1. Forces in Nature

2. Forces in Nature
There are many forces acting on an object in nature. These include:
Force due to gravity (weight)
Normal force
Frictional forces (resistive forces)
Applied force (tension)

3. Force of Gravity

4. Force due to Gravity Weight vs. Mass
Mass is the amount of matter and object contains and weight is that gravitational pull on the mass or object
Weight varies with location, mass does not.
As you move further from Earth, gravity decreases. As gravity decreases, so does your weight.
Mass is measured in kg
Weight = Fg Forces are measured in N

5. Calculating force due to Gravity
The force of gravity (Fg) is dependent on the mass of the object and the gravitational field of the larger object (for example a planet)
On Earth the gravitational field is 9.81 N/kg

6. Sample Problem
What is the weight of a 12.0 kg object near the surface of the earth?

7. Sample Problem
What is the acceleration due to gravity near the surface of the moon if an object that has a mass of 22.0 kg has a weight of 36.0 N near the moon’s surface?

8. Normal Force and Force of Friction

9. Normal Force
The Normal Force is the perpendicular force that forces two surfaces together
Normal force differs on horizontal and inclined surfaces

10. Normal Force - Horizontal
Horizontal surface  FN = mg

11. Normal Force - Incline
On an inclined plane  FN = mgcos

12. Frictional Forces Frictional Forces are always resistive forces
There are 2 types of frictional forces:
Static friction exists only while the objects are at rest relative to one another
Kinetic friction is the frictional force between surfaces when they are in relative motion to one another.
It is the roughness of the two surfaces in contact that causes friction (the surface roughness is called the coefficient of friction)

13. Calculating Force of Friction

14. Sample Problem
A 7.6 kg object is resting on a horizontal surface. What is the normal force acting on the object?
75N

15. Sample Problem
A 7.6 kg object is pulled along a horizontal surface. If the coefficient of friction between the surfaces is 0.20, what is the force of friction?
15N

16. Sample Problem
A 7.6 kg object is at rest on an inclined plane. If the inclined plane makes an angle with the horizontal of 33, what is the normal force acting on the object?
63N

17. Homework
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