ForceForceA Push or a PullA Push or a Pull

A Push or a Pull• A force is a push or a pull.

• Force is measured in newtons (N)– You exert about 1 N of force when you lift a

lemon

• Often more than one force is acting upon an object at one time. The combination of all forces acting on an object is called net force. – The net force determines if an object moves

and in what direction.

Net Force

Unbalanced and Balanced Forces

• Unbalanced forces acting on an object result in a net force and cause a change in the objects direction.

• Balanced forces acting on an object do not change the object’s motion.

Friction• Friction is the force that two surfaces exert

on each other when they rub against each other.– The strength of the force depends on two

factors• How hard the surfaces are• The types of surfaces involved

Types of Friction

• Static Friction– Friction that acts upon objects that are not

moving, it opposes the intended motion

• Sliding Friction– When two solid surfaces slide over each

other

• Rolling Friction– When an object rolls over a solid surface

• Fluid Friction– When a solid object moves through a fluid

Which is Which?

Gravity• Sir Isaac Newton concluded that a force acts to

pull objects straight down to the center of the Earth.– gravity is the force that pulls objects together– Newton realized that the force of gravity acts

between all objects in the universe • This is called the law of universal gravity.

Gravity• Two factors affect the gravitational attraction

between objects:– Mass-the amount of matter in an object (kg)

• The more mass, the greater its gravitational pull– Distance

• The further apart, the less gravitational pull

Weight and Mass

• Mass is sometimes confused with weight– Mass is the amount of matter in an object– Weight is a measure of the gravitational force

exerted on an object

• Weight varies depending on the strength of the gravitational force, while mass does not change

Did you know…you weigh less on the moon than you do on Earth?

Gravity and Motion

• When gravity is the only force acting on an object it is in free fall

• Objects falling through air experience a type of fluid friction called air resistance.– It is the upward force exerted on falling objects

Projectile Motion

• An object that is thrown is called a projectile.• Does it hit the ground at the same time as an

object that was dropped?????