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## What are Forces?

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### What are Forces?

Module 3

What are forces?

- In module 2 we learned that a force is a push or a pull.
- We learned that Newton’s 2nd Law states that a Force is equal to the mass of a moving object times its acceleration.
- We learned that Newton’s 3rd Law states that for every force there is an equal and opposite reaction force.

Newton’s Second Law of Motion

- Force = mass X acceleration
- F = ma
- Force and acceleration are directly proportional. The greater the force, the greater the acceleration.
- Mass and acceleration are inversely related. The greater the mass of an object the less the acceleration if the same force is applied.

What is friction?

- Friction is a force which opposes motion.
- Friction is an important force in our lives. We rely on friction in many ways. An athlete usually wears shoes which provide him or her with a greater friction between the shoe and the surface. We rely on friction as an important aspect of our motion. In what other ways does friction play a role in our everyday lives?

What causes friction?

- There are two factors which affect friction between two surfaces:
- Kind of surfaces in contact (rough or smooth)
- Amount of force pressing the surfaces together.

The rougher the surface and the stronger the force between the surfaces, the greater the amount of friction.

What is static friction?

- If you were trying to move a box like pictured below and the box did not move, this would be static friction.
- Static friction is friction between two surfaces which are not moving past each other.

What is sliding friction?

- Sliding friction occurs when a force is great enough to overcome the static friction.

- What is rolling friction?
- Rolling friction is the friction which enables wheels to turn and objects to move. If there were no friction, turning wheels would not enable an object to move.

Air Resistance – another type of friction??

- Air resistance is a force which opposes a moving object. This can be easily seen with falling objects. When a piece of paper and a crumple piece of the same type of paper are dropped, they fall at the same rate.
- A penny and a feather dropped in air will not fall at the same rate, but a penny and feather dropped in a vacuum (no air) will.

What is terminal velocity?

- Forces cause objects to accelerate (2nd Law).
- When the force of gravity on a falling object equals the force of the air resistance going against gravity, the forces balance out and the object stops accelerating.
- The object will travel at a constant velocity – the terminal velocity.

What is Gravity?

- Gravity is a force of attraction between two objects.
- Law of Gravitation – any two masses exert an attractive force on each other. The amount of attraction depends upon two things: the mass of the objects and the distance between the objects.

What is gravitational acceleration?

- When objects fall they accelerate toward the ground.
- Using Newton’s 2nd Law we can see this relationship: F = ma
- The force caused by gravity acts upon a certain mass to cause it to accelerate toward the ground at a constant rate.
- Acceleration due to gravity is 9.8 m/s2

Gravity causes weight

- The weight of an object is caused by the force of attraction between the earth and objects on the surface of the earth.

Gravitation force is equal to weight, therefore…

How much does a person with a mass of 70.0 kg weigh on the earth?

Weight = mass X 9.8 m/s2

Weight = 70.0 kg X 9.8 m/s2

Weight = 686 N

Remember that 1 N = (1 kg) (1m/s2)

What is weight?

- When an astronaut moves away from the earth they “lose” weight. Weight is caused by the force of attraction between the earth and objects on its surface. The greater the distance an object has from the earth, the less the force of attraction which would exist.

What is the difference between mass and weight?

- Mass is the amount of matter in an object. This does not change in an object as it moves away from the earth.
- Weight is caused by gravity. Therefore, the greater the distance, the less the force of attraction. The less the distance the greater the force of attraction.
- Your weight is dependent upon the mass of the earth. If you were able to go to the planet, Jupiter, your weight would be 27 time greater, because the mass of Jupiter is 27 times greater.

What causes “weightlessness”?

- Even in the space shuttle, there is a force of attraction exerted by the earth on the shuttle and its contents.
- When the space shuttle temporarily “falls” toward the earth, the contents of the space shuttle appear to be weightless, but in fact they are falling with the space shuttle. This is called “freefall”.

When an object is thrown into the air, two forces and two motions affect the movement of the object.

The object may be thrown horizontally with a certain force, but the vertical force of gravity combines to affect the motion.

Slow projectile - shoot a monkey

Fast projectile - shoot a monkey

What is centripetal force?

- “centripetal” means to move toward the center.
- Centripetal acceleration is the acceleration of an object toward the center of a curve or circular path.
- Centripetal force is a force toward the center of a circle which holds the moving object in its circular path.

What is Newton’s 3rd Law of Motion

- For every action there is an equal but opposite reaction.
- When one object exerts a force on a second object, the second one exerts a force on the first that is equal in size and opposite in direction.
- Action-Reaction Law

Two objects in contact with one another and moving must use the action-reaction law.

Rocket propulsion involves the action-reaction law. The explosion from one end of the rocket causes a force in that direction. This causes an unbalanced force in the opposite direction causing the rocket to accelerate. Video Clip!

What is momentum?

- The momentum of an object is equal to the product of its mass and its velocity. The unit for momentum is kg m/s

What is the Law of Conservation of Momentum?

- When object collide the momentum of one object is transferred to the other object.
- View this video to see a demonstration of conservation of momentum.
- Notice that the soccer ball has greater momentum since it has a larger mass. Its momentum is transferred to the smaller ball which has a smaller mass and will therefore have a larger velocity. Try this with a soccer ball and a tennis ball!

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