1. Chapter 2 Physical Science
The Laws of Motion
Chapter 2
Physical Science

2. Ch2 L.1 Gravity and Friction
What are some contact forces and some noncontact forces?
What is the law of universal gravitation?
How does friction affect the motion of two objects sliding past each other?

3. Ch2 L.2 Bellwork Force: a push or pull on an object
Contact force: a push or pull on one object by another that is touching it
Noncontact force: a force that one object can apply on another without touching it
Gravity: an attractive force that exists between all objects that have mass
Mass: the amount of matter in an object
Weight: the gravitational force exerted on an object
Friction: a force that resists the motion of two surfaces that are touching

4. Types of forces Contact force Noncontact force
One object applies force to another MUST be touching
Can be weak or very strong
Noncontact force
Gravitational force
Magnetic force
Electric force

5. Forces Have strength and direction Unit= Newtons
In arrows, longer arrow= stronger force
Unit= Newtons
1 N = 1 kg⋅m/s2

6. Gravity Gravity: attractive force between objects that have mass
Mass: amount of matter in an object (in kg)
Earth exerts attractive force on you and you exert attractive force on Earth

7. Law of universal gravitation
Sir Isaac Newton (1600’s English scientist and mathematician) developed this law
All objects are attracted to each other by gravitational force
How strong the force is depends on the mass of the objects

8. Law of universal gravitation cont’d
Gravitational force and mass
The greater the masses of the objects, the greater the gravitational force between them
EVEN if the objects have DIFFERENT masses, the force between them is the SAME for each one
Gravitational force and distance
The force decreases as the distance between the objects increases

9. Weight Weight: gravitational force exerted by an object
Biggest object near us: Earth.
On Earth, an object’s weight is the gravitational force exerted on the object by the Earth
Weight is proportional to mass. If mass doubles, so does the weight.
Mass is constant. Weight depends on location.

10. Friction
Friction: force that resists the motion of two surfaces that are touching
Three types
Static
Sliding
Fluid

11. Static friction Prevents surfaces from sliding past each other
Static means not moving
Static friction equals the applied force up until the limit
THIS MEANS THE STATIC FRICTION CAN CHANGE
Applied force needs to be greater than the static friction in order to get something to move

12. Sliding friction
Once you overcome static friction, the object will move
Opposes the motion of surfaces sliding past each other
DOES NOT CHANGE (as long as the object is sliding)
Increasing applied force just makes box move faster
Stop pushing= box slows down and stops moving because of sliding friction

13. Fluid friction Friction between a surface and any fluid that flows
water, air, oil, etc.
Resistance between surface and air= air resistance
Decreasing surface area decreases the resistance

14. What causes friction?
Microscopic dips/bumps on surfaces rub against each other
Slows sliding
Atoms on the opposing surfaces can form weak attractive forces and slow sliding

15. Reducing friction Reduce contact between bumps on surfaces
Reduce ability for attractive forces
Lubricants can do both
Reducing surface area reduces air resistance

16. Ch2 L.1 Homework p
Outline Ch2 L.1
Study for quiz

17. Ch2 L.2 Newton’s law What is Newton’s first law of motion?
How is motion related to balanced and unbalanced forces?
What effect does inertia have on the motion of an object?

18. Ch2 L.2 Bellwork
Net force: a combination of all the forces acting on an object
Balanced forces: forces acting on an object that combine and form a net force of zero
Unbalanced forces: forces acting on an object that combine and form a net force that is not zero
Newton’s first law of motion: if the net force on an object is zero, the motion of the object does not change
Inertia: the tendency of an object to resist a change in its motion

19. Identifying forces Net force- combining forces
When two or more forces act on an object, the forces combine
How they combine depends on the directions of the forces
Forces in the same direction: add forces to get net force
direction of net forces is the same as the direction of the individual ones
MUST specify a reference direction (usually to the right is positive)

20. Identifying forces cont’d
Combining forces in opposite directions:
Still the sum of the forces
One force will have a positive value and one will have a negative value depending on your reference direction

21. Balanced and unbalanced forces
When the net force on the object is zero
Same effect as if there were no forces acting on it
Do not change the motion of the object
Unbalanced
When the net force on the object is not zero

22. Newton’s first law of motion
If the net force on an object is zero, the motion of the object does not change.
Balanced forces
Cause no change in velocity of an object
Unbalanced forces
First law DOES NOT apply
These forces DO change an object’s velocity

23. Inertia
Tendency of an object to resist change

25. Why do objects stop moving?
Book on table
Stays still because of inertia
You push it
You overcome static friction
Book moves
You stop pushing
Friction stops book
IF* there were no friction on the table
You push book
Book continues to move forever because of inertia

26. Ch2 L.2 Homework p
Outline Ch2 L.2
Study for quiz

27. Ch2 L.3 Newton’s Second Law
What is Newton’s second law of motion?
How does centripetal force affect circular motion?

28. Ch2 L.3 Bellwork
Newton’s second law of motion: the acceleration of an object is equal to the net force acting on the object divided by the object’s mass
Circular motion: any motion in which an object is moving along a curved path
Centripetal force: in circular motion, a force that acts perpendicular to the direction of motion, toward the center of the curve

29. How do forces change motion?
Forces change an object’s motion by changing its speed, direction or both speed and direction.
Remember: balanced forces do not change an object’s velocity, only unbalanced forces do
Cyclist’s speed will not change unless the force on the petals overcomes the friction between the wheels and the road

30. Unbalanced forces on an object at REST
Object is originally not moving
Balanced forces keep object still
Unbalanced forces cause object to move
In direction of net force

31. Unbalanced forces on object in MOTION
Speeding up
If net force acting on object is in the SAME direction that the object is already moving
Slowing down
If net force is in the opposite direction that the object is already moving
Example: friction

32. Unbalanced forces on object in MOTION cont’d
Change direction
Object moves at constant velocity until it hits something and then it changes direction
Acceleration
When velocity changes
Unbalanced forces make an object accelerate by changing the speed, direction, or both

33. Newton’s second law of motion
The acceleration of an object is equal to the net force acting on the object divided by the object’s mass
Acceleration (m/s2)= Force(N)/mass(kg)
N= kg*m/s2

34. Math Practice
You throw a .5kg ball with a force of 10N. What is the acceleration of the ball?
A 24N net force acts on an 8kg rock, what is the acceleration of the rock?
A 30N net force on a skater produces an acceleration of .6m/s2, what is the mass of the skater?
What net force acting on a 14kg wagon produces an acceleration of 1.5m/s2?

35. Circular motion
Any motion in which an object is moving along a circular path
Centripetal force
Velocity= tangent to circular motion. Wants straight path because of inertia
Circular motion because string pulls ball inward
Force that acts perpendicular to direction of motion, to center of curve= centripetal force
Acceleration in direction of centripetal force

36. Circular motion cont’d
Motion of satellites and planets
Gravity (not a string) pulls the satellite inward towards the Earth
Moon a satellite of Earth
Earth a satellite of the Sun

37. Ch2 L.3 Homework p
Ch2 L.3 outline
Study for quiz

38. Ch2 L.4 Newton’s third law What is Newton’s third law of motion?
Why don’t the forces in a force pair cancel each other?
What is the law of conservation of momentum?

39. Ch2 L.4 Bellwork
Newton’s third law of motion: when one object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first object.
Force pair: the forces two objects apply to each other
Momentum: a measure of how hard it is to stop a moving object
Law of conservation of momentum: the total momentum of a group of objects stays the same unless outside forces act on the objects

40. Opposite forces Objects push against each other in opposite directions
Girl wants to move away from the wall so she pushes against the wall
Wall exerts equal but opposite force on the girl

41. Newton’s third law of motion
Equal but opposite rule
Equal magnitude of force
Opposite direction
Force pairs
The forces the two objects apply to each other
Don’t add up to be zero like balanced forces because aren’t applied to the same object

42. Newton’s third law of motion cont’d
Force pair
One force= action force
Other force= reaction force
Example: girl jumping off boat
Girl applies action force on boat
Boat applies reaction force on girl

43. How to use the third law
If reaction force results in an unbalanced force, there is a net force and the object accelerates
Swimming
Action force: Push arms against water
Reaction force: water pushes back on arms (in forward direction)
Jumping
Action force: push down on ground
Reaction force: ground pushes up on you plus gravity pushing down to make net force acting on you
Rocket
Action force: Engine pushes gas out downward
Reaction force: gas pushes upward on engine (needs to overcome gravity)

44. Momentum How hard is it to stop a moving object
In same direction as velocity
p=m*v
p= momentum (kg*m/s)
m= mass in kg
v= velocity (m/s)

45. Momentum practice
What is the momentum of a 12kg bicycle moving at 5.5 m/s?
What is the momentum of a 1.5kg ball rolling at 3.0 m/s?
A 55kg woman has a momentum of 220kg*m/s. What is her velocity?

46. Conservation of momentum
In any collision, one object transfers momentum to another object
TOTAL momentum does NOT change
Law of conservation of momentum: total momentum of a group of objects stays the same unless outside forces act on the objects ex.Friction

47. Types of collisions Elastic- colliding objects bounce off one another
Inelastic- colliding objects stick together after collision

48. Ch2 L.4 Homework p.75 1-9 Outline Ch2 L.4 Study for quiz
Study for test