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.