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Newton’s Laws
Targeted Skills for Newton’s Laws (Lecture ONLY)
1. Identify and apply Newton’s Laws of Motion to a variety of qualitative and quantitative problems.
2. Identify: Gravitational Forces (Fg), Tension
Forces (FT) Normal Forces (FN) and Frictional
Forces (Ff).
3. Draw free body diagrams (FBD).4. Analyze position versus time, velocity versus
time and acceleration versus time graphs for regions of zero and non-zero net force.
5. Solve dynamics problems.
Define Mass/Inertia
Inertia – A measure of a bodies resistance to a change in motion.
Mass = Inertia
Mass – A measure of a bodies resistance to a change in motion
Newton’s First Law of Motion
Describe the motion of an arbitrary object setting in the room.
Newton’s First Law of Motion
Describe the motion of an arbitrary object sitting in the room. What is required to change an object’s motion?
An UNBALANCED Force Define Newton’s First Law of Motion
(Also known as Law of Inertia)
An object will remain in a state of constant motion unless acted upon by an unbalanced force.
Newton’s Second Law of Motion
What is required to change an object’s motion?An UNBALANCED ForceUNBALANCED Force = NET ForceNET Force = F (Sum of forces)What results if an unbalanced force is applied to an object?
ACCELERATIONDefine Newton’s Second Law of MotionThe acceleration of an object is proportional to the net force applied to the object and inversely proportional to the object’s mass.
Newton’s Second Law of Motion
Equation of Newton’s Second Law of Motion
F = maQuestionsHow do we know if we have an unbalanced force? If there is an unbalanced force, in what direction is it acting?AnswerFREE-BODY DIAGRAMA diagram of the object involved in a problem and the forces exerted on the object.
Free-Body Diagram Construction
Horizontal / Vertical ScenariosA jet plane is gliding at a constant elevation at a constant velocity. Draw the Free-Body Diagram of the forces acting on the plane. NO air resistance.
A jet plane is flying at a constant elevation at a constant velocity. Draw the Free-Body Diagram of the forces acting on the plane. Consider Air Resistance.
Free Body Diagram Construction
Horizontal / Vertical ScenariosA jet plane is flying at a constant elevation with an increasing velocity. Draw the Free-Body Diagram of the forces acting on the plane. Consider Air Resistance.
A jet plane is flying at a constant elevation with a decreasing velocity. Draw the Free-Body Diagram of the forces acting on the plane. Consider Air Resistance.
Free Body Diagram Construction
Rules1. Draw an arrow representing the weight of the
object.2. Label the arrow Fg.
3. Draw additional arrows in the appropriate directions to represent any forces acting on the object. The length of the arrows should be proportional to the quantity of the force.
Free Body Diagram Construction
Rules5. Label arrows with appropriate names, e.g.:
• Force of Gravity, Fg
• Tension, FT
• Normal, FN
• Friction, Ff
6. Remember, ONLY the arrows constitute the free body diagram.
Free Body Diagram Worksheet
Example Problem 1
Two forces are applied to a 10 kg block. Calculate the net force block on the block if F1 equals 15 N and F2 equals 30 N.
F1
F2
10 kg
F = 30N –15 NF = 15 N to right
What is the block’s acceleration?
F = ma
Example Problem 1
Two forces are applied to a 10 kg block. Calculate the net force block on the block if F1 equals 15 N and F2 equals 30 N.
F1
F2
10 kg
F = 15 N to right
What is the block’s speed after 4 seconds if it was initially at rest?G:
U: vf = ________
E: F=ma
S:
S:
F = 15 N to right
Example Problem 2
Fred and Wilma push a stalled car at constant velocity along level ground. If Fred and Wilma push to the right with 395 N and 275 N respectively, what is the magnitude of the opposing force? Identify the opposing force.
F = 395N + 275 N + f = 0 N
G:
U: Ff = ______
E: F=ma
S:
S:
constant velocity--acceleration = 0
Identify the opposing force.
Example #3
A dirt buggy has a mass of 575 kg. The buggy uniformly accelerates from rest for 4 seconds and travels 35 meters. What’s the buggy’s acceleration?G: m = 575 kg
vi = 0
t = 4 sd = 35 m
U: accelerationE: d = vit + ½at2
S: See OverheadS: See Overhead
Example #3
A dirt buggy has a mass of 575 kg. The buggy uniform accelerates from rest for 4 seconds and travels 35 meters. How fast is the buggy traveling after accelerating for 4 seconds? G: m = 575 kg
vi = 0
t = 4 sd= 35 m
U: velocityE: vi = vf + at or vf
2 = vi2 + 2ad
S: See OverheadS: See Overhead
Example #3
A dirt buggy has a mass of 575 kg. The buggy uniform accelerates from rest for 4 seconds and travels 35 meters. What net force is applied to the buggy? G: m = 575 kg
vo = 0
t = 4 sd = 35 m
U: net forceE: F = maS: See OverheadS: See Overhead
Example #4
The maximum force a grocery sack can withstand and not rip is 250N. If 20 kg of groceries are lifted from the floor to the table with an acceleration of 5 m/s, will the sack hold? if F1
equals 15 N and F2 equals 30 N.
G: m = 20 kg
a = 5 m/s2
Fmax
= 250 N
F2= 30 NU: F=______E: F = ma + mgS: See OverheadS: See Overhead
Newton’s Third Law
Definition of Newton’s Third Law of Motion
When two bodies interact, the forces on the bodies from each other are always equal in magnitude and opposite in direction. These are referred to as Action-Reaction pairs of forces.
Horse-Cart Problem
Draw ALL the forces acting on the horse, cart and roadway.
Newton’s Third Law
Identify action-reaction pairs of forces.
Explain how the horse can move.
Newton’s Third Law
Explain how the horse-cart can move.
Behavior of ForcesInternal Forces – Come in pairs (action-reaction), cancel one another, and can NOT accelerate an object.
External (applied) Force – Individual forces which may accelerate an object (F > 0).
All type of forces (Fg, FN, FT, Ff) can behave as internal and external forces.