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:

10 kg

F1 = 20 N F2 = 20 N

Stationary:

What is the Fnet acting on this box:ΣF = Fnet = F1 – F2 = 20 N - 20 N = 0

A Fnet of zero means no change in movement. The box stays stationary.

FORCES ARE BALANCED

Fg

FN – The Normal Force- table pushing up on the box- THIS IS ALWAYS PERPENDICULAR TO THE SURFACE AND EQUAL TO THE Fg.

Let’s look at forces acting on this box:

10 kg

F1 = 20 N F2 = 40 N

Stationary:

What is the Fnet acting on this box:ΣF = Fnet = F1 – F2 = 20 N - 40 N = 20 N

Now the Fnet is not zero which means there is a change in movement. This box is not going to remain stationary.

FORCES ARE UNBALANCED

Fg

FN

Let’s look at forces acting on this box:

10 kg

F1 = 20 N F2 = 40 N

Stationary:

What is the Fnet acting on this box:ΣF = Fnet = F1 + F2 = 20 N + 40 N = 60 N

Again there is a non-zero Fnet which means there is a change in movement.

FORCES ARE UNBALANCED

Fg

FN

But what if the box had an initial motion:

10 kgF1 = 20 N F2 = 20 N

Moving with a constant velocity:

What is the Fnet acting on this box:ΣF = Fnet = F1 - F2 = 20 N - 20 N = 0

The Fnet is zero which means there is not a change in movement or direction. This box will continue to move with a constant velocity.

FORCES ARE BALANCED

Fg

FN

What forces are acting on this ball?

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?…

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.

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

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.

2. The famous tablecloth trick:

3. Sliding Cars:

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

http://www.youtube.com/watch?v=KyfjZlOSq2A

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?

aF

m Which turns into Newton’s 2nd equation: F ma

F manet kg m/s2

kg • m/s2 = Newton = N

A medium-sized apple weighs

about one Newton.

1 N = .22 lbs.

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

10 kg

Fg

FN

F1 = 20 N F2 = 40 N

a = ?

Fnet = ΣF = 20 N - 40 N = 20 N

m = 10 kg

aF

m

20

102 2N

kg

kg ms

kg

=

a

This leads us to the Fg:

F mag g = 9.81 m/s2 ~ 10 m/s2

What’s mass times the acceleration due to gravity?

THIS IS WEIGHT w mg

Use the weight equation to find your mass:

w mg

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

mw

g

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

10 kg

F1 = 20 N F2 = 20 N

Fg = ma = mg =

FN – EQUAL but OPPOSITE to the Fg = 100 N

100 N

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

ΣFy = Fnety = Fg – FN = 100 N - 100N = 0

ΣFx = Fnetx = F1 – F2 = 20 N - 20N = 0This box is not moving or changingdirection.

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?

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

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 acceleration of the box?

10 kg

F1 = 300 N F2 = 500 N

How far did the box travel in 10 s?

Remember this guy?He was pushing that car with a force of 200 N. With how much force is the car pushing

on him?

Let’s go back to our box example…

10 kg

Fg

FN

The force of gravity it pulling the box downward with a force of 100 N.

But the table is also pushing back on the box with the same amount of force in the opposite direction.

This brings us to Newton’s 3rd Law:

For every action there is an equal and opposite reaction (action/reaction law).

The air is moving out of the balloon in this direction causing it to move in the opposite direction.

http://www.youtube.com/watch?v=mNM5tHou4IQ

http://www.youtube.com/watch?v=7y7KjanpWOk