Chapter 4 NEWTONS LAWS Newton s 3 Laws Force Diagrams Balanced Forces Unbalanced Forces
Force: a push or a pull Measured in Newton Vector Quantity Contact Force: applied by direct contact Field Force: can be applied over a distance
Newtons 1 st Law: Law of Inertia A body at rest remains at rest, A body in motion remains in motion, at a constant velocity, in a straight line Until an Unbalanced force acts on it. INERTIA: Tendency to resist changes in motion. Inertia is a property of matter
Examples: Basics Table Cloth Trick Crashes Seat Belts
Newtons 2 nd Law An unbalanced force causes acceleration Acceleration is: directly related to force. inversely related to mass. a = F m 1 kg * 1m/s 2 = 1 Newton
Newtons 2 nd Law: Example: An 1800 kg bus accelerates from 0 to 25 m/s in 5 seconds. What is the force required?
Newtons 2 nd Law: Example: A 50 kg kid runs into a tree with and initial speed of 6 m/s. The tree exerts a force of 350 N. In what distance does the boy come to rest?
Weight: Force exerted by gravity F g = mg = mass 9.8 earth 10 kg = how many Newtons? How much ya bench? (175 lbs) (1kg = 2.2 lbs) Convert your weight to Newtons. What do you weigh on the moon? (g m = 1.6 m/s 2 )
Newtons 3 rd Law When one object exerts a force on a second object, the 2 nd object exerts an equal Force in the opposite direction on the first object. For every action there is an equal and opposite reaction. All Forces come in pairs, equal in magnitude, opposite in direction.
Newtons 3 rd law Examples Kicking Soccer ball: Action: you apply force to ball via the kick. Reaction: Ball exerts Force on Your Foot Air Resistance: Action: Ball pushes air out of the way. Reaction: Air pushes on ball to slow it down. Earth-Moon System: Action: Earth pulls on moon causing the moon to orbit the earth. Reaction: Moon pulls on earth, causing the tides.
Force Diagrams (Free Body Diagrams) Identify the object you will draw a diagram for. Identify all the forces acting directly on the object and the object exerting them. Draw a dot or line to represent the object of interest. Draw a vector from the dot to represent each force. Draw it in the direction the force is being exerted. Label it by (a) the type of force, and/or(b) the object exerting the force
Force Diagrams (Free Body Diagrams) A ball is dropped (no air resistance) A ball is dropped (with air resistance) Coffee mug sits on table. Car accelerates forward Car slows down Crate pushed along ground with friction.
Force Diagrams (Free Body Diagrams) Draw Force diagram of the people Draw Force diagram of the beam
Force Diagrams (Free Body Diagrams) Draw Free body diagram of the person. Draw free body diagram of the rope.
Force Diagrams (Free Body Diagrams) Determine the net Force on each object below, If the vectors should graphically add up to zero, the object is stationary or is moving at a constant velocity, If the object is accelerating, the sum of the vectors should produce a vector in the same direction as the acceleration.
Balanced Forces Equilibrium : Net Force = Zero Object is at rest or moving at a constant velocity Tacos 40 o 40 o (10 kg) PIZZA (5kg) Find the Tension on each chain
Balanced Forces The ninja has a mass of 50 kg, ϴ =30 o Find the Tension in Each Rope
Balanced Forces A 20 kg crate of mangos hang from two cables. The cables make an angle of 90 o with each other. One cable has 3 times the tension of the other cable. What is the tension on each cable?
Unbalanced Forces Worker pushes a 100 kg box with a Force of 85N. Friction exerts a Force of 65 N. What is the acceleration of the box? 1) Draw Force diagram 2) Determine Net Force 3) Solve the Problem
Unbalanced Forces A Rescue Helicopter lifts 2 people with a total mass of 200kg with an acceleration of 3.0m/s 2. What is the Tension on the Rope holding the people? 1) Draw Force diagram 2) Determine Net Force 3) Solve the Problem
Riding in an elevator 1) Draw Force diagram 2) Determine Net Force 3) Solve the Problem A 50 kg woman is standing on a scale in an elevator. The scale reads 55kg. What is the acceleration of the elevator? A 50 kg woman is standing on a scale in an elevator. The scale reads 45kg. What is the acceleration of the elevator? A 50 kg woman is standing on a scale in an elevator. The scale reads 0 kg. What is happening to the elevator?
Unbalanced Forces: If the boat below as a mass of 500kg. What is the acceleration? 1) Draw Force diagram 2) Determine Net Force 3) Solve the Problem
Unbalanced Forces: A pair of fuzzy dice hang from the rear view mirror o a car. The car begins to accelerate forward. The dice now make an angle of ϴ =30 o with the vertical. What is the acceleration of the dice? 1) Draw Force diagram 2) Determine Net Force 3) Solve the Problem