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Forces and Newton’s 3 Laws

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Forces and Newton’s 3 Laws

- Push or pull
- Produce changes in motion or direction

- 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.

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.

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.

10 kg

UNBALANCED FORCES

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.

This is also known as the law of INERTIA.

INERTIA is an objects resistance to change in motion.

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

accident your body wants to keep moving at

the speed you were going.

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

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?

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

FN

F1 = -20 N

F2 = 40 N

10 kg

Fg

a = ?

Fnet = ΣF = -20 N + 40 N = 20 N

m = 10 kg

=

g = 9.81 m/s2 ~ 10 m/s2

What’s mass times the acceleration due to gravity?

THIS IS WEIGHT

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

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.

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?