1. Forces Contact vs. Noncontact
© Copyright All rights reserved.

2. Mass vs. Weight The amount of matter in an object. Unit: Grams
Does not depend on gravity.
Measured with a triple beam or digital balance.
Measure of the Earth’s gravitational pull (a force).
measured in lbs. or N
Changes depending on the gravitational force.
Measured using a spring scale.
Image credits:
Triple beam balance, by Flikr user biologycorner, (CC BY-NC 2.0)
Spring scale, (public domain)
© Copyright All rights reserved.

3. Gravity
Mass and distance affect the force of gravity. The greater the mass and the closer the object, the stronger its gravitational pull.
Image credits:
Human silhouettes walking on globe, Pixabay user geralt, (CC0 Public Domain)
Gravity Probe B and Space-Time, artist’s conception from NASA, (public domain; government publication)
Newton under the apple tree by Frits Ahlefeldt, (CC BY-NC-ND 2.0)
“What is Weight,” NASA, (public domain; government publication)
© Copyright All rights reserved.

4. Force: A push or Pull Contact Force Non-Contact Force
A force that can cause or change motion of an object by touching it. Motors and gears operate through contact forces.
Force that results when the two objects are not in physical contact with each other, yet are able to exert a push or pull.
Contact Force Examples:
Friction
Normal Force
Air Resistance
Applied Force
Non-Contact Force Examples:
Gravitational Force
Electrical Force
Magnetic Force
© Copyright All rights reserved.

5. Contact Forces
Applied Force: A force typical push or pull, pushing a door open, pulling a rope.
Friction: Happens when two objects rub against each other. Friction opposes motion.
Normal Force: The normal force is the support force exerted upon an object that is in contact with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book.
Air Resistance: The air resistance is a type of frictional force that acts upon objects as they travel through the air.
© Copyright All rights reserved.

6. NON-Contact Forces
Magnetic Force: A kind of non-contact force; materials such as iron, nickel, cobalt and other magnets just need to be near the magnet to be attracted or repelled.
Electric Force: Non-contact force of electrical charges.
Gravitational Force: The force of attraction between any two objects in the universe. All objects have gravitational force. The mass of an object and its distance determines the strength of its gravitational force.
© Copyright All rights reserved.

7. Quick Write/Draw activity
Draw an example of a contact force AND a non contact force.
Briefly describe the force in EACH drawing.

8. Classify the Forces 1. A foot kicking a soccer ball
2. An apple falling off a tree
3. A paper clip sliding towards a magnet
4. A bat striking a baseball
5. Two magnets pushing apart from one another
6. The moon orbiting the Earth
7. A compass needle turning North
8. A hand pushing an object
9. Shoes rubbing against the floor
10. A book sitting on a desk
Have students specify contact or non- contact as well as specific type of force. For example: A foot kicking a soccer ball is a contact force: applied force.
Answers:
1. Applied
2. Gravity
3. Magnetic
4. Applied
5. Magnetic
6. Gravity
7. Magnetic
8. Applied
9. Friction
© Copyright All rights reserved.

9. Check your answers Answers: 1. Contact - Applied
2. Non contact - Gravity
3. Non contact - Magnetic
4. Contact - Applied
5. Non contact - Magnetic
6. Non contact - Gravity
7. Non contact - Magnetic
8. Contact - Applied
9. Contact - Friction
10. Contact - Normal

10. Contact & non-contact forces in action

11. Balanced & unbalanced forces
Balance forces are two forces acting in opposite directions on an object, and equal in size. Anytime there is a balanced force on an object, the object stays still or continues moving at the same speed and in the same direction.
Unbalanced forces occur when more force is exerted from one direction. Anytime there is an unbalanced force, there is a change in the motion of an object.

12. Measuring motion Speed is distance divided by time.
Velocity is speed in a given direction. 
Acceleration is change in velocity divided by time.

13. Newton’s First Law
An object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an unbalanced force.

14. What does this mean?
Basically, an object will “keep doing what it was doing” unless acted on by an unbalanced force.
If the object was sitting still, it will remain stationary. If it was moving at a constant velocity, it will keep moving.
It takes force to change the motion of an object.

15. What is meant by unbalanced force?
If the forces on an object are equal and opposite, they are said to be balanced, and the object experiences no change in motion. If they are not equal and opposite, then the forces are unbalanced and the motion of the object changes.

16. Some Examples from Real Life
A soccer ball is sitting at rest. It takes an unbalanced force of a kick to change its motion.
Two teams are playing tug of war. They are both exerting equal force on the rope in opposite directions. This balanced force results in no change of motion.

17. Newton’s First Law is also called the Law of Inertia
Inertia: the tendency of an object to resist changes in its state of motion
The First Law states that all objects have inertia. The more mass an object has, the more inertia it has (and the harder it is to change its motion).

18. More Examples from Real Life
A powerful locomotive begins to pull a long line of boxcars that were sitting at rest. Since the boxcars are so massive, they have a great deal of inertia and it takes a large force to change their motion. Once they are moving, it takes a large force to stop them.
On your way to school, a bug flies into your windshield. Since the bug is so small, it has very little inertia and exerts a very small force on your car (so small that you don’t even feel it).

19. If objects in motion tend to stay in motion, why don’t moving objects keep moving forever?
Things don’t keep moving forever because there’s almost always an unbalanced force acting upon it.
A book sliding across a table slows down and stops because of the force of friction.
If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

20. In outer space, away from gravity and any sources of friction, a rocket ship launched with a certain speed and direction would keep going in that same direction and at that same speed forever.

21. Inertia in action

22. Force equals mass times acceleration.
Newton’s Second Law
Force equals mass times acceleration.
F = ma
Acceleration: a measurement of how quickly an object is changing speed.

23. What does F = ma mean?
Force is directly proportional to mass and acceleration. Imagine a ball of a certain mass moving at a certain acceleration. This ball has a certain force.
Now imagine we make the ball twice as big (double the mass) but keep the acceleration constant. F = ma says that this new ball has twice the force of the old ball.
Now imagine the original ball moving at twice the original acceleration. F = ma says that the ball will again have twice the force of the ball at the original acceleration.

24. More about F = ma
If you double the mass, you double the force. If you double the acceleration, you double the force.
What if you double the mass and the acceleration?
(2m)(2a) = 4F
Doubling the mass and the acceleration quadruples the force.
So what if you decrease the mass by half? How much force would the object have now?

25. What does F = ma say?
F = ma basically means that the force of an object comes from its mass and its acceleration.
Something very massive (high mass) that’s changing speed very slowly (low acceleration), like a glacier, can still have great force.
Something very small (low mass) that’s changing speed very quickly (high acceleration), like a bullet, can still have a great force. Something very small changing speed very slowly will have a very weak force.

26. 2nd Law of motion in action

27. For every action there is an equal and opposite reaction.
Newton’s Third Law
For every action there is an equal and opposite reaction.

28. What does this mean?
For every force acting on an object, there is an equal force acting in the opposite direction. Right now, gravity is pulling you down in your seat, but Newton’s Third Law says your seat is pushing up against you with equal force. This is why you are not moving. There is a balanced force acting on you– gravity pulling down, your seat pushing up.

29. Think about it . . .
What happens if you are standing on a skateboard or a slippery floor and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force.
Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).

30. 3rd Law of Motion in action