FORCES & NEWTON’S LAWS

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!

Book held in your hand

Book pushed across a desk by your hand

Book pulled across a desk by a string

A ball hanging from a string

Same ball just after the string breaks

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?

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 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)

Push vs. Pull

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