Forces

What is a force?• Forces are described by their strength and

direction.

What is a force?

• Forces are a push or a pull.

• One object can exert a force on another

object.

Units of Force

• The SI unit of force is the

newton (N), named for Issac

newton.

Measuring Forces

30 N

• Arrows always point in the direction of the force. The longer the arrow…the

greater the force.

5 N

Measuring Forces

7 N 7 N 14 N

8 N 5 N14 N

• Forces are added when they go in the same direction. They are subtracted when they go in opposite directions. The result

is a net force.

Measuring Forces

7 N 7 N0 N

• In this case both forces cancel each other out because they are equal.

There is NO net force.

Unbalanced Forces

• Unbalanced forces can cause an object to start moving, stop moving or change

direction.

• Unbalanced forces acting on an object result in a net force and cause a change in

the objects motion.

Unbalanced Forces

• This net force causes the box to move to the right.

Unbalanced Forces

• Two equal forces result in no net change – the box does not move.

Unbalanced Forces

• Two equal forces result in no net change – the box does not move.

Balanced Forces

• Equal forces acting on one object in opposite directions.

• They do not change the objects motion.

Friction and Gravity

Friction

• The force that two surfaces exert on each other when they rub against

each other is friction.

• There are a few types of friction.

Friction

The strength of the force of friction depends on two factors:

• How hard the surfaces push together.• The type of surface involved.

Friction and Surfaces

• Some things have a low coefficient of friction, like

ice. It is smoother the coefficient is low.

• If it is bumpy or rough there is a high coefficient

of friction.

Types of Friction: Static Friction

• Static friction is a type of friction that acts on objects that are NOT moving.

• It takes extra force to move something that is not already moving.

• Must exert a force GREATER than the static friction.

Types of Friction: Static Friction

It really takes a lot of force to get past the amount of static friction here!

Types of Friction: Sliding Friction

• Sliding friction occurs when two solid surfaces slide over each other.

A ballet dancer will put powder on their slippers to prevent the sliding friction

from occurring.

Types of Friction: Rolling Friction

• When an object rolls across a surface – rolling friction occurs.

A skateboarder is demonstrating rolling

friction…how?

Types of Friction: Fluid Friction

• Fluid friction occurs when an object pushes a fluid.

A surfer’s board is helping them overcome the force of fluid

friction.

Gravity

• Gravity is a force that pulls objects toward each other

It is also a force that can be

measured. On Earth Gravity is

different than on the moon.

Factors affecting gravity

• Two factors that affect the

gravitation attraction between objects are MASS

and DISTANCE

Gravity

The force of gravity acts between all objects

If mass increases, the force of gravity increases.

Gravity

If distance increases, the force of gravity decreases.

Mass VS Weight

• Mass is a measure of the amount of matter in an object.

• Weight is a measure of the gravitational force exerted on an object. Weight = Mass X Gravity

Free Fall

• When the only force acting on an object is gravity, the object is

said to be in free fall.

• In free fall, the force of gravity is an

unbalanced force. This causes an object

to accelerate.

Acceleration Due to Gravity

• Near the surface of Earth – acceleration due to gravity is 9.8

m/s

Acceleration Due to Gravity

• All objects in free fall accelerate at the same rate regardless of their masses.

• These rocks and this feather would fall at the same rate!

Air Resistance

• Air resistance creates a type of fluid friction

Air Resistance

• Air resistance increases with more surface area.

• Air resistance increases with increasing velocity.

• The greatest velocity a falling object reaches is called its terminal velocity. This is reached when the force of air resistance equals the weight of

the object.

Projectile Motion

An object thrown is called a projectile.

The force of gravity will act on the projectile.

At first it goes upward, but eventually gravity pulls it back to the ground.

Projectile Motion