Forces in one Dimension

Chapter 4

Force: push or pull. It is a vector.

Contact Force: when object touches system(friction, tension, spring,normal...)

Field Force: exerted without contact( gravitational, magnetic, electric)

Net Force: Vector sum of all forces on object.

Net

First: An object at rest will remain at rest, and an object that is moving will continue to move in a straight line at the same speed, if the net force acting on the object is zero.(Inertia).

Second:

Third:

Newton’s Laws

F = maFA on B = −FB on A

FT = Force of TensionFF = Force of FrictionFN = Normal(90 deg rees) contact ForceFg = Force of Gravity

Net Force

5N 8N

Fnet = 8 − 5 = 3N

If the forces are at an angle to each other you need to split the forces into x and y components. It’s your choice about

the x and y directions.

If we have a slope x is along slope.y is perpendicular to slope.

Using Newton’s Second Law

If you know the mass, you also know the weight

Weight = mass x acceleration

If you know the weight, you also know the mass

Fnet = ma

net force

Newton’s Second Law

Gravity

• Earth’s gravity acts downward toward the center of the Earth.

• Gravity causes objects to accelerate downwards, whereas air resistance acts in the opposite direction to motion.

If two forces are perpendicular you have to use Pythagoras to find the net force.

Say the mass of the bucket is 10kg

Weight = Fg = mgFg = 10(9.8) = 98N 98N

Say the acceleration is +2m/s2 upward

This tells us the net force Fnet = maFnet = 10(2) = 20N

20N

We now know the tension in the rope

118N

M = 50kg M = 70kg

a = 4m / s2 towards the boy

N’s third law says that action &reaction are equal and opposite

So, the force toward the boy equals the force toward the girl

Fboy = maFboy = 70(4) = 280N

Fgirl = ma280 = 50(a)5.6m / s2 = a

FrictionFN = normal force

Fg = mi9.81

FFfriction

Static Kinetic

Fs = µs iFN

Fk = µk iFNThis gets bigger and bigger until it reaches a maximum and the object

moves.

Fg

FNThis is your basic

slope diagram.If the object is

moving down the slope you will need to put in

friction going up the slope

Adj. =Fg Cos

Opp.=Fg Sin