1. Physics 218
Lecture 6: Dynamics
Alexei Safonov

2. Basics: Reminders
Acceleration always implies a non-zero net force and vice versa
Numerically, F=ma
Mass m is essentially a measure of how much a particular body “resists” a force applied
Light objects give in easily (large a for a given F), heavy objects resist a lot (smaller a for the same F)
Forces are applied by objects onto other objects, each experiences the same force
Action = Reaction
Today we will do more exercising in using these concepts

3. Clicker Question
A force F is applied to a small block, that pushes a larger block. The two blocks accelerate to the right. Compare the NET FORCE on the block with mass M, to the net force on the block with mass 5M.
A) FM < F5M
B) FM = F5M
C) FM > F5M a 5M F M 3

4. Clicker Question a 5M F M Net Force
A force F is applied to a small block, that pushes a larger block. The two blocks accelerate to the right. Compare the NET FORCE on the block with mass M, to the net force on the block with mass 5M.
A) FM < F5M
B) FM = F5M
C) FM > F5M
Net Force
Same acceleration, so larger mass has larger net force. a 5M F M 4

5. You tie a brick to the end of a rope and whirl the brick around you in a horizontal circle. Which best describes the path of the brick after you suddenly let go of the rope.
The brick drops directly straight down to the ground.
The brick continues in its circular path.
The brick spirals outward.
The brick flies off in a straight line.

6. Reminder: Newton’s Third Law
“Whenever one object exerts a force on a second object, the second exerts an equal and opposite force” OR
“To every action there is an equal and opposite reaction”

7. Pre-Lecture Question 2 A B C D E

8. Pre-Lecture Question 2

9. CheckPoint
You are driving a car with constant speed around a horizontal circular track. The net force acting on your car
A) Points radially inward toward the center of the circular track
B) Points radially outward, away from the center of the circular track
C) Points forward in the same direction your car is moving
D) Points backward, opposite to the direction your car is moving
E) Is zero. 9

10. CheckPoint Responses
You are driving a car with constant speed around a horizontal circular track. The net force acting on your car
A) Points radically inward toward the center of the circular track
B) Points radically outward, away from the center of the circular track
C) Points forward in the same direction your car is moving
D) Points backward, opposite to the direction your car is moving
E) Is zero.
A) Force is in the same direction as acceleration (in this case, centripetal).
B) the acceleration is outwards, therefore the force is outwards
C) The car is moving forward so the net force must
be forward.
E) Because the car is moving at a constant speed, so there is no a. We get the net force is zero. 10

11. Aside: Centripetal acceleration and force
1) Objects moving in a circle always have a component of acceleration, called centripetal, which is toward the center of the circle.*
2) Centripetal acceleration must be caused by a force:
Friction, gravity – whatever force keeps it moving in a circle.
This force is often called the “centripetal force”
3) There is no “new” kind of force here.
4) There is no such thing as centrifugal force.
* They can have also have tangential acceleration if their speed is not constant

12. Free Body Diagrams Same tricks as in Chapters 1-3:
Draw a diagram: Draw each force on an object separately! Force diagram!
Break each force into the X and Y-components, THEN sum!!!
Show your TA that you know the difference between a force, and a component of force
GREAT way to pick up partial credit

13. Pulling a box
A box with mass m is pulled along a frictionless horizontal surface with a force FP at angle Q as given in the figure. Assume it does not leave the surface.
What is the acceleration of the box?
What is the normal force? FP Q

14. If a rope is massless and doesn’t stretch, this means:
Rope Problems
If a rope is massless and doesn’t stretch, this means:
The acceleration of any part of a rope is the same as any other part
The magnitude of the acceleration of the two things it attaches are equal
The tension (Force) exerted by each end is the same

15. 2 boxes connected with a string
Two boxes with masses m1 and m2 are placed on a frictionless horizontal surface and pulled with a Force FP. Assume the string between doesn’t stretch and is massless.
What is the acceleration of the boxes?
What is the tension of the strings between the boxes? M2 M1

16. Clicker Question
A cart with mass m2 is connected to a mass m1 using a string that passes over a frictionless pulley, as shown below. The cart is held motionless. m2 m1 g a
The tension in the string is
A) m1g
B) m2g
Let’s talk more about tensions and strings 16

17. Closer Look at Elevators
A box of mass m is hung with a string from the ceiling of an elevator that is accelerating upward. Which of the following best describes the tension T in the string:
A) T < mg
B) T = mg
C) T > mg
Lets walk over this step by step a

18. Clicker Question
You are traveling on an elevator up the Sears tower. As you near the top floor and are slowing down, your acceleration
A) is upward
B) is downward
C) is zero

19. Clicker Question
You are traveling on an elevator up the Sears tower, and you are standing on a bathroom scale.
As you near the top floor and are slowing down, the scale reads
A) More than your usual weight
B) Less than your usual weight
C) Your usual weight

20. A crate is suspended from the end of a vertical rope
A crate is suspended from the end of a vertical rope. The tension in the rope is greater when:
The crate is at rest.
The crate is moving up at a constant speed.
The crate is moving down at a constant speed.
The tension for A, B and C are the same.

21. Spring Scale
What is the reading in the spring scale?

22. Next time
Will talk about friction, tension, circular motion, and more on applying Newton Laws