1. Prentice Hall Physical Science: Concepts in Action Chapter 12
Forces and Motion
Prentice Hall
Physical Science: Concepts in Action
Chapter 12

2. May the Force be with you
Forces exist all around us
There are 4 types of forces: gravitational, electromagnetic and strong and weak nuclear forces
Which do you think would be the strongest?
Gravity is actually the weakest but has a far reaching effect and nuclear forces (attractions) are the strongest with an effect only on things nearby

3. What is Force?
Force: a push or pull that acts on an object by changing its speed or direction
Can cause a resting object to move
Can accelerate a moving object

4. How is force measured? Spring scale Unit of Force
Stretch of the spring depends on the mass of the object acting on it
Ex: vegetable scale at the grocery store
Unit of Force
Newton (N)
1 kg to accelerate 1 m/s2
(named after Sir Isaac Newton)

5. How is force represented?
Use arrows
Direction
Strength
Length represents
strength or
magnitude
The scale with more apples, greater mass, has a longer arrow. The arrow is pointed downward due to mass is below the balance pulling downwards.

6. Combining Forces Have you ever tried to push a broken down car alone?
Did you ask for help? Why?
Forces in the same direction are added together
Force in the opposite direction are subtracted
Net Force the overall force acting on an object after all the forces are combined

7. Balanced vs. Unbalanced Forces
Combine to produce a net force of zero
No change in the object’s motion
Ex: tug of war, arm wrestling
Unbalanced
Net force equals the size of the larger force minus the size of the smaller force
Net force does not equal zero
Causes an object to accelerate

8. Representing Forces
Forces can add together or subtract from one another
A- 2 forces acting in the same direction add
B- forces in opposite directions subtract from one another
C – forces that are equal in size and opposite in direction result in no net force

9. Friction All moving objects are subject to friction
Friction is a force that opposes the motion of objects that touch as they move past each other
Acts at the surface where objects are in contact
4 types of friction:
Static friction
Sliding friction
Rolling friction
Fluid friction

10. 4 Types of Friction Static friction Sliding friction
Force that acts on objects that are not moving
Always acts in the direction opposite to that of the applied force
Ex: taking a step
Sliding friction
Force that opposes the direction of motion of an object as it slides over a surface
Less force is needed to keep an object moving than to start it moving
Ex: pushing a potted plant

11. 4 Types of Friction Rolling friction
The friction force that acts on rolling objects
Causes change in shape at the point of rolling contact
Ex: furniture
Fluid friction
The force that opposes the motion of an object through fluid
Increases the speed of the object moving through the fluid
Ex: cake batter
Fluids (gas and liquids)
Fluid friction acting on an object moving through the air is known as air resistance

12. Gravity Gravity is a force that acts between two masses
Attractive force that pulls objects together
Earth’s gravity exerts a force of attraction on every other object that is near Earth
Acts downwards towards the center of the earth
Because the boulder is at rest, what do you
Know about the net force acting on it?

13. Gravity and Falling Objects
Gravity causes objects to accelerate downward
Air resistance (fluid friction) acts in the direction opposite to the motion and reduces acceleration
The flying squirrel to the right takes advantage of air resistance to slow its fall by increasing it’s surface area.
Terminal velocity is the constant velocity of a falling object when the force of air resistance equals the force of gravity. (in other words, top speed)

14. Projectile Motion
Projectile Motion: motion of a falling object after given an initial forward velocity
Air resistance and gravity are the only forces
The combination of an initial velocity and the downward vertical force of gravity causes the ball to follow a curved path
The 2 balls fall with the same acceleration and hit the ground at the same time

15. 12:2 Newton’s 1st Law of Motion
Sir Isaac Newton studied motion and gave us the basic laws of motion
The 1st Law of Motion states that the motion of an object does not change as long as the net force acting on the object is zero.
Sometimes also called the Law of Inertia
Inertia Tendency of an object to resist change in its motion
In other words… An object at rest stays at rest, an object in motion stays in motion at the same direction and speed (until something acts on it)

16. Newton’s 1st Law of Motion
Crash Dummy: Example of inertia

17. Newton’s 2nd Law of Motion
How do unbalanced forces affect the motion of an object?
It causes the velocity to change (accelerate)
Ex: you apply a net force to a ball when you throw it
The harder you throw, the more the ball accelerates.
The acceleration is directly proportional to the net force acing on it.
Newton’s Second Law of Motion The acceleration of an object is equal to the net force acting on it divided by the objects mass

18. Newton’s 2nd Law of Motion
Mass is a measure of inertia of an object and depends on the amount of matter the object contains
If you double the mass of an object, it cuts the acceleration in half
The acceleration of an object is always in the same direction as the net force
Newton’s 2nd Law applies when a net force acts in the opposite direction of object’s motion
Force produces deceleration and reduces speed
Ex. Seat belts
Force decelerates the passenger in order to prevent injury
Units for Acceleration are equivalent (mean the same thing)
N/kg=m/s2

19. Newton’s 2nd Law of Motion

21. Weight and Mass Are weight and mass the same?
Weight & Mass are Different
Weight The force of gravity acting on an object
Product of the mass and acceleration due to gravity
Unit is Newtons (N)

22. Weight and Mass

23. Newton’s 2nd Law of Motion
How would the acceleration of a chain of two carts compare with the acceleration of a single cart if the same force acted on both?
When the same force acts, the single cart accelerates 8x faster than the chain of eight carts
As mass increases, acceleration has to decrease in proportion to the increase in mass

24. Newton’s 2nd Law of Motion
How would the force have to change in order to have the same acceleration for the eight carts as for one cart?
The force would have to be 8x greater
How would another force directed to the left on the cart affect the cart’s acceleration?
The acceleration would depend on the net force.
The net force would be the force acting to the right minus the force acting to the left.

25. Newton’s 2nd Law of Motion
1.A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N?
2.What is the upward acceleration of a helicopter with a mass of 5000 kg if a force of 10,000 N acts on it in an upward direction?
a=F/m
= N/5000 Kkg
= 2 m/s2
a=F/m
= 60.0 N/40.0 kg
= 1.50 m/s2

26. Newton’s 2nd Law of Motion
3.An automobile with a mass of 1200 kg accelerates at a rate of 3.0 m/s2 in the forward direction. What is the net force acting on the automobile? (Hint: Solve the acceleration formula for force.)
a=F/m F=ma
= 1200 kg(3.0 m/s2)
= 3600 N
4.A 25-N force accelerates a boy in a wheelchair at 0.5 m/s2 What is the mass of the boy and the wheelchair? (Hint: Solve Newton's second law for mass.)
a=F/m m=F/a
= 25 N/0.50 m/s2
= 50 k/=g

27. Newton’s 3rd Law of Motion & Momentum
Newton’s 3rd Law – when an object exerts a force on a second object, that object exerts an equal and opposite force on the first object
A force cannot exist alone. Forces always exist in pairs.
These are called action and reaction pairs (forces)

28. Action and Reaction Forces
Pressing your hand against a wall produces a pair of forces
Your hand exerts a force on the wall – the action force
The wall exerts an equal and opposite force against your hand – the reaction force
Using a hammer to drive a nail into a piece of wood produces force pairs
Hammer applies force to the nail
Action forces drives nail in wall
Nail exerts equal and opposite force
This force brings the motion of the hammer to a stop

29. Action and Reaction Forces Do Not cancel
These forces that are produced by the two objects are action and reaction forces.
Sometimes action and reaction forces result in motion – sometimes they don’t.
Action and reaction forces do NOT cancel themselves out.
Only when equal and opposite forces act on the same object do they result in a net force of zero

30. Momentum Momentum is the product of an object’s mass and its velocity
An object at rest has a momentum of zero.
Momentum = Mass x Velocity
Measured in kilogram-meters/second (kg m/s)
p = mv (p is momentum)
An object has a large momentum if the product of it’s mass and velocity is large

31. Law of Conservation of Momentum
What happens to momentum when objects collide?
Momentum is conserved – the total momentum does not increase or decrease
If no net force acts on a system, then the total momentum of the system does not change
A ‘system’ is part of the world under investigation (ie two cars colliding)
The rest of the world around the system is called the ‘surroundings’
A closed system of objects means that there are no outside forces or objects acting on the objects in the system.
In a closed system, the loss of momentum by one object equals the gain of momentum by the second object.

32. Law of Conservation of Momentum

33. Universal Forces- Electromagnetic
Observations of planets, stars or other galaxies suggest that the four forces exist throughout the universe
Electromagnetic force is associated with charged particles
Electric and magnetic force are 2 different parts of the electromagnetic force
Electric and magnetic are the only forces that both attract and repel

34. Electric and Magnetic Forces
Electric forces act between charged objects such as electrons or protons
Objects with opposite charges attract
Objects with like charges repel
Clothes often acquire electric charges in the dryer
Clothes with opposite charges tend to cling together
Magnetic forces act on some metals, poles of magnets and moving charges

35. Nuclear Forces
If the nucleus of an atom has protons (+) packed into a small space, why don’t they repel each other and break apart the nucleus?
Two forces, the strong-nuclear force and the weak-nuclear force act within the nucleus to hold it together.
The strong NF overcomes the electric force of repulsion between the protons in the nucleus
Acts only on neutrons and protons. 100x strong than repulsive forces
The weak nuclear force is involved in radioactive processes and is weaker than the strong NF

36. Gravitational Force
Gravitational force an attractive force that acts between any two masses
Weakest universal force
Most effective over large distances
Newton’s law of universal gravitation
Says that every object in the universe attracts every other object
The greater the mass of an object, the greater the gravitational force.
The more distance between two objects, the less the gravitational force
Gravitational forces quicklab

37. Centripetal Force
How is the moon kept in orbit around earth? It has inertia and should move in a straight path until acted on by a force
Earth’s gravitational force continuously pulls the object toward it
Centripetal force: center-directed force that constantly changes the direction of an object and causes it to move in a circle

38. Centripetal Force
If the Earth causes the moon to stay in a circular orbit, what force does the moon apply on the earth?
Gravitational pull from moon causes two bulges in earth’s oceans
Earth rotates 2x a day below these bulges resulting in two high and two low tides per day on Earth
Examples affected by centripetal force:
Satellites
Examples (there are too many to list) of satellite uses include:
communication
tracking Earth’s weather and climate