Momentum P
Aperçu
A bullet leaves a rifle at four hundred metres per second and a cricket ball leaves a bat at thirty. Which one is harder to stop? The honest answer is the cricket ball, and the quantity that says so is momentum. Speed on its own tells you almost nothing about how much effort an object will cost you to bring to rest. Mass on its own tells you almost nothing either. Multiply them together and you have the single number that governs every collision, every recoil and every crash on the planet.
This lesson builds that number from scratch and then uses it. You will learn to treat momentum as a directed quantity with a sign rather than a bare figure, apply the one law that lets you predict the outcome of a collision you have never seen, and finish with the equation that explains why a gymnast lands on a thick foam mat and a cricketer draws the hands back to take a catch. It is one of the few topics in the whole course where three short equations let you answer questions that look, at first sight, impossible.
Objectifs
- All moving objects have momentum. The relationship between momentum, p, mass, m, and velocity, v, is: p = m×v
- In a closed system the total momentum before an interaction is equal to the total momentum after the interaction. This is called conservation of momentum. Students may be required to complete calculations involving two objects. Examples of interactions are collisions and explosions.
- The relationship between force, F, change in momentum, ∆p, and time, t, is: F = ∆p / t ie the force equals the rate of change of momentum. Students should be able to use this relationship to explain qualitatively car safety features such as air bags, seat belts, side impact bars, crumple zones. Also, gymnasium crash mats, cushioned surfaces for playgrounds and cycle helmets.
Carte mentale
Ce theme est schematise pour montrer comment les idees se relient.
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Note de cours
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Évaluation de la leçon
Félicitations, vous avez terminé la leçon sur Momentum P. Maintenant que vous avez exploré le concepts et idées clés, il est temps de mettre vos connaissances à lépreuve. Cette section propose une variété de pratiques des questions conçues pour renforcer votre compréhension et vous aider à évaluer votre compréhension de la matière.
Vous rencontrerez un mélange de types de questions, y compris des questions à choix multiple, des questions à réponse courte et des questions de rédaction. Chaque question est soigneusement conçue pour évaluer différents aspects de vos connaissances et de vos compétences en pensée critique.
Utilisez cette section d'évaluation comme une occasion de renforcer votre compréhension du sujet et d'identifier les domaines où vous pourriez avoir besoin d'étudier davantage. Ne soyez pas découragé par les défis que vous rencontrez ; considérez-les plutôt comme des opportunités de croissance et d'amélioration.
- What is the unit of momentum? A. N B. J C. kg m/s D. m/s Answer: C
- Which pair of quantities are both vectors? A. Mass and speed B. Momentum and velocity C. Distance and time D. Energy and mass Answer: B
- A trolley of mass 4.0 kg moves at 2.5 m/s. What is its momentum? A. 1.6 kg m/s B. 6.5 kg m/s C. 10 kg m/s D. 20 kg m/s Answer: C
- Two identical carts roll towards each other at the same speed and stick together on impact. What is the total momentum of the pair immediately after the collision? A. Zero B. Twice the momentum of one cart before the collision C. The same as the momentum of one cart before the collision D. Greater than it was before the collision Answer: A
- A car is brought to rest in a collision. The change in momentum is the same, but the car is designed so that the collision lasts longer. What happens to the average force on the car? A. It increases B. It decreases C. It stays the same D. It becomes zero Answer: B
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