Force, Friction, Gravity and Newton’s Laws

Chapter 3 Lessons 4-6

I can... •identify when forces add or subtract.

•calculate the Net Force.

•explain the difference between balanced and unbalanced

forces.

force: a push or a pull on an object.

Newton: unit used to measure force

net force: combination of all forces acting on an object

friction: force that two surfaces exert on each other

when they rub against each other

sliding friction: occurs when solid surface slide over

each other

static friction: acts between objects that aren't

moving; bonds form between non-moving objects and

surfaces

fluid friction: occurs when a solid object moves

through a fluid

rolling friction: occurs when an object rolls over a

surface

gravity: force that pulls objects toward each other

(force of attraction between all objects that have

mass)

inertia: resistance to changes in motion

Balanced and Unbalanced Forces Balanced forces:

•forces on object cancel each other out

•net force is zero

•object does not change motion

•object does not accelerate

Unbalanced forces:

•don’t cancel out

•net force is not zero

•object’s motion changes

•acceleration occurs

An object will speed up, slow down, or turn

ONLY if something is applying an unbalanced

force to the object.

A force is a push or a pull on an object.

The unit used to measure force is the Newton (N)

Net force: all the forces acting on an object combined

The net force determines how the force will affect an

object

10 N

10 N

If the forces are in the same direction, they add

together to form the net force.

20 N

10 N 10 N

If forces are in opposite directions, then the net force

is the difference between the forces, and it is in the

direction of the larger force.

If forces are in opposite directions, then the net force

is the difference between the forces, and it is in the

direction of the larger force.

10 N 15 N

5N

10 N

Forces at different angles partially add and partially

cancel each other out

8 N

6 N

8 N

6 N 16 N

I can….

•Solve for net force

•Identify types of friction

•Explain that gravity depends on mass and distance

Friction • force that brings nearly everything to a stop - resists

motion

• changes kinetic energy into thermal

• Static: bonds form between non-moving objects and surfaces

• Sliding: slows down objects that are moving (due to roughness of surfaces)

• Rolling: occurs between ground and the part of the tire touching ground

• Fluid: occurs when a solid object moves through a fluid

Gravity • Force between any two objects that have mass • Pulls objects together • Depends on the mass of the objects

– more mass = stronger – less mass = weaker

• Depends on distance – farther away = weaker – closer = stronger

• Weight is affect of Earth's gravity upon mass

I can… •state the two factors that affect gravity •explain that gravity affects weight and not your mass •summarize Newton’s 3 laws of motion •use Newton’s 2nd law to solve for force or acceleration

60N

Newton's First Law of Motion (Law of Inertia)

•An object at rest remains at rest unless an unbalanced

force acts on it and causes it to move.

•A moving object will continue to move in a straight line

and at constant speed unless an unbalanced force acts

on it and causes it to change.

•Simplified – unbalanced forces are required to cause

any acceleration

Newton’s 2nd Law

•connects force, mass, and acceleration.

•An object acted upon by a force will accelerate in

the direction of the force.

A = Fnet

m

Fnet = m • Ÿ a

Acceleration (m/s2) = net force (N)

mass (kg)

Net force (N) = mass (kg) •Ÿ acceleration (m/s2)

How much force is required to accelerate a 2000kg

car at 4 m/s2 East?

A 2 kg object is pushed with a 10 N forward force,

how fast will it accelerate?

Newton's 3rd law:

•Forces always act in equal and opposite pairs.

•Action and reaction forces do not cancel out because

they act on different objects.

A 500 kg cannon holds a 1 kg cannon ball.

When the gunpowder ignites, the explosion

provides 1000 N of force in all directions.

What is the acceleration of the cannon and

cannonball?

I can… •summarize Newton’s 3 laws of motion •use Newton’s 2nd law to solve for force or acceleration

•What force is required to move a 4000 kg object at

an acceleration of 3 m/s2 North

•How much will a 200 kg object accelerate when

there is a net force of 40 N South?

If Captain Science's pet magic doggie pushes a 5 kg toy dump truck to the mountain with a force of 100 N Southwest, how fast will the dump truck accelerate?

How much force is required to accelerate a special 120 kg hot air balloon at 4 m/s/s to get Evil Minion's evil cow down to the evil farm?