1. Lesson 4.1 Changes in Motion
Essential Question: How do you describe how force affects an object?

2. Think about opening your car door, what happens
Think about opening your car door, what happens? How do you get it to open?

3. Forces are in your everyday world
Forces are in your everyday world! You use force to open doors, type at a keyboard, steer a car, drive a bulldozer through a wall, climb stairs, take your wallet out of your pocket, and more…

4. What is force?
It describes the interactions between an object and its environment.
Force is an action exerted on an object which may change the object’s state of rest or motion
Examples: throwing or kicking a ball, sitting on a chair

5. Force is a vector just like displacement, velocity and acceleration.
It has both magnitude and direction.

6. What can forces do?
A force can cause a stationary object to move or a moving object to change direction.
Force is responsible for a change in velocity with respect to time, thus causing an acceleration.

8. How do forces work?
Forces can act through contact or at a distance.

9. What are contact forces?
Contact forces are pushes and pulls.
For example: pull on a spring, pull on a wagon, catching a football
Contact forces result from physical contact between two objects.
These are usually easily identifiable.

10. What are field forces?
Field forces are forces that do not involve physical contact between two objects
For example: gravitational force
An object falls to earth after being thrown even though earth isn’t touching it.
Example: attraction or repulsion between electric charges
If you rub a balloon against your hair, the balloon will attract pieces of paper nearby

11. Electric Force The electric field around the rubbed balloon exerts an attractive electric force on the pieces of paper.

12. What is weight?
A measure of the magnitude of the gravitational force exerted on the object.
It is the result of the interaction of an object’s mass with the gravitational field of another object, such as Earth.

13. How do these forces differ?
The distinction between contact forces and field forces is useful when dealing with forces that we observe at the macroscopic level; those we can see.
Every force can be categorized as one of four fundamental field forces.

14. What are the four fundamental field forces?
Gravity
Electromagnetism
Weak nuclear force
Strong nuclear force

15. Gravitational and electromagnetic forces cause the most observable interactions. Interactions caused by nuclear forces are rarely observed because the forces exist only inside the nucleus of an atom.

16. Gravity Force Has the farthest reach on objects Weakest in magnitude
Attractive force that exists between all masses

17. Electromagnetism Forces between charged particles
Very large force compared to gravity

18. Nuclear Forces Strong nuclear force Weak nuclear force
Holds the nucleus of an atom together
Very strong force
Weak nuclear force
Also inside the nucleus of an atom
A form of an electromagnetic force
Mainly acts within the particles of the nucleus

19. How do you measure force?
The newton is the SI unit for force.
The newton (N) is defined as the amount of force that, when acting on a 1 kg mass, produces an acceleration of 1 m/s2

20. How do you represent a force?
Forces on an object are represented using a force diagram.
A force diagram is a diagram that shows force vectors as arrows.

21. How do you draw the arrows?
The tail of an arrow is attached to the object on which the force is acting.
A force vector points in the direction of the force and its length is proportional to the magnitude of the force.
ALL forces are drawn as if they act at a point; assume the point is the center of the object.

22. What is a free-body diagram?
A free-body diagram shows the forces that are ACTING on that object.
A free-body diagram is used to analyze only the forces affecting the motion of a single object.
Free-body diagrams are constructed and analyzed just like other vector diagrams.

23. What is the difference between the diagrams?
In a force diagram, vector arrows represent all forces acting in a situation.
A free-body diagram shows only the forces acting on the object of interest.

24. Force Diagrams Versus Free-Body Diagrams a) In a force diagram, vector arrows represent all forces acting in a situation. b) A free-body diagram shows only the forces acting on the object of interest – in this case, the car.

25. The photograph shows a person pulling a sled
The photograph shows a person pulling a sled. Draw a free-body diagram for this sled. The magnitudes of the forces acting on the sled are 60 N by the string, 130 N by the Earth (gravitational force) and 90 N upward by the ground.

26. A truck pulls a trailer on a flat stretch of road
A truck pulls a trailer on a flat stretch of road. The forces acting on the trailer are the force due to gravity ( N downward), the force exerted by the road ( N upward), and the force exerted by the cable connecting the trailer to the truck (20000 N to the right). Draw a free-body diagram of the trailer.

27. A truck pulls a trailer on a flat stretch of road
A truck pulls a trailer on a flat stretch of road. The forces acting on the truck are the force due to gravity (80000 N downward), the force exerted by the road (80000 N upward), the force exerted by the cable (20000 N to the left), and the force causing the truck to move forward (26400 N to the right). Draw a free-body diagram of the truck.

28. A physics book is at rest on a desk
A physics book is at rest on a desk. Gravitational force pulls the book down. The desk exerts an upward force on the book that is equal in magnitude to the gravitational force. Draw a free-body diagram of the book.

29. How do you describe how force affects an object?