# Forces Standard: S8P3 b. Demonstrate the effect of balanced and unbalanced forces on an object in terms of gravity, inertia, and friction.

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Forces Standard: S8P3 b. Demonstrate the effect of balanced and unbalanced forces on an object in terms of gravity, inertia, and friction.

What are forces? Inertia Gravity Newton’s Laws Projectile motion

FORCES

What is a force? A push or a pull May be unbalanced or balanced

What does a force do? Gives energy to an object
Can set an object in motion, make it stop, or change its motion

Unbalanced forces change motion

Unbalanced forces change motion
The truck motion is changed, but the ladder keeps going

Unbalanced forces change motion

What are forces represented by? Forces are represented by vectors
motion of book friction

What is friction? sliding fluid rolling
A force which opposes motion, bringing objects to rest Types: sliding fluid rolling

Inertia

What is inertia? I have more inertia but we fall at the same rate
The tendency of an object to stay at rest or in motion. Objects with greater mass have greater inertia but we fall at the same rate

Inertia keeps astronauts in motion

Seat belts protect against inertia

Gravity

Gravity The attractive force between all objects in the universe

F = M1 M2 d2 More mass means greater gravity
More distance means less gravity My gravitational attraction is small no matter what the distance! Massive objects have lots of gravity

The earth exerts a force on the student
and the student exerts a force on the earth because of GRAVITY!

Gravity warps the fabric
of space-time

Gravity

gravity pulls it in inertia keeps it going What forces are acting on the moon? Inertia

Newton’s Laws of Motion
1. Inertia 2. F=ma 3. Action-reaction

Newton’s First Law of Motion
The law of inertia: An object at rest stays at rest an object in motion stays in motion unless acted upon by an outside force.

Inertia keeps astronauts in motion

Inertia keeps satellites in orbit

Inertia keeps you at rest

Newton’s Second Law of Motion
Force = Mass x acceleration A force is needed for acceleration Acceleration will occur in the direction of the force.

Which cars are accelerating?
If you watch carefully, the green and blue cars are accelerating. The red car is traveling at constant speed.

A force is needed for acceleration
Will these accelerate? A skier going downhill? What force is acting on the skier? An astronaut on a spacewalk? Are any forces acting on the astronaut?

Which of the following will accelerate?
A bullet shot from a gun? What forces are acting on the bullet? A hammer and feather dropped on the moon? What force is acting?

Newton’s 2nd Law F_____ M A
Force = mass x acceleration F_____ M A Force is measured in newtons. A newton is a kg m s2 Example: A car with mass 1000kg hits a tree and decelerates at 50m/s/s. What force does it exert on the tree?

Answer: F_____ M A 1000kg * 50m/s/s = 50,000 kgm or 50,000N s2

F = ma A car is traveling at constant speed. Is a force acting on it?
Answer: although there are forces acting on the car (gravity, friction, air resistance,etc…) since there is no acceleration, the net force in the direction of motion must be zero.

More 2nd Law Problems: 1. What is the acceleration on a mass of 50kg if a force of 10N is applied? 2. An object accelerates due to gravity at a rate of 10m/s/s. If its mass is 15kg, what force is acting on the mass?

Answers: _F_ M A 1. _10N_ 0.2 n/kg= 0.2 kg m = 0.2 m/s2 50kg A s2/kg
15kg 10m/s2 150 kgm/s2 or 150N

Gravity is a force that causes an acceleration
On earth, ALL objects accelerate at 9.8m/s2 (ignoring air resistance) because of gravity. No matter what the mass, ALL objects on earth accelerate at 9.8 m/s2

Newton’s 2nd Law F__ m a An object of mass 3kg falls off a cliff. How much force is acting on the object? Answer: F_______ 3kg m/s2 29.4N

Free fall without air resistance
Both the elephant and the feather land at the same time!

Free fall with air resistance
The feather takes lots longer to hit the ground!

Newton’s Third Law For every action, there is an equal and opposite reaction

For every action, there is an equal and opposite reaction
In rocketry…… The gases being expelled is the action. The rocket propelling forward is the reaction.

How do Newton’s laws describe the motorcycle rider?
1st: Inertia keeps the rider moving forward. 2nd: The force with which the rider hits the ground will be equal to his mass times his deceleration. 3rd: The action of hitting the ground will equal the reaction of the dirt.

Projectile motion

Projectile motion An object moves forward due to inertia and accelerates downward due to gravity.

In this video, the ball is dropped straight down and moves forward due to inertia

Projectile motion of a ball fired vertically
The motion of the truck causes the ball to project forward. What would happen if the truck stopped?

Package dropped from plane
The forward motion of the plane causes the package to land in front of where it’s dropped.

Monkey in gravity-free
environment

With gravity- monkey lets go when banana is fired

with gravity- cannon shoots at slow speed

Satellites Projectile with no gravity with gravity- launched
at < 8000m/s with gravity- launched at > 8000 m/s

The End

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