Resultant Forces
Aperçu
Stand still and nothing appears to be happening to you. In fact the floor is pressing upwards on your feet with a force of several hundred newtons, gravity is pulling you downwards with exactly the same amount, and the two are cancelling so precisely that you never notice either of them. Physics has a single word for that cancelling: the resultant force on you is zero. Change it by even a few newtons and you would begin to move.
This lesson is about the one quantity that decides whether anything in the universe speeds up, slows down or turns. You will learn to collapse a whole crowd of forces into one, in a straight line by arithmetic and at an angle by scale drawing. You will meet all three of Newton's laws of motion in the order that makes them easiest to see, put numbers into \(F = m \times a\), and find out why a raindrop and a skydiver both stop speeding up long before they reach the ground. It is also the one topic in this section where the two tiers genuinely part company, and this note says exactly where.
Objectifs
- Whenever two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction. This is Newton’s Third Law.
- A number of forces acting on an object may be replaced by a single force that has the same effect on the motion as all the original forces acting together. This single force is called the resultant force. Students should be able to determine the resultant of opposite or parallel forces acting in a straight line and determine the resultant of two coplanar forces by scale drawing.
- A non-zero resultant force acting on an object causes it to accelerate.
- Acceleration is the rate of change of velocity. An object can accelerate by changing its direction even if it is going at a constant speed. Deceleration is a negative acceleration. The average acceleration, a, of an object is given by the equation: where ∆v is the change in velocity and t is the time taken for the object to accelerate.
- The acceleration of an object can be calculated from the gradient of the velocity-time graph.
- The distance travelled by an object can be calculated from the area under a velocity-time graph.
- If the resultant force acting on an object is zero: a moving object will continue to move at the same velocity; a stationary object will remain at rest. This is Newton’s First Law.
- If the resultant force on an object is not zero, the object will accelerate in the direction of the resultant force. The relationship between the resultant force, F, acting on an object, its mass, m, and the acceleration caused, a, is: This is Newton’s Second Law.
Carte mentale
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Note de cours
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Évaluation de la leçon
Félicitations, vous avez terminé la leçon sur Resultant Forces. 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.
- A box has a force of 12 N acting to the right and a force of 5 N acting to the left. What is the resultant force on the box? A. 7 N to the left B. 7 N to the right C. 17 N to the right D. 60 N to the right Answer: B
- A resultant force of 18 N acts on an object of mass 6.0 kg. What is the acceleration of the object? A. 0.33 m/s2 B. 3.0 m/s2 C. 12 m/s2 D. 108 m/s2 Answer: B
- Which statement about a pair of forces described by Newton's Third Law is correct? A. The two forces act on the same object B. The larger object experiences the larger force C. The two forces are equal in magnitude and opposite in direction D. The two forces always cancel to give a zero resultant Answer: C
- The resultant force on a moving car is zero. What happens to the car? A. It stops immediately B. It slows down steadily C. It continues to move at the same velocity D. It accelerates in the direction it is already moving Answer: C
- A raindrop falls from a cloud and after a while it falls at a constant velocity. Which statement about the raindrop is correct while it falls at a constant velocity? A. Its weight is greater than the air resistance on it B. The resultant force on it is zero C. Its acceleration is 9.8 m/s2 D. No forces act on it Answer: B
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