# Forces and Newton’s 3 Laws - PowerPoint PPT Presentation

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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

### 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.

• 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:

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.

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.

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:

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?

10 kg

UNBALANCED FORCES

### 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 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:

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

accident your body wants to keep moving at

the speed you were going.

### 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 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:

m/s2

kg

kg • m/s2 = Newton = N

### 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 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:

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:

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 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?

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?

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?

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