1. How Forces Affect Motion
Unit 2 - Dynamics
How Forces Affect Motion

2. Unit 2 Part 1 – Newton’s Laws of Motion
Book Chapter 4

3. Motion Theories The old way looked at the what
The new way looked and looks at the why
4th century BC - Aristotle
Did not experiment
Used only logic and observation
Two states of motion natural and violent
Earth at rest everything else moves in complete circles

4. Motion Theories 1600 Galileo 1670 Newton Used experiments with ramps
Discussed friction came up with the idea of inertia: property of all matter
1670 Newton
Built off of Galileo’s idea

5. 1st Law of Motion: The Law of Inertia
Newton said, “An object at rest wants to stay at rest unless acted on by a net (unbalanced) force, and an object in motion wants to stay in motion in a straight line at a constant speed unless acted on by an net (unbalanced) force.
Forces do not cause motion they change motion
Inertia is a property of all matter
Discuss inertial reference frame.

6. 1st Law of Motion: The Law of Inertia
Inertia is related to the mass of an object
Mass is the measure of inertia or how easy or hard it is to change an objects position
Mass can also be how much stuff makes up an object.
Units of mass kilograms or slugs
1 slug = kg
Discuss inertial reference frame.

7. 1st Law of Motion: The Law of Inertia
Discuss inertial reference frame.
Which one has more inertia?

8. 1st Law of Motion: The Law of Inertia
Discuss inertial reference frame.

9. Forces and Free Body Diagrams
A force is defined as any push or pull
To display forces free body diagrams are used.
Include only the forces acting on the object.
Force is a vector! Use an arrow to indicate a force.
Contact forces: occur between two object in contact ie friction
Non-contact: object do not have to touch ie gravity, magnetism, attraction
Draw an example of a simple free body diagram showing weight and normal force

10. Forces and Free Body Diagrams
Fit in two main categories
Contact forces between 2 objects
Shoot a basketball
Push a shopping cart
Friction
Non-contact or at a distance
GRAVITY
Electricity
Magnetism
Contact forces: occur between two object in contact ie friction
Non-contact: object do not have to touch ie gravity, magnetism, attraction
Draw an example of a simple free body diagram showing weight and normal force

11. Forces and Free Body Diagrams
Equilibrium
All forces are equal up = down , lefts = rights
If any one force is not equal then there is a non- zero net force on the object
Contact forces: occur between two object in contact ie friction
Non-contact: object do not have to touch ie gravity, magnetism, attraction
Draw an example of a simple free body diagram showing weight and normal force

12. Forces and Free Body Diagrams
Equilibrium involving gravity
Contact forces: occur between two object in contact ie friction
Non-contact: object do not have to touch ie gravity, magnetism, attraction
Draw an example of a simple free body diagram showing weight and normal force

13. Proportions Directly Proportional
Means two quantities are on opposite sides of the equal sign, either both in the numerator or both in the denominator
In the equation s= 𝑑 𝑡 , s and d are directly proportional
What happens to s when d increases?
When d decreases?

14. Proportions Indirectly Proportional
Means two quantities are on opposite sides of the equal sign, one in the numerator and the other in the denominator
In the equation 𝑠= 𝑑 𝑡 , s and t are indirectly proportional
What happens to s when t increases?
When t decreases?

15. Newton’s 2nd Law: Law of Acceleration
Newton came up with is in 1680
Law of acceleration is a proportion
Deals with the sum of the forces, acceleration and mass
𝑎 = 𝐹 𝑚 = 𝐹 𝑛𝑒𝑡 𝑚 on AP equation sheet
𝐹 =𝑚 𝑎 is the equation rearranged.
Straight line motion
At an angle motion

16. Newton’s 2nd Law: Law of Acceleration
“When a net external force acts on an object of mass m, there is an acceleration that results.”
The acceleration is directly proportional to the force applied and inversely proportional to the mass.
Straight line motion
At an angle motion

17. Newton’s 2nd Law: Law of Acceleration
Units of force are Newtons and pounds
A Newton is a kg m/s2
A pound is a slug ft/s2
Straight line motion
At an angle motion

18. Newton’s 2nd Law: Law of Acceleration
WDhvw
Straight line motion
At an angle motion

19. Example Problem
Two people are pushing a stalled car. The mass of the car is 1850 kg. One person applies a force of 275 N to the car, while the other person applies a force of 395 N in the same direction. A third force of 560 N also acts on the car, but in the opposite direction. (This force is due to the friction of the tires on the pavement.) Find the acceleration of the car.

20. Newton’s 3rd Law of Action/Reaction
Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.
“for every action there is an equal and opposite reaction”
Hammer hits nail and nail stops hammer
Swimming: you push the water backward and the water pushes you forward

21. Newton’s 3rd Law of Action/Reaction
If this is true, then why doesn’t the cannon move as far as the cannonball?
Remember that 𝑎= 𝐹 𝑚 and that a and m are indirectly proportional.
The cannon has larger m, smaller a.
The cannonball has smaller m, larger a.

22. Weight and Normal Force
Weight is the force of gravity on an object.
It depends on where an object is. (On the moon, in space, at Earth’s sea level, at high altitude…)
It is directly proportional to mass, but not the same thing!
𝑔 = 𝐹 𝑔 𝑚 on the AP equation sheet (towards the bottom)

23. Weight and Normal Force
More About Weight
𝑊𝑒𝑖𝑔ℎ𝑡=𝑚𝑔 is the equation rearranged
This is really still the 𝐹=𝑚𝑎 equation!
The weight vector always points DOWN in a free-body diagram.
The SI unit for weight is the Newton.

24. Weight and Normal Force
The normal force is the perpendicular component of the force that a surface exerts on an object with which it is in contact.
Does NOT necessarily point directly upwards!
Equal and opposite of the weight IF no other forces are acting and the object is on a horizontal surface.
Think of “atomic springs” in the surface compressing to produce the normal force.
Show balancing diagram with no extra forces, a push down, and a pull up.

25. True Weight vs. Apparent Weight
The apparent weight is the force that an object exerts on a scale.
If the scale and the object are either at rest or moving at a constant speed (a=0), then True=Apparent.
If the scale and object are accelerating, then apparent weight will be different!

26. True Weight vs. Apparent Weight
If the elevator is accelerating upwards, the apparent weight will be more than the true weight. (You get a heavier feeling as the elevator starts to move up.)
If the elevator is accelerating downwards, the apparent weight is less than the true weight. (You feel lighter as the elevator starts to move down.)
If the elevator is in free fall (𝑎=𝑔) then apparent weight is 0. (weightlessness!)
Show balancing diagram with no extra forces, a push down, and a pull up.

27. Friction
Friction is the parallel component of the force that a surface exerts on an object with which it is in contact.
Two types:
If the object IS NOT moving, then the force is called static friction.
If the object IS moving, then the force is called kinetic friction.

28. Friction 𝑓=𝜇𝑁 The FUN equation to calculate the frictional force:
µ is the coefficient of friction and depends on the materials involved and whether it is static or kinetic friction.
Rubber on Dry Concrete: 𝜇 𝑠 =1.0, 𝜇 𝑘 =0.8
Teflon on Teflon: 𝜇 𝑠 =0.04, 𝜇 𝑘 =0.04
The higher the coefficient, the greater the friction.

29. Friction
Static friction: 𝑓 𝑠 𝑀𝐴𝑋 = 𝜇 𝑠 𝑁 calculates the maximum static frictional force.
If a force exerted is greater than this, the object will start moving!
If the force exerted is less than this, the object remains stationary and the frictional force is equal to the force exerted.
Kinetic friction: 𝑓 𝑘 = 𝜇 𝑘 𝑁