1. SPH4U – Grade 12 Physics Unit 3
Dynamics Review
SPH4U – Grade 12 Physics
Unit 3

2. Tribal Challenge!
An airplane flies due north over Sudbury with a velocity relative to the air of 235 km/h and with a wind velocity of 65 km/h [N 45º E]. Calculate the speed and direction of the airplane.
1st Correct Answer – 3 points
2nd Correct Answer – 2 points
Every Other Correct Answer – 1 point

3. Tribal Challenge!
An airplane flies due north over Sudbury with a velocity relative to the air of 235 km/h and with a wind velocity of 65 km/h [N 45º E]. Calculate the speed and direction of the airplane.
Solution: 280 km/h [E 81º N]

4. Tribal Challenge! - full solution

5. Review - Dynamics
Dynamics is the study of forces and the effects that forces have on the velocities of other objects.

6. Review – Forces
A force is a push or a pull. Forces are vectors, and always have a direction.
Forces are measured in Newton's. 1 N = 1 kgm/s2
The net force (also called resultant force) is the sum of all the forces acting on an object. It is denoted as or

7. Review – Free Body Diagrams
A FBD (free body diagram) of an object’s motion shows all the forces acting on that object. It is an important tool for solving physics problems!

8. Practice:
A box sits stationary on a ramp that has an angle of elevation of 30º. Draw a free body diagram of this situation with all relevant forces.

9. Practice:
A box sits stationary on a ramp that has an angle of elevation of 30º. Draw a free body diagram of this situation with all relevant forces.

10. Review – Newton’s Laws
Draw a line between each description and the law it describes
Newton’s First Law
If the external net force on an object is not zero, the object accelerates in the direction of the net force. The acceleration is directly proportional to the net force and inversely proportional to the object’s mass.
The Action – Reaction Law
Newton’s Second Law
If the net force acting on an object is zero, the object maintains its state of rest or uniform motion.
Law of inertia
For every action force there is a simultaneous force equal in magnitude but opposite in direction.
Newton’s Third Law

11. Review – Newton’s Laws
Draw a line between each description and the law it describes
Newton’s First Law
If the external net force on an object is not zero, the object accelerates in the direction of the net force. The acceleration is directly proportional to the net force and inversely proportional to the object’s mass.
The Action – Reaction Law
Newton’s Second Law
If the net force acting on an object is zero, the object maintains its state of rest or uniform motion.
Law of inertia
For every action force there is a simultaneous force equal in magnitude but opposite in direction.
Newton’s Third Law

12. Review – Friction Friction is a force that always opposes motion
Friction occurs when one substance moves past another. There is friction between solids, liquids, gases – any two molecular substances in contact with each other will experience friction.
The amount of friction is dependent on the types of substances. Some substances (like ice) provide less friction for our feet than others (like sandpaper).

13. Review – Friction
Kinetic friction is the friction that occurs between two objects that are in motion relative to each other. ( )
Static friction is the friction that occurs between two objects that are not in motion relative to each other. ( )

14. Review – Friction
The magnitude of friction can be calculated using the equations :
Fk = μkFN (kinetic) or Fs = μsFN (static)
Recall that:
Fk or Fs are the forces of friction
FN is the normal force
μ is the coefficient of friction (a value that changes depending on whether we have static or kinetic friction. It also depends on the substance. See table 1 on pg. 85)

15. Review – Friction
The force of static friction opposes the force applied to an object and will increase as the force applied to the object increases until a maximum static friction is reached.
This maximum is called the limiting static friction.
At that moment, the object begins to move and will experience kinetic friction instead.

16. Homework
Read Sections 2.1 & 2.2 (Supplement your notes with this reading)
Complete the following questions:
Pg. 69 # 1, 4, 5, 6, 7
Pg. 76 # 3, 4, 6, 7, 8