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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 1st 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 2nd LawAn unbalanced force causes acceleration
Acceleration is:
• directly related to force.
• inversely related to mass.
𝑎 = 𝐹𝑚
1 kg * 1m/s2 = 1 Newton
Newtons 2nd Law:
Example: An 1800 kg bus accelerates from 0 to 25 m/s in 5 seconds. What is the force required?
Newtons 2nd 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
𝐹𝑔 = 𝑚𝑔 = 𝑚𝑎𝑠𝑠 ∗ 9.8earth
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? (gm = 1.6 m/s2)
Newtons 3rd Law
• When one object exerts a force on a second object, the 2nd 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 3rd 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(10 kg) PIZZA (5kg)
40o 40o
Find the Tension on each chain
Balanced Forces The ninja has a mass of 50 kg, ϴ =30o
Find the Tension in Each Rope
Balanced Forces• A 20 kg crate of mangos hang from two cables. The cables make an
angle of 90o with each other. One cable has 3 times the tension of the other cable. What is the tension on each cable?
Unbalanced ForcesWorker 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 ForcesA Rescue Helicopter lifts 2 people with a total mass of 200kg with an acceleration of 3.0m/s2.
What is the Tension on the Rope holding the people?
1) Draw Force diagram2) Determine Net Force3) Solve the Problem
Riding in an elevator
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?
1) Draw Force diagram2) Determine Net Force3) Solve the Problem
Unbalanced Forces: If the boat below as a mass of 500kg. What is the acceleration?
1) Draw Force diagram2) Determine Net Force3) 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 =30o
with the vertical. What is the acceleration of the dice?
1) Draw Force diagram2) Determine Net Force3) Solve the Problem