1. Ch. 4 Forces

2. Objectives Develop and interpret free-body diagrams.
Calculate and describe the affect of forces on objects.
Understand the relationship between force and acceleration.

3. Forces Force – the interaction of an object with its environment
A push or a pull
Agent – source of force

4. Forces Can change an object’s velocity
by increasing or decreasing acceleration
Can change an object’s direction

5. Forces
SI unit is the Newton (the amount of force that when acting on a 1 kg mass, produces an acceleration of 1 m/s2)
1 N = 1kg•m s2

6. Forces
Two categories of forces
Contact Force
Field Force

7. Contact Force Involve contact between two or more objects
ex. You hit a ball

8. Field Force Does not involve contact between two objects
Ex. Magnetic field – attraction or
repulsion between magnetic
objects

9. Field Force The presence of an object affects the space around it.
A force is exerted on
any other object within
that space

10. Field Force The region of influence is called a field
Objects exert forces
on each other when
their fields interact

11. Force Diagrams Force is a vector
The effect of force depends on both the magnitude and direction
Net forces – all forces acting on an object
Balanced and unbalanced forces

12. Force Diagrams Forces are represented by arrows
The arrow is drawn from the center of the object

13. Force Diagram Called a Free Body Diagram
The object the forces are acting on is removed from it’s environment
Normal Force opposes the Gravitational Force
Frictional Force opposes the Applied Force

14. Free Body Diagram
Normal Force (FN) Applied Force (Fa) Frictional Force (Fs or Fk) Gravitational Force (Fg)

15. What is Net Force? F1
When more than one force acts on a body, the net force (resultant force) is the vector combination of all the forces, i.e., the “net effect.” F2 F3
Fnet

16. A tow truck is exerting a force of 5800 N on a car
A tow truck is exerting a force of 5800 N on a car. The gravitational force on the car is 14,700 N directed toward the center of the Earth. The Earth directs a force of 13,690 N on the car. The road exerts a backward friction force of 775 N on the car. Draw a free-body diagram of the forces on the car.

18. 5800 N

19. 5800 N N

20. 13690 N 5800 N N

21. 13690 N 5800 N 775 N N

22. 13690 N = FN Fa= 5800 N 775 N = Fs N = Fg