of material. 4. Fs = -Fa

Coefficient of Friction (Table 4-2) A value of defines the relationship between two surfaces relative to the force normal, Fn. Fn = Fw =mg Coefficient of Friction: ratio of Ff to Fn μ = Ff / Fn Coefficient of Static Friction: ratio of Fs to Fn μ s = Fs / Fn Coefficient of Sliding Friction: ratio of Fk to Fn μ k = Fk / Fn

Friction Sampler 1. A man pulls a sled at an angle of 15 o from the horizontal at a constant speed with a force 25 N. The sled has a mass of 5.4 Kg. What is the coefficient of sliding friction?

Friction Sampler 2. A 8 Kg box is pulled across a rough surface at a constant acceleration of 0.25m/s 2. What is the coefficient of sliding friction?

Friction Sampler 3. Describe the motion of an 80 Kg mass with a coefficient of static friction = 1 ? TLS: #17-26.

Resolution of Forces 2D w/ Friction Relationships used: Fg = Fw = mg Fy = Fn = FwCos A Fx = Fp = FwSin A µf = Fk / Fn F = ma where the force is the sum of all forces, ∑F = Fp – Fk so, Fp – Fk = ma or ∑F = ma

Friction Sampler A 25 Kg box setting on an incline of 23 o is released and slides down the incline that has a coefficient of sliding friction or What is the acceleration of the box?

Friction Sampler A 35 Kg object sets on an incline of 35o when it begins to slide at a rate of acceleration of 4.34 m/s2. What is the coefficient of sliding friction? #17-21 page 128 and #18-26, page 130