AQA A-level Physics Kinematics
This page covers the following topics:
The equations of motion given in the diagram can be used to calculate displacement, initial and final velocity, acceleration and time, provided that three of these variables are known. These equations can be used only when the acceleration is uniform and when the object is travelling in a straight line. For an object falling vertically downwards, acceleration can be moddeled to be equal to g = 9.8 m/s². For an object that is dropped, the inital velocity can be assumed to be 0.
The normal force is the support force exerted onto an object by another stable object that it is in contact with. The friction force is the force which opposes the direction of motion exerted by a surface onto an object moving or trying to move across it. The maximum value of the friction force F = μR, where μ is the coefficient of friction and R is the normal force, is reached when the body it is acting on is in limiting equilibrium, ie. about to move.
Problems involving objects on slopes can be solved by resolving forces perpendicularly and parallelly to the slope, rather than horizontally and vertically. The angle the slope makes with the horizontal is equal to the angle the weight of the object makes with the line perpendicular to the slope.
An object thrown into the air follow a parabolic path called a trajectory; this is called a projectile. This object will move under the influence of gravity. Since gravity acts downwards, it only affects the vertical velocity of the object. Equations of motion can be used to calculate the vertical velocity. There are no horizontala forces acting on the projectile, since air resistance is usually neglected, so the horizontal velocity remains constant during the time of flight and the equation velocity = distance/time can be used. The object travels upwards at a decreasing rate, until its velocity becomes 0 and it starts to accelerate towards the ground. Gravity also affects objects that are falling vertically. At the beginning, the only force acting on the object is its own weight, so the object accelerates downwards. As the velocity of the object increases, the drag forces acting on it increases. At some point, the drag force will balance the weight force and there is no resultant force acting on the object, therefore by Newton's First Law, it will continue to fall at constant velocity; this is called terminal velocity.
A painting is suspended in equilibrium by two inextensible strings, both making 45° with the horizontal. Given that the tension in one of the strings is 48 N, calculate the mass of the painting. Use g = 9.8 m/s², give your answer to 3 significant figures.
A box of mass 3 kg is being held by two ropes as shown in the diagram: one perpendicular to the wall and another at 60˚ to the horizontal attached to the ceiling. Calculate the tension in the two ropes, T1 and T2. Use g = 9.8 m/s².
A particle of mass 600 g lies on a rough slope angled at 37° to the horizontal. The particle is held in limiting equilibrium by a force P acting up the slope, in the direction parallel to it. Given that the coefficient of friction between the particle and the slope is 0.19, calculate the magnitude of P. Use g = 9.8 m/s² and give your answer to 3 significant figures.
A box of mass 0.9 kg is being pulled up a rough slope angled at 35˚ to the horizontal by a rope exerting a tension force of 10 N parallel to the slope. Given that the box starts to move from rest and that the coefficient of friction between the box and the slope is 0.25, calculate the time taken for the box to reach 10 m/s. Use g = 9.8 m/s².
A sky-diver jumps out of a plane and keeps free falling until he reaches terminal velocity. He then opens his parachute. Explain the acceleration and the forces acting on the sky-diver at each stage.
A ball is kicked at an angle to the horizontal and follows a parabolic path. The ball lands on the ground 23 m from its original position. The time of flight of the ball is 4 seconds. Given that g = 9.8 m/s², calculate the initial speed that the ball is kicked with.
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