What is Force?

What is Force?• What is force?• A push or pull on an object that changes the motion of

the object.• Force is measured in Newtons• SI unit symbol is N• 1 N = the gravity on Earth of 100g of mass

• There has to be something receiving the force as well as something applying the force.

What is Force?

• You can not always see the force being exerted– Gravity– Static electricity– Others

Net Force

• The combination of all the forces acting on an object

• Example

4 N

4 N Net Force = 8N

Net Force

• The combination of all the forces acting on an object

• Example

4 N 1 N

Net Force = 3N

Balanced Forces• The combination of all forces acting on an

object.• Example

4 N

4 N

4 N

4 N

4 N

4 N

4 N

4 N

Net Force = 0 N

Balanced Forces• The combination of all forces acting on an

object.• Example

5 N

8 N

10 N

5 N

10 N

8 N

4 N

4 N

Net Force = 0 N

Unbalanced Forces• The combination of all forces acting on an

object are not equal.• Example

4 N

4 N

4 N

5 N

4 N

4 N

4 N

4 N

Net Force = 1 N

Unbalanced Forces• The combination of all forces acting on an

object are not equal.• Example

5 N

6 N

10 N

5 N

10 N

6 N

4 N

8 N

Net Force = 4 N

Unbalanced Forces• The combination of all forces acting on an

object are not equal.• Example

5 N

6 N

10 N

1 N

10 N

6 N

4 N

8 N

Unbalanced Forces

• Unbalanced forces can cause a change in motion

What is Friction?

• Friction is the force that opposes motion• Friction occurs when two objects slide over

each other• The amount of friction depends on several

factors– The roughness of the surfaces– The force pushing the surfaces together

What is Friction?

• The amount of friction depends on several factors– The roughness of the surfaces– The force pushing the surfaces together• Includes the downward force and the horizontal

force• .

Types of Friction

• There are two types of friction–Kinetic• Kinetic friction is the force that tries to

slow a moving object down– Static• Static friction is the force that tries to

prevent an object at rest, from moving

Overcoming the affects of Friction

• There are other types of friction–Sliding• Sliding friction happens when two objects

slide over each other• Example–Pushing a crate across the floor

Overcoming the affects of Friction

–Rolling• Rolling friction happens when an object

slides on rollers or wheels• Example–Skates, –a wagon, –markers under a book

Overcoming the affects of Friction

– Fluid friction• Fluid friction happens when an object

slides over a surface that has a layer of fluid on it• Example–Water on the floor–Oil in an engine.

Gravity

• Isaac Newton• 1665 under an apple tree

Gravity

• Isaac Newton– Two questions?• Why do things fall to the Earth?• Why do the planets stay in motion?

Gravity

• Gravity–The force of attraction between two

bodies–All matter has mass, therefore all

matter has gravity

Gravity

• The Law of Universal Gravitation– Gravitational force increases as mass increases– Gravitational force decreases as distance increases

Gravity

• Gravity and Motion– Galileo dropped two cannon balls from the tower at

Pisa .• Each cannon ball had a different mass• They both hit the ground at the same time

–Gravity accelerates all objects at the same rate• 9.8 m/s/s (4.9m/s first second and 9.8 every second after)• .

V = 0m/s

V = 9.8m/s

V = 19.6m/s

V = 29.4m/s

4.9m

14.7m

24.5m

Gravity

–Gravity accelerates all objects at the same rate• 9.8 m/s/s (4.9m/s first second and

9.8 every second after)–Air Resistance• Similar to friction in that it opposes

the motion of a falling object in air

Gravity

–Terminal Velocity•When the force of air resistance is equal to the force of gravity and a falling object no longer accelerates

Gravity

–Terminal Velocity . . When the force of air resistance is equal to the force of gravity and a falling object no longer accelerates –Free fall • Only the force of gravity is pulling on an

object

Gravity

–Free fall .• Only the force of gravity is pulling on an object

–Orbits . . .• A combination of forward motion and the

downward pull of gravity

–Projectile Motion• The curved path that an object takes when thrown

or launched

Projectile MotionThe curved path that an object takes when thrown or launched