1. Chapter
8th Grade

2. Galileo Galilei Force Italian Astronomer
Suggested that once an object is in motion, no force is needed to keep it moving.
Force
Needed to change the motion of the object.

3. Newton’s 1st Law of Motion
Restates Galileo’s ideas about force and motion
An object at rest will remain at rest, and an object moving at a constant velocity will continue moving at a constant velocity unless it is acted upon by an unbalanced force.

6. Newton’s 1st Law of Motion
This means that if an object is not moving, it will not move until a force acts on it.
If an object is already moving, it will continue to move at a constant velocity until a force acts to change either its speed or direction.
Gravity and friction are unbalanced forces that often change an object’s motion.

7. Inertia
An object moving or not moving it resists any change to its motion.
Inertia
The tendency of an object to resist a change in motion

8. Inertia Newton’s 1st Law
Also called the Law of Inertia
The greater mass of an object, the greater its inertia, and the greater the force needed to change its motion.

9. Newton’s 1st Law

10. Newton’s 2nd Law of Motion
Acceleration
depends on the object’s MASS and the NET FORCE acting on the object.
Net force / mass
Units = Newtons (N)

11. Newton’s 2nd Law of Motion

12. Newton’s 2nd Law of Motion
How do you increase acceleration?
Change the FORCE!
If mass is constant, acceleration and force change in the same way.
Decrease in mass = increase in acceleration

13. Section 10.3 Assessment What does Newton’s 1st Law of motion state?
An object at rest will remain at rest and an object in motion will remain in motion, unless acted upon by an unbalanced force.
Why is Newton’s 1st law of motion sometimes called the law of intertia?
Inertia is a measure of an object’s tendency to resist a change in its motion.
Use what you know about inertia to explain why you feel pressed back into the seat of a car when it accelerates?
Because of your inertia, your body tends to remain in place. The car seat causes you to accelerate, therefore, by exerting a force on your back.

14. Section 10.3 Assessment
State Newton’s 2nd law of motion in your own words.
The greater the force= the greater the acceleration.
The greater the acceleration or the greater the object’s mass = the lower the acceleration
How could you keep an object’s acceleration the same if the force acting on the object were doubled?
You could double the object’s mass

15. Section 10.3 Assessment
Why does a car with a large mass might use more fuel than a car with a smaller mass.
A greater force is required to accelerate a more massive car, therefore, more engine power is required and more fuel is needed to produce that power.
Find the force it would take to accelerate an 800kg car at a rate of 5 m/s2
F=ma
800 kg X 5m/s2= 4,000 N

16. Section 10.3 Assessment
What is the net force acting on a 0.15 kg hockey puck accelerating at a rate of 12 m/s2?
F=ma
(0.15 kg x 12 m/s2) = 1.8 N

17. Newton’s 3rd Law of Motion
Whenever one object exerts a force on a second object, the second object exerts a force of equal strength in the opposite direction on the first object

18. Newton’s 3rd Law of Motion
“Action-Reaction” forces
Ex. When a dog leaps it pushes down on the ground. The reaction force of the ground pushes the dog into the air.
Do action-reaction pairs
Cancel?
No! They are acting on
Different objects.

19. Momentum “quantity of motion”
It is the characteristic of a moving object related to the mass and velocity of the object
Equation: mass x velocity
Units = kg* m/s

20. Momentum
The more mass an object has the more momentum it has, so the harder it is to stop the object.
Ex. A car moving 20 m/s is harder to stop than a baseball moving at 20 m/s because the car has a greater mass and greater momentum.

21. Momentum
Amount of momentum objects have is conserved when they collide.
It can be transferred from one object to another, but none is lost.
Law of conservation of momentum
The total amount of momentum of objects that interact does not change in the absence of outside forces.
Ex. Friction

22. Determining Acceleration Word Problems
1. A speedboat pulls a 55-kg water skier. The force causes the skier to accelerate at 2.0 m/s2. Calculate the net force that causes this acceleration.

23. Determining Acceleration Word Problems
2. What is the net force on a 1,000kg object accelerating at 3m/s2?

24. Determining Acceleration Word Problems
3. What net force is needed to accelerate a 25 kg cart at 14 m/s2?

25. Determining Acceleration Word Problems
4. Find the force it would take to accelerate an 800 kg car at a rate of 5 m/s2?

26. Determining Acceleration Word Problems
5. What is the net force acting on a 0.15 kg hockey puck accelerating at a rate of 12 m.s2?

27. Momentum Word Problems
Which has more momentum: a 3.0- kg sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s?
Momentum = Mass X Velocity
Smaller : 3.0 kg X 1.5 m/s = 4.5 kg x m/s
Larger : 4.0 kg X 0.9 m/s = 3.6 kg x m/s

28. Momentum Word Problems
A golf ball travels at 16 m/s, while a baseball moves at 7 m/s. The mass of the golf ball is kg and the mass of the baseball is 0.14 kg. Which has a greater momentum?
Golf Ball: kg x 16 m/s = 0.72 kg x m/s
Baseball: 0.14 kg x 7 m/s = 0.98 kg x m/s
The baseball has greater momentum

29. Momentum Word Problems
What is the momentum of a bird with a mass of kg flying at 15 m/s?
0.018 kg x 15 m/s = 0.27 kg x m/s

30. Momentum Word Problems
What is the momentum of a 920 kg car moving at a speed of 25 m/s?
920 kg x 25 m/s = 23,000 kg x m/s

31. Momentum Word Problems
Which has more momentum? A 250 kg dolphin swimming at 4 m/s or a 350 kg manatee swimming at 2 m/s?
Dolphin : 250 x 4 = 1,000 kg x m/s
Manatee: 350 x 2 = 700 kg x m/s
The dolphin has more momentum!