Forces (Ch 6) • Why do things move? What is the cause of this motion we have studied? • Explained by Sir Isaac Newton • 3 Laws of Motion • Explain the way forces influence motion - • what is needed for objects to move • “how much” is needed • what happens when objects interact
What is a Force? • Force (F) = push or a pull exerted on an object • Has magnitude & direction • Contact forces – act only on an object by touching it • Long-range forces – act on an object without touching it
What causes a force? • The agent is the immediate cause of a force • There has to be an agent for a force to exist • What is the agent: • A book in your hand? • A book on a desk? • A book falling from the desk?
What kind of forces are there? • Friction (Ff) – contact force opposing motion between surfaces • Normal (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)
What kind of forces are there?(continued) • Tension (FT) – force exerted by a string, rope or cable when it is attached to an object & taut • Spring (Fsp) – force a spring exerts on an object
Newton’s 1st Law of Motion • Law of Inertia • Inertia is an object’s tendency to resist a change in its motion • Law states… • Objects in motion will stay in motion unless a net force acts on them • Objects at rest will stay at rest unless a net force acts on them
Newton’s 2nd Law of Motion • Force = mass X acceleration • Fnet = ma • Force 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.
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
How do we measure force? • One “unit of force” causes a 1 kg mass to accelerate 1 m/s2 • Since F = ma, • (1 kg)(1 m/s2) = 1 kg●m/s2 • We call this “unit of force” a Newton (N)
Friction • Force that opposes motion • Static Friction – opposes the start of motion • Kinetic Friction – opposes motion between 2 surfaces • Coefficient of Friction ( μ)– a ratio of the magnitude of the 2 forces • It’s a constant depending on the 2 surfaces • No unit! • Calculating Friction • Ff = μFN
What is a Net Force? • The net force is the sum of all forces acting on a body • The net force causes the acceleration of the object • Acceleration is always in the direction of the Fnet • So, 2nd law: Fnet = ma • When the Fnet = 0 – system is in equilibrium
How do you find Fg? • Recall that F = ma (2nd law) • So, the force of weight Fg = mg • g = 9.8 m/s2 on earth • Mass in KILOGRAMS!
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 direction • Let’s Practice!
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 = Fg • Why? • If accelerating, Fnet = ma
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 = Ff • Why? • If accelerating horizontally, Fnet = ma
Let’s Practice # 1 • A 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.
Let’s Practice # 2 • A 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?
Let’s Practice # 3 • A 6000 kg helicopter accelerates upward at 3 m/s2. • What is the net force? • What is the weight force? • What lift force does the air exert on the propellers ?
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!
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)
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 the frictional force? Why?
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-components • Mass needs to be in kg! • Formulas: • Fnet = ma = sum of all forces acting on the object • Fg = mg • Ff = μFN