Resultant Forces
Visão Geral
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 pull two different answers out of the same velocity-time graph: the acceleration from its gradient and the distance travelled from the area beneath it.
Objetivos
- 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: a = ∆v / t 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: F = m× a This is Newton’s Second Law.
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- 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
- What does the area under a velocity-time graph represent? A. The acceleration of the object B. The distance travelled by the object C. The resultant force on the object D. The mass of the object Answer: B
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