Physics Unit Four

Forces that Affect Motion

Force

• A push or a pull.• Measured in newtons with a spring scale.• 1 newton (N) = 1 kg m/s2

• An apple weighs about 1 newton.• Calculate your weight in newtons:

Weight in pounds = _____ 2.2 lbs/kg

Mass in kg = _____ x 9.8m/s2 =

Weight in newtons _____ N

Forces that affect motion• Fa Applied Force –a force exerted on an object by

other objects in it’s environment.• Ff Frictional Force – a force that opposes

motion between the object and the surface it rests upon• Fg Gravitational Force – the force of attraction

between two objects with mass (ex. Between an object and the Earth, for example)

• Fn Normal Force – the supportive force of the surface an object rests upon. It is equal to the force of gravity acting on the object.

Normal means “perpendicular”

If forces are balanced…No Motion

No net (unbalance) forces.

FgFn

Ff

Ff

Fg

Fn

If forces are balanced…No Motion

• No net (unbalance) forces.

Fg Fn Fg

Fn 50N, up

50N, down

If forces are unbalanced…Motion

• Net (unbalanced) force.

Fg

Fn

Direction of motion

Fn

Direction of motion

Fg

Force Equations

• Acceleration– According to Newton’s 2nd Law of motion, the

acceleration of an object is equal to the net force acting on it divided by the object’s mass

– Formula:

A = F / m

– Therefore, doubling the mass of an object cuts its acceleration in half

– The unit for acceleration is m/s2

• Force – Force on an object is equal to its mass and

acceleration– Formula:

F= m(a)

Force Equations

• Weight– Weight is the force of gravity acting on an

object– Formula:

W= m(g)

– The unit for weight is also newtons because it is a force

Force Equations

• Momentum– Momentum is the product of an object’s mass and its

velocity– An object has a large momentum if the product of its

mass and velocity is large.– Formula:

Momentum (P)= m(v)

– The unit for momentum is kg*m/s

Force Equations