1. What are Newton’s Laws?

3. The Nature of Forces

4. What are Balanced Forces?
Have you ever had an arm wrestling competition with someone?
If you compete against someone who is just about as strong as you
There will probably be a time when both of you are pushing as hard as you can, but your arms stay in the same place.
This is an example of balanced forces.

5. Unbalanced Forces
Imagine that you are competing against Mr. Zim - a world famous arm wrestler.
Chances are that his force will be much greater than yours.
Your arms will move in the direction he is pushing with a force that is equal to his force minus your force. It will look like this.

6. Force can add to each other
Imagine that your family's car breaks down on the road and you have to push it into a parking lot.
If you and your brother both push on the car, the resulting force on the car will be the sum of your forces
The figure below shows how this would work.

7. Who was Isaac Newton?
Isaac Newton (a 17th century scientist) put forth three laws which explain why objects move (or don't move).
These three laws have become known as Newton's three laws of motion.

8. Isaac Newton’s 1st Law of Motion
An object at rest tends to stay at rest
And an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an another force.

9. Pass the Water
If students participate in a relay race carrying a plastic container of water
the water will have a tendency to spill from the container at specific locations
the container is at rest and you attempt to move it
the container is in motion and you attempt to stop it
the container is moving in one direction and you attempt to change its direction.

10. Should you wear seatbelts?

11. Check Your Understanding
1. Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose an astronaut in that place throws a rock. The rock will:
a) gradually stop.
b) continue in motion in the same direction at constant speed.

12. Check Your Understanding
If you were in a weightless environment in space, would it require a force to set an object in motion?

13. Check Your Understanding
Mr. Zim is being chased through the woods by a bull moose which he was attempting to photograph. The enormous mass of the bull moose is extremely intimidating. Yet, if Mr. Zim makes a zigzag pattern through the woods, he will be able to use the large mass of the moose to his own advantage. Explain this in terms of inertia and Newton's first law of motion.

14. Check Your Understanding
Two bricks are resting on the edge of a lab table. She acquires an intense desire to know which of the two bricks is more massive. Since Shirley is vertically challenged, she is unable to reach high enough and lift the bricks; she can, however, reach high enough to give each brick a push.
Discuss how the process of pushing the bricks will allow Shirley to determine which of the two bricks is more massive.

15. What is Inertia
Inertia is the tendency of an object to resist changes in its state of motion.
Inertia is the tendency of an object to resist changes in its velocity.

16. What is Inertia
An object at rest has zero velocity and (in the absence of an unbalanced force) will remain at a zero velocity; it will not change its state of motion
An object in motion with a velocity of 2 m/s, East (in the absence of an unbalanced force) will remain in motion with a velocity of 2 m/s, East; it will not change its state of motion (i.e., its velocity).

17. Newton's 2nd Law of Motion
F=ma
Force need = mass x acceleration

18. Newton's Third Law of Motion
According to Newton's third law, for every action force there is an equal (in size) and opposite (in direction) reaction force. Forces

19. Newton’s 3rd Law of Motion
Blow up a balloon
Hold the opening downward and release the balloon.
Repeat this several times, and observe what happens.
Now describe what happened using Newton's third law of motion.

20. Try these Problems
1.Two tugboats are moving a barge. Tugboat A exerts a force of 3000 newtons on the barge. Tugboat B exerts a force of 5000 newtons in the same direction. What is the combined force on the barge?
2.Draw arrows showing the individual and combined forces of the tugboats in #1.

21. Try these Problems
Now suppose that Tugboat A exerts a force of 2000 newtons on the barge and Tugboat B exerts a force of 4000 newtons in the opposite direction. What is the combined force on the barge?
Draw arrows showing the individual and combined forces of the tugboats in #3.

22. Try these Problems
5.Could there ever be a case when Tugboat A and Tugboat B are both exerting a force on the barge but the barge doesn't move? Draw arrows showing the individual and combined forces in such a situation.

23. Which law is this?