1. Forces
Chapter 11

2. 11.1 Laws of Motion

3. Newton’s First Law
An object at rest remains at rest and an object in motion maintains its velocity unless it experiences an unbalanced force.
It is also called the law of inertia
Inertia is the tendency of an object to resist being moved or changed
All objects resist changes in motion, so all objects have inertia

4. Inertia is related to an object’s mass
Mass is the measure of inertia
An object with a small mass has a smaller amount of inertia, so it is easier to change its motion because it requires a small force
An object with a large mass has a larger amount of inertia, so it requires a larger force to change its motion.

5. Seat belts and car seats provide protection
Why does a seat belt help you stop when a car stops suddenly?
Why do we place babies in a special backward-facing car seat?

6. Newton’s Second Law of motion
The unbalanced force acting on an object equals the object’s mass times its acceleration.
Force = mass x acceleration
F=ma

7. Force is measured in newtons, which is the SI base unit for Force
1 N = 1 kg x 1 m/s2

8. Newton Second Law Practice Problem
Zookeepers lift a stretcher that holds a sedated lion. The total mass of the lion and stretcher is 175 kg, and the lion’s upward acceleration is m/s2. What is the unbalanced force necessary to produce this acceleration of the lion and stretcher?

9. 11.2 Gravity

10. Gravity – any two masses that exert an attractive force on each other.
Gravity depends on mass & distance between objects
Law of Universal Gravitation- All objects in the universe attract each other through gravitational force.

11. Law of Universal Gravitation
The greater the object mass, the stronger the gravitational force on it.
The greater the distance, the weaker the gravitational force.

12. Free fall – the motion of an object when only the force of gravity is acting on the object
Free fall acceleration near Earth’s surface is constant
Since we live on Earth we assume that objects accelerate toward Earth at 9.8m/s2

13. weight= mass x free-fall acceleration
•Weight – gravitational force exerted on an object; measured in units called Newtons
weight= mass x free-fall acceleration
w = mg
g=9.8m/s2

14. Weight is different from mass
Mass is the amount of matter an object has and will not change
Weight is the amount of gravitational force on an object and will change depending on where you are.
Weight influences shape

15. Velocity is constant when air resistance balances weight
Must have balanced forces
Terminal velocity – the constant velocity of a falling object when the force of air resistance is equal in size and opposite in direction to the force of gravity
Example: Skydivers

16. Projectile Motion
Projectile is anything that is thrown or shot through the air.
A projectile follows a curved path that has both horizontal and vertical motion
Horizontal motion- motion parallel to Earth’s surface. Caused by a force.
Vertical motion- motion perpendicular to Earth’s surface. Caused by gravity.

17. Horizontal and vertical velocities are independent on each other.
Horizontal velocity
Depends on inertia
Remains constant
Vertical velocity
Depends on gravity
Accelerate downward at 9.8 m/s2

18. •A moving truck launches a ball vertically (relative to the truck)
•A moving truck launches a ball vertically (relative to the truck). If the truck maintains a constant horizontal velocity after the launch, where will the ball land (ignore air resistance)?
A) In front of the truck
B) Behind the truck
C) In the truck

19. 11.3 Newton’s Third Law

20. Newton’s third Law
For every action force, there is an equal and opposite reaction force.
Also called the Law of Action and Reaction

21. Forces always occur in pairs called force pairs.
They will be equal in size and opposite in direction
Force pairs do not act on the same object and their names will be opposites of each other
Equal forces don’t always have equal effects, think of gravity

22. Action and reaction pairs are everywhere
List 5 examples of action and reaction pairs you see in your everyday life.

23. Momentum
Momentum is a property of moving objects resulting from its mass and velocity.
Momentum describes the tendency of objects to keep going in the same direction with the same speed.
Changes in momentum result from forces or create forces.

24. Momentum = mass x velocity
p=mv
Units for momentum are kg m/s

25. Momentum Example problem
Calculate the momentum of a 6.00 kg bowling ball moving at m/s down the alley toward the pins.

26. Momentum is conserved in collisions
Total momentum before a collision is the same total momentum after the collision

27. When one object hits another object, some or all of the momentum of the first object is transferred to the second object and the rest of the momentum stays with the first object.

28. Example of momentum being transferred
The cue ball is moving forward with momentum. The billiard ball’s momentum is zero.
When the cue ball hits the billiard ball, the action force makes the billiard ball move forward. The reaction force stops the cue ball.
Because the cue ball’s momentum was transferred to the billiard ball, the cue ball’s momentum after the collision is zero.