3 Forces (Ch 6)Why do things move? What is the cause of this motion we have studied?Explained by Sir Isaac Newton3 Laws of MotionExplain the way forces influence motion -what is needed for objects to move“how much” is neededwhat happens when objects interact
4 What is a Force? Force (F) = push or a pull exerted on an object Has magnitude & directionContact forces – act only on an object by touching itLong-range forces – act on an object without touching it
5 What causes a force? The agent is the immediate cause of a force There has to be an agent for a force to existWhat is the agent:A book in your hand?A book on a desk?A book falling from the desk?
6 What kind of forces are there? Friction (Ff) – contact force opposing motion between surfacesNormal (FN) – contact force exerted by a surface on an object (perpendicular to surface)Weight (Fg) – long-range force due to gravitation attraction of 2 objects (usually Earth & an object)
7 What kind of forces are there? (continued) Tension (FT) – force exerted by a string, rope or cable when it is attached to an object & tautSpring (Fsp) – force a spring exerts on an object
8 Newton’s 1st Law of Motion Law of InertiaInertia is an object’s tendency to resist a change in its motionLaw states…Objects in motion will stay in motion unless a net force acts on themObjects at rest will stay at rest unless a net force acts on them
9 Newton’s 2nd Law of Motion Force = mass X accelerationFnet = maForce and acceleration are directly proportional. The greater the force, the greater the acceleration.Mass and acceleration are inversely related. The greater the mass of an object the less the acceleration if the same force is applied.
10 Newton’s 3rd Law of Motion For every action force there is an equal and opposite reaction force.Two interacting objects become action-reaction pairs
11 How do we measure force?One “unit of force” causes a 1 kg mass to accelerate 1 m/s2Since F = ma,(1 kg)(1 m/s2) = 1 kg●m/s2We call this “unit of force” a Newton (N)
12 Friction Ff = μFN Force that opposes motion Calculating Friction Static Friction – opposes the start of motionKinetic Friction – opposes motion between 2 surfacesCoefficient of Friction ( μ)– a ratio of the magnitude of the 2 forcesIt’s a constant depending on the 2 surfacesNo unit!Calculating FrictionFf = μFN
13 What is a Net Force?The net force is the sum of all forces acting on a bodyThe net force causes the acceleration of the objectAcceleration is always in the direction of the FnetSo, 2nd law: Fnet = maWhen the Fnet = 0 – system is in equilibrium
14 How do you find Fg? Recall that F = ma (2nd law) So, the force of weight Fg = mgg = 9.8 m/s2 on earthMass in KILOGRAMS!
15 Free-Body Diagrams Use a dot (or box) to represent the object Represent each force acting on the object with an arrow (vector)Remember to point it in the right directionLet’s Practice!
21 Calculating Lift Problems Is the object being lifted at a constant speed? Or is it accelerating?If constant speed, the forces are balanced. So, Fappied = FgWhy?If accelerating, Fnet = ma
22 Calculating Friction Problems Is the object moving at a constant speed? Or is it accelerating horizontally?If constant speed, the forces are balanced. So, Fapplied = FfWhy?If accelerating horizontally, Fnet = ma
23 Let’s Practice # 1A horizontal force of 45 N is needed to keep a 125 N block sliding at a constant speed over a horizontal surface. Calculate the coefficient of friction.
24 Let’s Practice # 2A force of 40 N accelerates an 8 kg block 2 m/s2 along a horizontal surface.What is the net force?What is the frictional force?What is the normal force?What is the coefficient of friction?
32 What lift force does the air exert on the propellers ?
33 Now, we are working with a 2-D force! What if???What if the object is moving horizontally, but the applies force is at an angle?Now, we are working with a 2-D force!
34 Breaking down a single vector into 2 or more vectors A force includes magnitude & direction, making it a VECTOR.So, we can break it down!Resolution of Forces –Breaking down a single vector into 2 or more vectors(SOH CAH TOA)
36 A car is pulled with a force of 60N at angle of 37° from the horizontal. Find the vertical and horizontal components of the applied force . If the cart is rolling with constant velocity, what is thefrictional force? Why?
37 Reminders – Don’t forget your kinematic equations (motion) Remember to ID all forces acting on the object. Draw the diagram!If a force is applied at an angle, break it into its x- and y-componentsMass needs to be in kg!Formulas:Fnet = ma = sum of all forces acting on the objectFg = mgFf = μFN