Physics 1A Lecture 4B. "Fig Newton: The force required to accelerate a fig inches per second. --J. Hart

Transcription

1 Physics 1A Lecture 4B "Fig Newton: The force required to accelerate a fig inches per second. --J. Hart

2 Types of Forces There are many types of forces that we will apply in this class, let s discuss a few. 1) Gravitational Force: Newton found that every body in the universe attracts every other body in the universe given by the equation: where G is the universal gravitational constant.

3 Gravitational Force The direction of the force will be along the line connecting the two masses; it will always be attractive. For a human on the surface of the Earth:

4 Gravitational Force For a mass near the surface of the Earth: F gravity = mg This force is directed downward, towards the center of the Earth. This is also known as weight (measured in N). Mass and weight are different. Mass is a scalar and will remain the same value no matter where you are in the universe. Weight is a vector which will change magnitude and direction depending on where you are.

5 2) Tension Force: Normal Force When a cable or rope pulls an object, this applies a force to the tied object known as the tension force. 3) Normal Force: As gravity pulls down on an object, whatever that object is resting upon will push back. This push back is called the normal force and is perpendicular to the surface. The compression of the surface is what leads to the normal force.

6 What is the direction of the normal force of the hill on the sled? Normal Force

7 Solving Force Problems Guidelines: 1) Choose an appropriate coordinate system. (You may have to make a clever choice.) 2) Make a free-body diagram. (Label forces as clearly as possible.) 3) Break force vectors into perpendicular components. (If not already.) 4) Choose appropriate Newton s Law to apply. (You may need to apply more than one.) 5) Perform algebra or math techniques.

8 Normal Force Example A crane exerts an upward force of 1,000N on a stationary 1,000kg concrete block that lies on a pier. What is the normal force of the pier on the concrete block? Answer First, you must define a coordinate system. Let s choose up as positive.

9 Normal Force Answer Next, we draw a free-body diagram: F tension, crane on block F normal, ground on block block F gravity, Earth on block No need to break the forces into components, so we can turn to Newton s Laws. a y = 0 ΣF y = 0 F tension + F normal = F gravity F normal = F gravity - F tension F normal = mg - (1,000N) F normal = (9,800N) - (1,000N) = 8,800N

10 Frictional Force 4) Frictional Force: When an object attempts to move over a surface, there will be a resistance known as friction. When an object is at rest and you are attempting to move it, the resistance is known as static friction, f s. Static friction will increase as the applied force increases until it reaches a maximum static friction given by: where μ s is the coefficient of static friction and F N is the normal force.

11 Frictional Force In general, the static friction can be written as:

12 Frictional Force The static force will be in the direction opposite the applied force that is attempting the motion. Before you can get an object to move you must overcome the maximum static friction. Once you have an object moving over a surface, the friction will become kinetic friction, f k. Kinetic friction is less than the maximum static friction for a given surface.

13 Frictional Force To calculate kinetic friction use: where μ k is the coefficient of kinetic friction and F N is the normal force. If an object moves through the air, there will be resistance due to the air. This resistance is called air resistance or drag. The direction of air resistance is opposite the direction of the velocity of the moving body.

14 Conceptual Question You ve just kicked a light box, and it is NOW sliding across the flat ground about 2 meters in front of you. Which of these forces currently act on the box? A) Gravity, acting downward. B) The normal force, acting upward. C) The force of the kick, acting in the direction of motion. D) All of the above forces currently act on the box. E) Only choices A and B above are correct.

15 Newton s 3rd Law Newton s Third Law Whenever one body exerts a force on a second body, the second body exerts an oppositely directed force of equal magnitude on the first body. Also stated as: For every action, there is an equal, but opposite reaction. If this is so, how can a horse ever pull a cart forward if the cart pulls back on the horse equally in magnitude and opposite in direction? Let s look at a game of Tug-Of-War.

16 Conceptual Question Who wins a game of Tug-Of-War between a 80kg person and a 60kg person? A) A tie, Newton s Third Law tells that neither can win, because the force between them is equal and opposite. B) The 80kg person, because the 80kg person will always exert a greater force than the 60kg person. C) The 60kg person, because the force exerted by this much mass is greater than the 80kg person. D) You can t tell until you draw your force diagram for this situation.

17 Newton s 3rd Law Draw a force diagram for each person separately. F normal, ground on 80kg F friction, ground on 80kg F gravity, Earth on 80kg 80kg F tension, 60kg on 80kg F tension, 80kg on 60kg F normal, ground on 60kg 60kg F gravity, Earth on 60kg F friction, ground on 60kg The winner of the tug-of-war contest is not who is the strongest (F A on B = F B on A ), but whoever had the most friction with the ground.

18 Newton s 3rd Law The action force and the associated reaction force is usually called a Third Law Pair. F A on B and F B on A are called Third Law Pairs. But normal force and gravitational force on a chair are not each other s Third Law Pairs (F ground on chair and F Earth on chair ). Use all of Newton s Laws together effectively: Newton s 1st Law: one object, ΣF predicts motion. Newton s 2nd Law: one object, ΣF predicts motion. Newton s 3rd Law: two objects, one force; no prediction of resulting motion.

19 For Next Time (FNT) Finish the Chapter 4 HW Start reading Chapter 5