Forces and newton s 3 laws
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Forces and Newton’s 3 Laws. What is a force?. Push or pull Produce changes in motion or direction. Net force:. The net force is a combined total force acting on an object. F net ΣF

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Forces and Newton’s 3 Laws

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Forces and newton s 3 laws

Forces and Newton’s 3 Laws


What is a force

What is a force?

  • Push or pull

  • Produce changes in motion or direction


Net force

Net force:

  • The net force is a combined total force acting on an object.

  • Fnet

  • ΣF

  • We represent force by using vectors- arrow symbols that represent magnitude and direction by their length and which way they point.


Let s look at forces acting on this box

Let’s look at forces acting on this box:

FN – The Normal Force- table pushing up on

the box- THIS IS ALWAYS PERPENDICULAR

TO THE SURFACE AND EQUAL TO THE Fg.

Stationary:

F1 = -20 N west

F2 = 20 N east

10 kg

Fg

What is the Fnet acting on this box:

ΣF = Fnet = -20 N + 20 N = 0

FORCES ARE BALANCED

A Fnet of zero means no change in movement.

The box stays stationary.


Forces and newton s 3 laws

Let’s look at forces acting on this box:

FN

Stationary:

F1 = 20 N west

F2 = 40 N east

10 kg

Fg

What is the Fnet acting on this box:

ΣF = Fnet = -20 N + 40 N = 20 N east

FORCES ARE UNBALANCED

Now the Fnet is not zero which means there is a change

in movement. This box is not going to remain stationary.


Forces and newton s 3 laws

Let’s look at forces acting on this box:

FN

Stationary:

F1 = 20 N west

F2 = 40 N east

10 kg

Fg

What is the Fnet acting on this box:

ΣF = Fnet = 20 N + 40 N = 60 N east

FORCES ARE UNBALANCED

Again there is a non-zero Fnet which means there is a change

in movement.


But what if the box had an initial motion

But what if the box had an initial motion:

FN

Moving with a constant velocity:

F1 = -20 N west

10 kg

F2 = 20 N east

Fg

What is the Fnet acting on this box:

ΣF = Fnet = -20 N + 20 N = 0

FORCES ARE BALANCED

The Fnet is zero which means there is not a change

in movement or direction. This box is continues to move

with a constant velocity.


So what needs to happen to make this box move

So what needs to happen to make this box move?

10 kg

UNBALANCED FORCES


What about when the box is already moving with a constant velocity

What about when the box is already moving with a constant velocity?

What would cause the box to stop?

10 kg

UNBALANCED FORCES

What would happen if there weren’t unbalanced forces? Would the box ever stop?…


Not according to newton s 1 st law of motion

…Not according to Newton’s 1st Law of Motion:

An object at rest has a natural tendency to stay at rest, or an object in motion will stay in motion, unless a force is acting upon it.

This is also known as the law of INERTIA.

INERTIA is an objects resistance to change in motion.


Examples of inertia

Examples of INERTIA:

1. Not wearing your seatbelt- if you get into an

accident your body wants to keep moving at

the speed you were going.


2 the famous tablecloth trick

2. The famous tablecloth trick:

http://www.youtube.com/watch?v=D-a3kwvY0WM


The relationship between mass and inertia

The relationship between mass and inertia:

MASS IS A MEASURE OF INERTIA- the more massive the object, the more that object tends to resist changes in its state of motion.

What would be easier to push a small car or a semi?


Adding mass into the mix brings us to newton s 2 nd law of motion

Adding mass into the mix brings us to Newton’s 2nd Law of Motion:

If I push both vehicles with the same amount of force

which one would accelerate more? Why?

Which turns into Newton’s 2nd equation:


Forces and newton s 3 laws

m/s2

kg

kg • m/s2 = Newton = N


A medium sized apple weighs about one newton

A medium-sized apple weighs about one Newton.


Let s find the magnitude of the acceleration for this box if the following forces are applied

Let’s find the magnitude of the acceleration for this box if the following forces are applied:

FN

F1 = -20 N

F2 = 40 N

10 kg

Fg

a = ?

Fnet = ΣF = -20 N + 40 N = 20 N

m = 10 kg

=


This leads us to the f g

This leads us to the Fg:

g = 9.81 m/s2 ~ 10 m/s2

What’s mass times the acceleration due to gravity?

THIS IS WEIGHT


Use the weight equation to find your mass

Use the weight equation to find your mass:

This needs to be in N. Use 1 N = .22 lbs


Let s go back and talk about our box again this time analyzing all the forces

Let’s go back and talk about our box again this time analyzing all the forces:

FN – EQUAL but OPPOSITE to the Fg = 100 N

F1 = -20 N

F2 = 20 N

10 kg

-100 N

Fg = ma = mg =

Now we are dealing with forces acting in two directions (x and y).

ΣFy = Fnety = 100 N + -100N = 0

ΣFx = Fnetx = -20 N + -20N = 0

This box is not moving or changing

direction.


Practice with newton s 2 nd

Practice with Newton’s 2nd:

1. A tractor pulls a loaded wagon with a constant force of 400 N. If the total mass of the wagon is 200 kg, what is the wagon’s acceleration?


Forces and newton s 3 laws

2. A broken down car is being pushed to the side of the road with a force of 200 N which is causing it to accelerate at .2 m/s2. What is the mass of the car?


Forces and newton s 3 laws

3. The car below was moving with an initial velocity of 50 m/s until F2 was applied to slow the car down. What is the deceleration of the box?

F1 = 300 N

F2 = -500 N

10 kg

What is the distance the box travels before it comes to a stop?


Forces and newton s 3 laws

4. A student weighs 600 N. What is his mass?


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