An overview

What is Force?

• Force is a push or pull• Force acts in a certain direction• There are many examples of force in

nature:– Electrical force– Magnetic force– Gravitational force– Weight

What is Gravity?• Gravity is a force of attraction between all

objects in the universe.

What is Gravity:

Gravity, Mass and Distance

• The amount of gravitational force of an object depends on its mass– An object with more mass has more

gravitational force• The amount of gravitational force of an

object depends on its distance– The force of gravity between 2 objects

decreases as the distance between them increases

Measuring Force

• Force is measured using a NEWTON– 9.8N are needed to lift a 1kg mass on Earth– Mass does not change, but weight depends

on gravity• You weigh less on the Moon than you do on Earth,

but your mass does not change

Mass and Weight

Air Resistance and falling objects

• Air resistance is the force that opposes the movement of an object in the air– Lighter objects feel more air resistance than

heavier objects– Objects with greater surface area feel more

resistance than objects with smaller surface area

• Terminal Velocity is the speed at which air resistance and gravity acting on a falling object are equal

Falling Objects

Free Fall

• There is no air in a Vacuum • There is no air resistance in a vacuum• In a vacuum, all objects fall at the same

speed

weightlessness

The law of Universal Gravitation

• Inertia is the tendency of an object to stay at rest or in motion

• A Force is a push or pull needed to move or stop an object

Newton’s First Law of Motion:–An object at rest will remain at rest

or move at a common speed in a straight line unless it is acted on by an unbalanced force

Friction• To slow down a

moving object, a force must act in the direction opposite to that of the motion. This is called:

• Friction – a force that opposes the motion of an object

There are different types of Friction.

For examples:• Sliding friction

– Like a book sliding across a table

• Air resistance– A parachute slowing

down a sky diver• Rolling friction

– Bike wheels on the road

To summarize Friction

• Friction makes motion possible.• Friction makes stopping or slowing down

possible.• Friction can cause heat• Friction can wear things down

Inertia must be overcome in order to move an object at rest or stop an object in motion. The force that slows things down, produces heat, and wears things down is friction.

Reducing Friction

• Friction makes it hard to move an object.• To overcome friction takes force.• Reducing friction makes it easier to move

an object

Reducing Friction• There are many ways to reduce friction.

For example:•Use a Lubricant – a substance used to reduce friction.

•Change sliding friction to rolling friction by using wheels

• In addition to changing the amount of force needed to move an object, sometimes we want to change its direction.

• The change of motion of an object

depends on the mass of the object and the force applied.

Newton’s 2nd Law- force, mass and momentum

The formula for the 2nd law

Force equals mass times accelerationor F = MxA

Reminder:• Force is measured in Newtons (N)• Mass is measured in Kilograms (kg)or grams (g)• Acceleration is measured in meters/second

(m/s)

Solve these 1. What force is needed to accelerate a 2-kg

mass at 1 m/sec/sec?

2. How hard would you have to push a 50kg skater to increase her speed by 2 m/sec/sec?

3. What is the mass of an object if a force of 10N causes it to accelerate at 5 m/sec/sec?

Write:

• Make up 3 problems based on the formula F=MA