Dynamics is a subset of mechanics, which is the study of motion. Whereas kinetics studies that motion itself, dynamics is concerned about the CAUSES of motion. In particular, it involves the concepts of force, momentum and energy. This video gives an overview of what dynamics is, and is useful as an intro to its study, or as a review
Momentum and energy are intricately connected to the concept of force. In fact in Newton's Principia he described his second law of motion in terms of momentum.
In classical physics the concept of momentum is relatively straightforward. It's simply an object mass times its velocity. So for example a heavy object if moving will have a large momentum due to its mass. But I like object can also have a large momentum if it's velocity is particularly high. So for example a bullet can have a relatively large momentum by way of its velocity.
The concept of energy took a little bit more time to develop. In essence, it's the ability to do work.The concept of energy took a little bit more time to develop. In essence, it's the ability to do work. In the context of classical mechanics, this energy could be related to an object in motion or its position in a field.
Although they are intimately connected we are going to discuss them separately below, at least initially.
In classical physics the concept of momentum is relatively straightforward. It's simply an object mass times its velocity. So for example a heavy object if moving will have a large momentum due to its mass. But I like object can also have a large momentum if it's velocity is particularly high. So for example a bullet can have a relatively large momentum by way of its velocity.
The concept of energy took a little bit more time to develop. In essence, it's the ability to do work.The concept of energy took a little bit more time to develop. In essence, it's the ability to do work. In the context of classical mechanics, this energy could be related to an object in motion or its position in a field.
Although they are intimately connected we are going to discuss them separately below, at least initially.
Momentum
1. What is impulse?
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Since in the concept of dynamics we are interested in changes in motion, we are often more interested in an object's change in momentum rather than its momentum itself. Impulse is simply a change of momentum.
Using the 'eggsample' of an egg falling (get it), as well as cars safety, I discuss momentum and impulse. |
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1. Calculate the magnitude of the momentum in the following cases:
(a) a proton (mass = 1.67 x 10^-27 kg)) moving with a speed of 5 x 10^6 m/s, (8.35 x 10^-21 Ns)
(b) a 15 g bullet moving with a speed of 500 m/s,
(c) a 75 kg sprinter running with a speed of 12 m/s,
(d) The earth (mass = 5.98 x 10^24 kg) moving with an orbital speed of 2.98 x 10^4 m/s.
2. An 18,000 kg van is moving with a speed of 15 m/s. If Superman is to stop the van in 0.5 s, what average force must he exert on the van? (-540,000 N)
3. A 0.15-kg baseball is thrown with a speed of 20 m/s. It is hit straight back at the pitcher with a final speed of 22 m/s.
(a) What impulse is delivered to the ball? (6.3 Ns towards pitcher)
(b) Find the average force exerted by the bat on the ball if the two are in contact for 2 x 10^-3 s. (3,150 N towards pitcher)
(a) a proton (mass = 1.67 x 10^-27 kg)) moving with a speed of 5 x 10^6 m/s, (8.35 x 10^-21 Ns)
(b) a 15 g bullet moving with a speed of 500 m/s,
(c) a 75 kg sprinter running with a speed of 12 m/s,
(d) The earth (mass = 5.98 x 10^24 kg) moving with an orbital speed of 2.98 x 10^4 m/s.
2. An 18,000 kg van is moving with a speed of 15 m/s. If Superman is to stop the van in 0.5 s, what average force must he exert on the van? (-540,000 N)
3. A 0.15-kg baseball is thrown with a speed of 20 m/s. It is hit straight back at the pitcher with a final speed of 22 m/s.
(a) What impulse is delivered to the ball? (6.3 Ns towards pitcher)
(b) Find the average force exerted by the bat on the ball if the two are in contact for 2 x 10^-3 s. (3,150 N towards pitcher)
Here another worksheet on impulse, includes answer - worked out solutions
2. Deriving conservation of momentum
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Using the crash between a bus and a bee I show how we derive and understand Conservation of Momentum |
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- A 40-kg child standing on a frozen pond throws a 0.5-kg stone to the east with a speed of 5 m/s. Find the recoil velocity of the child. (0.0625 m/s)
- A rifle with a weight of 30 N fires a 5-g bullet with a speed of 300 m/s. Find the recoil speed of the rifle, if it is held loosely by the shooter. (0.49 m/s)
- A 65-kg boy and a 40-kg girl, both wearing skates, face each other at rest on a skating rink. The boy pushes the girl, sending her to the east with a speed of 4 m/s. What is the velocity (speed and direction) of the boy? (2.46 m/s west)
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Explore momentum and energy in an explosive setting. By Tom Walsh
3. Car crash Physics
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It is always helpful to attempt the problem first before looking at the sample solution
There's a lot of physics involved in car crashes. This video looks at the basics: acceleration, forces, momentum and energy. |
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Energy
4. What is work and energy?
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In this video I discuss the relationship between work and energy: the work energy theorem. It includes how the formulas are derived.
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Download worksheets on work
5. Conservation of Energy Explained
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Using some ramps, and an experiment, I discuss the Conservation of energy
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Using a Newton's Cradle and an air track, with which I apply video analysis, I discuss energy as they relate to elastic and inelastic collisions |
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- A graphing problem - using graphs from a datalogger, students need to interpret elastic vs inelastic collisions
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Use the Skate Park at pHET to explore Conservation of Energy
6. Work and Power
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Coming Soon
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- pHET graphing animation - this interactive from the University of Colorado pHet team is a great way to demonstrate the relationship between motion and its graphical analysis. That why I used it in my video. At this time its Java based so will only work on PC/Mac
7. How well do you know momentum and energy?
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Test your understanding of conservation laws. So do the Quiz and try to get full marks Then check your understanding if necessary with the video |
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