1. Newton’s Laws of Motion

2. What is a Force???

3. Forces:
The strong force: (holds quarks and gluons together to form protons and nuetrons, 1038 times stronger than the gravitational force)
The weak force: (affecting leptons, quarks, and nuetrinos – 1011 times weaker than the electromagnetic force and 1013 times weaker than the strong force)
Electromagnetic force: (holds e- and protons together in atoms, and holds atoms together in molecules)
gravitational force: (gravitational attraction b/w any two bodies that have mass
1. What is a Force?
2. What causes one object to exert a force on another?
3. Give as many examples of force as you can:
4. What are the SI units for force?
5. Is force a vector or a scaler?
A force causes an acceleration (meaning a change in an objects velocity or direction)
Through contact:
a push or a pull
At a distance:
field forces (examples gravitational or electromagnetic)
Newton (N=(kg*m) /s2
Vector – Force has a magnitude and a direction

4. Forces can change motion
Start movement, stop movement, or change the direction of movement
Cause an object in motion to speed up or slow down

5. Forces Diagrams and Free Body Diagrams:
Forces are vectors (magnitude and direction).
Force diagram (a)
Shows all forces acting during an interaction
On the car and on the wall
Free-body diagram (b)
Shows only forces acting on the object of interest
On the car

6. Net Force - the Sum of the Forces
This car is moving with a constant velocity
Fforward = road pushing the tires
Fresistance = force caused by friction and air
Forces are balanced
Velocity is constant because the net force (Fnet) is zero

7. Equilibrium The state in which the net force is zero
All forces are balanced
Object is at rest or travels with constant velocity
In the diagram, the bob on the fishing line is in equilibrium
The forces cancel each other
If either force changes, acceleration will occur

8. Newton’s First Law:
Experimentation led Galileo to the idea that objects maintain their state of motion or rest.
Newton developed the idea further, in what is now known as Newton’s first law of motion:
An object at rest remains at rest and an object in motion continues in motion with constant velocity unless acted upon by an outside force

9. Newton’s First Law: This is also called the Law of Inertia Inertia
Tendency of an object to not accelerate
Mass is a measure of inertia
More mass produces more resistance to a change in velocity
Which object in each pair has more inertia?
A baseball at rest or a tennis ball at rest
Answer: the baseball
A tennis ball moving at 125 mi/h or a baseball at rest

10. Newton’s 1st Law: (inertia)

11. Newton’s Second Law ΣF=ma
The acceleration of an object is directly proportional to the force acting on the object and inversely proportional to the mass
ΣF=ma

12. Newton’s Second Law
Increasing the force will increase the acceleration
Which produces a greater acceleration on a 3-kg model airplane, a force of 5 N or a force of 7 N?
Answer: the 7 N force
Increasing the mass will decrease the acceleration
A force of 5 N is exerted on two model airplanes, one with a mass of 3 kg and one with a mass of 4 kg. Which has a greater acceleration?
Answer: the 3 kg airplane

13. Newton’s 2nd Law: F=ma

14. Practice Problem
Space-shuttle astronauts experience accelerations of about 35 m/s2 during takeoff. What force does a 75 kg astronaut experience during an acceleration of this magnitude?
Answer: 2600 kg•m/s2 or 2600 N

15. What do you think?
Two football players, Zach and Jake, collide head-on. They have the same mass and the same speed before the collision. How does the force on Alex compare to the force on Jason? Why do you think so?
Two football players, Jordan and Jake, collide head-on. Suppose Jake has twice the mass of Jordan. How would the forces compare?
Suppose Jake has twice the mass and Jordan is at rest. How would the forces compare?
When asking students to express their ideas, you might try one of the following methods.
(1) You could ask them to write their answers in their notebook and then discuss them.
(2) You could ask them to first write their ideas and then share them with a small group of 3 or 4 students. At that time you can have each group present their consensus idea. This can be facilitated with the use of whiteboards for the groups.
The most important aspect of eliciting student’s ideas is the acceptance of all ideas as valid. Do not correct or judge them. You might want to ask questions to help clarify their answers. You do not want to discourage students from thinking about these questions and just waiting for the correct answer from the teacher. Thank them for sharing their ideas. Misconceptions are common and can be dealt with if they are first expressed in writing and orally.
This question will likely produce a wide variety of responses. Some students may believe that the forces are always equal. Many will believe they are equal for the first example but not so for the second and third examples (next slide).

16. Newton’s Third Law Forces always exist in pairs
You push down on the chair, the chair pushes up on you
Called the action force and reaction force
Occur simultaneously so either force is the action force

17. Newton’s Third Law
For every action force there is an equal and opposite reaction force
The forces act on different objects
Therefore, they do not balance or cancel each other
The motion of each object depends on the net force on that object