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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.

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

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FORCES

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

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**What does a force do? Gives energy to an object**

Can set an object in motion, make it stop, or change its motion

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**Unbalanced forces change motion**

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**Unbalanced forces change motion**

The truck motion is changed, but the ladder keeps going

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**Unbalanced forces change motion**

crosswind tailwind headwind

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**What are forces represented by? Forces are represented by vectors**

motion of book friction

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**Forces are added with vectors**

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**What is friction? sliding fluid rolling**

A force which opposes motion, bringing objects to rest Types: sliding fluid rolling

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Inertia

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**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

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**Inertia keeps astronauts in motion**

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**Seat belts protect against inertia**

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Gravity

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Gravity The attractive force between all objects in the universe

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**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

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**The earth exerts a force on the student**

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

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**Gravity warps the fabric**

of space-time

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Gravity

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

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**Newton’s Laws of Motion**

1. Inertia 2. F=ma 3. Action-reaction

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**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.

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**Inertia keeps astronauts in motion**

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**Inertia keeps satellites in orbit**

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**Inertia keeps you at rest**

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**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.

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**Which cars are accelerating?**

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

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**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?

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**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?

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**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?

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

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**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.

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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?

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**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

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**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

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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

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**Free fall without air resistance**

Both the elephant and the feather land at the same time!

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**Free fall with air resistance**

The feather takes lots longer to hit the ground!

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

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**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.

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**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.

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Projectile motion

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

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

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**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?

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**Package dropped from plane**

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

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**Monkey in gravity-free**

environment

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With gravity- monkey lets go when banana is fired

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with gravity- cannon shoots at slow speed

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**Satellites Projectile with no gravity with gravity- launched**

at < 8000m/s with gravity- launched at > 8000 m/s

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The End

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