Scott's Physics of Sports Blog

7.4-7.6 and Questions

scottbirne — Sat, 13 Feb 2010 06:21:23 +0000

The text begins by mentioning how if someone wishes to change an object’s momentum, an impulse must be exerted on it. However, the impulse must be exerted from an external force. Without an outside force, there is no momentum. Momentum is referred to as a vector quantity, meaning it can be canceled. The main idea stressed is that without a net force or impulse acting on a system, the system’s momentum cannot change. The term coined for no momentum change is “conserved,” which leads to the law of conservation of momentum. The law is evident in collisions where the net momentum (before collision) = net momentum (after collision). An elastic collision is when objects collide without being deformed or creating heat. As we discovered in our lab, the data is different in collisions when the masses are changed. The text continues in saying that momentum conservation can be looked at when momentum comes from any direction by using vector techniques. The vectors will prove the conservation of momentum.

22. a. The speed will be 3 m/s since the momentum is transferred between objects with the same mass. They must have equal momenta before and after the collision.

b. The speed will be 1.5 m/s if the blob at rest if 4 kg. Continuing with the idea that net momenta must be equal before and after the collision, we find the velocity to equal 1.5 m/s.

27. a. The spring exerts a greater impulse because it requires more impulse to stop an object and then push it back.

b. The force exerted by the spring is still greater because it not only used a force to stop the motion of the ball, but exerted another force to push it back, whereas the pillow only used a force to stop the ball.

Ch. 7 Questions 8, 9, 11, 33

scottbirne — Wed, 10 Feb 2010 03:22:19 +0000

8. By extending the time, the impact is less severe and therefore the wreck is less disastrous to the passengers. The momentum will decrease, and the force is of impact is smaller.

9. If the impact in a collision is extended by four times, the force of impact will decrease by four times. They work indirectly.

11.

a. By throwing a ball upward, an impulse is experienced

b. An impulse is experienced when a ball of the same speed is caught as well.

c. Another impulse is created by catching the ball and then throwing it up again.

d. The greatest impulse is experienced by catching the ball and then throwing it back up again. You have to reduce the momentum of the ball to zero, then quickly increase its momentum by throwing it in the air.

33. The force of impact on the car can be determined. The impulse cannot be determined without knowing the force. By multiplying mass by speed, we find the force of impact to be 20,000 N.

7.1-7.3

scottbirne — Sat, 30 Jan 2010 22:44:29 +0000

The books introduces the new subject of momentum, which is said to be inertia in motion. The formula for momentum is momentum = mass x velocity. The formula explains why a big truck will have the more momentum than a small car going the same speed. Conversely, if the masses of the two vehicles were the same, and the car had a greater speed, it would be more momentum. The greater the force on an object, the greater acceleration it will have, which increases the velocity and the momentum with it. The time the force is applied is important too. The longer a force is applied, the greater change in momentum. When force x time interval occurs, the result is an impulse. An impulse equals a change in momentum, so force x time = change in momentum (mv). The formula corroborates what we found in the lab. It is important to differentiate between impact and impulse. An impact is just a force that is used in the impulse equation. The longer the impact time, the longer the time it makes a force zero. If there is a short impact time, there is much more force applied to the object at the moment of impact. When objects bounce, there is greater impulse. It requires more impulse to catch and object and throw it up again than to just catch it. Momentum and impulses are new facets in our physics knowledge and can account for why objects react the way they do. Another item the book does not touch on, but something we found in the lab, was that with an impulse vs. change in velocity relationship slope = mass. We found that theory to be true in our lab, but since the book does not mention the equation, it would be that our experiment was shaped to prove that theory, and it may not be 100% correct. It is an interesting item to note between the lab and text.

Reflection on Energy Model

scottbirne — Thu, 28 Jan 2010 23:11:17 +0000

The energy models allow us to better understand how Work and other variables determine the amount of energy. Likewise, the models can help us find variables such as velocity and height. Knowing that energy is transferred, we can use different models together for certain problems. For instance, if an object has an initial position with all Eel, then the final position is the object with Eg, we can use the model of Eel to find the Eg. The models derived from energy experiments are as follows:

Change in Eg = mg x change in height

Eel =1/2kx^2

Fsp = -kx

Ek = 1/2mv^2

Those formulas can be applied to problems with energy movement. We can make many discoveries with the formulas and use them to discover how each variable can change. The laws of energy along with the formulas give us a better understand of how energy affects and is affected by the motion of an object.

8.5-8.6

scottbirne — Mon, 25 Jan 2010 23:33:22 +0000

As we learned in our labs, if an object is moving and doing work, it is making kinetic energy. The text shares the same formula for KE as we found in our lab: KE = 1/2 x mv^2. The text also introduces another part to the formula by showing how Work relates to an object speeding up and slowing down. It is given by the formula: Fd = 1/2 mv^2. Work determines the amount of kinetic energy. Without energy, there is no work. Those ideas work under the Work-Energy Theorem, which says Work = Change in Energy. Energy also transforms. While the amount of energy by slightly differ from the transfer of potential energy to kinetic energy, one can still notice the effect of the energy transformation. For instance, heat can determine that amount of energy transferred between objects. Energy can neither be gained or lost, nor created or destroyed, just transformed. Those principles rest under the umbrella of the Law of Conservation of Energy. The text confirms theories brought up from our labs and investigates further the behavior of energy.

Model from Pull-up-Hill Lab

scottbirne — Tue, 12 Jan 2010 21:47:11 +0000

We can further our understanding of how force, angle, and distance interact. Whether pulling a car up manually or letting a spring launch the car, the same task is happening. The car is moving up. The same amount of energy is produced from either method because there is the same change in height from the car spring lab. Just because they have the same energy, however, doesn’t mean they have the same force. The product of force x distance does not change. We also determined that when we were using the spring scale, we were doing work. When setting w = fd, we determine that energy is being transferred by a force. To check our slope from the distance vs. force graph, we created the equation Change in energy of in gravitational field = mg x change in height. When tested at an angle of 90 degrees, we found the answer to be 15, which is very close to the 16 from the graph. We then were able to determine that the higher up on the ramp the car is, the higher the energy in the gravitational field. Moving objects have energy, and therefore there is energy in the gravitational field, the car, and the spring.

First Unit in POS Review

scottbirne — Tue, 12 Jan 2010 01:37:24 +0000

Since learning the first unit, I have come to understand different vectors and their resultant lines. I learned about different principles such as Bernoulli’s and discovered how spin affects a ball. I discovered how projectiles go through the air in terms of x and y position and velocity. Also, I learned how air resistance can and cannot influence the flight of a projectile. The learning of the first unit set a good foundation to further advance my studies in physics of sports. Before the first unit, I had a different perception of what the graphs of x and y position and velocity would look like, but after the labs concerning projectiles, I have come to understand how and why the graphs are formed. I approached my studies with an open mind and wanted to understand what we were trying to find out and why we found the results we had. For the next unit, I can continue to work hard at understanding the why and how of each situation. I will work hard and hopefully gain more knowledge in physics.

8.1-8.4

scottbirne — Tue, 12 Jan 2010 00:03:32 +0000

Work can be described as force x distance. Whenever work is done, there is the application of force and the movement of something from that force. Despite different activities, the amount of work done can stay the same depending on the distance. Work also occurs when it is done against another force and when work changes the speed of an object. Work is measured in joules (J). The length of time it takes to complete work depends on power. Power = work done/time interval. tripling power means tripling the work done in the same amount of time. Power is measured in watts (W). Energy, also measured in joules, allows an object to do work. The most common form of energy is mechanical energy, which is the energy due to the position or movement of something. It can be either potential or kinetic energy. Potential energy is stored to do work. In order to raise an object against earth’s gravity requires gravitational potential energy. Gravitational potential energy = weight x height. Work has many aspects that affect measurements and energy of objects.

Rotational Motion vs. Linear Motion

scottbirne — Tue, 05 Jan 2010 23:12:37 +0000

Both rotational and linear motion measure different types of speeds. Linear speed measures distance moved per unit of time. On the contrary, rotational speed measures the number of rotations per unit of time. With linear motion, an object further away from the axis will move a greater distance than an object closer to the axis. All points on a rotating object will have the same rate of rotation because they rotate around the axis in the same amount of time. Rotational speed can be measured in rotations per minute (RPM). Both linear and rotational speeds are tangential speeds because the direction of motion is always tangent to the circle. Rotational speed will always be the same, but the linear speed varies.

In relation with rotational inertia and analogue mass, I=mr^2. This occurs when the mass of an object stays the same distance from a rotational axis. If the mass spreads out, the rotational inertia will decrease. The rotational inertia changes with all difference objects because they have different masses and radii.

11.4-11.7

scottbirne — Tue, 05 Jan 2010 22:57:31 +0000

The text begins by noting that rotational inertia is the restistance of an object to changes in its rotational motion. Torque is the necessary force to chagne the rotational state of motion in an object. The greater the distance between the bulk of the mass of an object and the axis around which the rotation occurs, the greater the rotational interita. A long bat will have more rotational intertia than a short bat. however, the shorter bad makes it easier to swing. The formula given for rotational intertia when the mass of an object stays the same distance from the axis is I=mr^2. As we have learned in class, the larger the radius, the greater the rotational inerita, such as with a figure skater. The rotational intertia is 3 times greater when arms are outstreched, so you will move 3 times faster when you pull your arms in. The human body has many different axis, and they all affect the rotational intertia of a person’s movement. Mass and velocity of moving objects create linear momentum. Rotational intertia creates angular momentum. When a direction is assigned to the angular momentum, it is called a rotational velocity. The formula given is: angular momentum = rotational inertia x rotational velocity. Angular momentum also equals linear momentum x the radius. The text then states the law of conservation of angular momentum, which states angular momentum will remain constant if torque does not act on the rotating object. Rotational inertia and rotational speed operate indirectly, and as one increases, the other will decrease.