Physics 10. Lecture 3A

Transcription

1 Physics 10 Lecture 3A "Your education is ultimately the flavor left over after the facts, formulas, and diagrams have been forgotten." --Paul G. Hewitt

2 Support Forces If the Earth is pulling down on a book sitting on a table how is it remaining at rest? The table exerts an upward force which is called the support force (or normal force). This support force is equal in magnitude and opposite in direction to the force of gravity. But what if I put another book on top of the previous book, will the table still exert the same force as before?

3 Support Forces No, the table will exert more upward force in order to balance out the extra downward force by the extra book. But how does the table know how much force to exert upwards? The atoms in the table act as a spring that will compress depending on how much weight is put upon them.

4 Support Forces Since the support force is equal and opposite to the force of gravity this will lead to mechanical equilibrium. This is how a scale reads your weight. The scale measures the amount of support force is needed for balance.

5 Equilibrium for Moving Things Does an object in mechanical equilibrium have to be at rest? No, equilibrium just means that the state of motion is not changing. So when an object is moving in a straight line with the same magnitude of velocity it is in equilibrium.

6 Motion Chapter 3 will focus on linear motion. Any description of motion involves three concepts: 1) Speed 2) Velocity 3) Acceleration In order to better describe motion, we need to define these terms.

7 Motion Speed is the distance covered per amount of travel time. Speed = distance covered travel time Be careful, the units of speed are given in many forms: miles per hour or mph or mi/h. kilometers per hour or km/h. meters per second or m/s.

8 Motion Velocity is different from speed. It not only takes into account how fast the object is moving, but also in what direction it is traveling. An object that has constant speed does not go faster or slower. An object that has constant velocity has constant speed and moves in a straight line. For example, the Earth traveling around the Sun has constant speed but not constant velocity.

9 Motion Acceleration is defined as the rate of change of velocity. change of velocity Acceleration = time interval When as object is speeding up, this is acceleration. When an object is slowing down, this is deceleration or negative acceleration. The units of acceleration is usually given as m/s/s which is also written as m/s 2. But acceleration units can also be written as km/h/s.

10 Motion You can usually think of acceleration as change in velocity per second. But what if I am in an airplane moving at constant speed and just decide to change the direction of motion, is this acceleration? Yes it is, since acceleration is how velocity is changing and velocity is a vector. If you change the direction of a velocity vector you are accelerating the object.

11 Galileo s Experiments Galileo also performed many experiments with rolling balls on various inclines. He found that balls would speed up going down an incline and slow down going up an incline. From this he reasoned that a ball would horizontally roll forever if friction was missing (essentially Newton s First Law).

12 Galileo s Experiments Galileo found that a ball rolling down an inclined plane picks up the same amount of speed in successive seconds. These balls rolled with unchanging acceleration. Thus, Galileo found that he could calculate the velocity acquired of the ball by knowing the acceleration and the amount of time. Velocity acquired = acceleration x time v = at

13 Falling Bodies It didn t take long for Galileo to also predict the distance traveled by a moving object under constant accleration. Distance traveled = 1/2 (acceleration x time x time) d = 1/2(at 2 ) He then applied his theories to falling bodies. Objects moving under the influence of only gravity are said to be in free fall. Galileo was the first to predict that falling bodies on Earth would fall with a distance proportional to time squared (d t 2 ).

14 Falling Bodies Galileo surmised through repeated experiment that: at a given location on the Earth and in the absence of air resistance, all objects fall with the same uniform acceleration. Galileo felt that earlier work neglected the effect that air friction (i.e. resistance) had on different objects. The uniform acceleration Galileo talked about was the acceleration due to gravity, g.

15 Falling Bodies In SI units, g is generally regarded as being 9.80m/s 2. In this class we will stick with the close approximation of g = 10m/s 2. g varies slightly depending on where you are on Earth, but it always points downward towards the Earth.

16 Falling Bodies An object in free fall will gain velocity as it drops. Since acceleration due to gravity is 10 m/s 2, this means that the amount of velocity the object gains each second is 10 m/s.

17 Falling Bodies If you toss an object up in the air, its velocity will decrease 10 m/s every second. It will eventually reach a point at the top where its velocity is zero, then change direction.

18 Conceptual Question 1 A tennis player on serve tosses a ball straight up. When the ball is at its maximum height, the ball s: A) acceleration is zero. B) acceleration is g. C) acceleration is much less than g. D) acceleration is much greater than g.

19 For Next Time (FNT) Read Chapter 4. Finish up the homework for Chapter 2 and start the homework for Chapter 3. Clickers start on Monday.