Chapter 12: Forces and Motion

Objectives:1. Define forces and explain how they affect an

object’s motion2. Relate Newton’s laws to real life circumstances3. Identify the universal forces

12.1 Forces• Force- _________________________________________________

– Causes objects to accelerate

– Vector quantity

– SI unit is the ______________ (N) • 1N= the amount of force that causes a 1kg object to accelerate at

1 m/s²

– Weight is a force • the amount of gravitational force acting on an object’s mass

Combining Forces• Forces can be represented as ________________

– Length of arrow shows the _________________ of the force– The direction of the arrow show the direction that the force is acting

on

• _____________________- the total of all of the forces acting on an object– Use vector addition– Same direction = __________– Opposite direction = __________– Two completely different directions = Pythagorean theorem– Ex: If a car broke down and you had to push the car to the side of the

road what would you do to make the job easier?

• Balanced Forces: net force is __________________.– Forces are acting in equal but _________________ directions

– Ex: tug of war, arm wrestling – neither side is winning it is at a stand still

• Unbalanced Forces: net force is not zero– Produces ________________________

– Ex: tug of war, arm wrestling – one side wins because they exert a greater force

Friction• Force that opposes the motion of objects that touch

1. _______________ Friction- friction force that is acting on objects that are not moving. Always in the opposite direction of the applied force

Ex: You are trying to push a large potted plant across the floor but it is not moving. The static friction between the plant and the floor is opposing the force that you are exerting on it. This would be a balanced force.

2. _________________ Friction- friction force that is acting on a moving object as it slides across a surface.

Ex. You get a friend to help you push the plant across the floor. The plant slides across the floor. This would be an ______________________ force. Note sliding friction has a smaller magnitude than static friction.

3. ____________________ Friction- the force that is acting on rolling objects– Much ______________ than static or sliding friction. This is why it is

easier to move the pot if it were on wheels.

4. ____________________ Friction- the force that resisting the movement of objects through fluids (water) or gases(air).– Fluid friction acts on a spoon as you are trying to stir cake batter.

– Fluid friction acting on object moving through the air is called _____________________________________.• This is why bicyclists were spandex suits and crouch low on the

bike, to reduce air resistance.

Gravity

• A force that acts between __________________________________.

• Gravity doesn’t only pull me to the ground but also attracts me to other nearby objects

• Decreases as distance _____________________

– An object’s ___________________ is determined by gravity

• A person will weigh more in Louisiana than they do at the top of a mountain in Colorado. (The closer to the center of mass the greater the force of gravity)

• This is why you accelerate when you fall (free fall). As you get closer to the center of gravity the force increases causing object to accelerate.

Falling Objects• When an object is in free fall the force of gravity is causing it to accelerate

down (_________________________________________)

• What force opposes the acceleration due to gravity when a person is sky diving and pulls his parachute?

• _______________________________- when the force of gravity is equal to the force by air resistance.

– the object has now stopped accelerating. It has reached the highest velocity that it can go.

Projectile Motion• The motion of an object that was given an initial forward (or

________________________motion) before it started falling.

– Ex: You roll a ball of the end of a table

• Follows a _____________________ path.

• Still falls with an acceleration of _____________________, and therefore will hit the ground at the same time as an object that was dropped straight down.

Newton’s 1st law of Motion• ______________________________- the motion of an object does not

change as long as the net force acting on it is ______________.

• ____________- the tendency of an object to resist a change in motion.

– an object at rest will tend to remain at rest (soccer ball resting in the grass) and an object in motion will remain in motion (car crash) until an unbalanced force acts on it.

– Ex. If an astronaut pushes off the side of the shuttle he will continue to move away from the shuttle indefinitely until a force opposes his motion.

Newton’s 2cd Law of Motion• Acceleration =

• An ________________________________causes an object’s velocity to change, and thus accelerate.

• Ex: If you throw a ball you are exerting a force and the ball will accelerate. The harder you throw the more it will accelerate.

• The ____________________ of the object also determines it’s acceleration.

• If the ball was twice as large and you exert the same force on it, then it will accelerate ________________ the amount.

Practice:1.An automobile with a mass of 1000kg accelerates when the traffic light

turns green. If the net force on the car is 4000 N. What is the car’s acceleration?

Remember: 1N = 1kg x ms²

2.An automobile with a mass of 1200kg accelerates at a rate of 3m/s². What is the net force acting on the object?

Weight vs. Mass

• Weight = mass x acceleration due to gravity• ______________________• Weight is a force, measured in __________• Ex: If an astronaut has a mass of 112kg what is his weight on Earth?

• Ex. What would be the astronaut’s weight on Mars where the acceleration due to gravity = 3.7m/s²?

• Why is Mar’s acceleration due to gravity smaller than Earth’s?

Newton’s 3rd Law of Motion• When one object exerts a force on a second object the second object

exerts a force of ____________________ magnitude but ______________________direction on the first object.

• Ex: You push against a wall and the wall is pushing in the opposite direction as your hand.

______________________________: mass x velocity• Determines how easily a moving object may be _________________• What is easier to stop? A loaded shopping car moving at a speed of 5m/s

or a marble moving at 5m/s

Conservation of Momentum

• If two objects collide then the momentum must be _____________________________. The momentum lost by one object equals the momentum ______________ by another object.

1. Two cars each have a mass of 300 kg. Car 1 speed = 10m/s. Car 2 speed = 5m/s. Car 1 runs into the back of car 2. The cars bounce off one another and continue moving forward at what speeds?

2. Car 1 speed = 10m/s. Car 2 speed = 0m/s. Car 1 runs into the back of car 2. The cars bounce off one another and continue moving forward at what speeds?

3. Car 1 speed = 10m/s. Car 2 speed = 0m/s. Car 1 runs into the back of car 2. The cars stick together and continue moving forward at what speeds?

Universal Forces1. _________________________ Nuclear- holds protons and neutrons in

nucleus of atoms together

2. Electromagnetic- only force that can both ________________________. Opposite charges/poles attract, while same charges/poles repel.

– Seen everyday. Determines how substances react. If you are hold a ball in your hand what stops that ball from melting into your hand?

3. Weak Nuclear- also acts on nucleus and responsible for ______________________________ decay

4. Gravitational- every object in the universe attracts every other object. – The larger the object the _____________________________. – Acts over ___________________ distances, unlike the other 3 forces– Determines the orbit of the moon and tides