MOTION: Forces, momentum, work and energy

Newton’s First Law of Motion: A body will remain in its state of rest or uniform motion in a straight line unless acted upon by a resultant/net force Newton’s Second Law of Motion: The rate of change of momentum of a body is directly proportional to the resultant/net force acting on it and takes place in the direction of the resultant/net force.Newton’s Third Law of Motion: When two bodies A and B interact, the force that A exerts on B is equal and opposite to the force that B exerts on A.Objects moving with constant velocity experience zero net force.Net force can cause an object to change its velocity without changing its speed (e.g. circular motion). In general net force can cause change in either speed or direction.1 Newton: The force acting on 1kg of mass, producing an acceleration of 1m/s2.In two-box scenario, consider all bodies as 1 body, all must move at the same acceleration.Force of object on inclined plane:. Hence:Power required to give body of mass m a forward acceleration a when it is moving with a velocity v up a frictionless track inclined at an angle to the horizontal:Two bodies of different masses moving down the same slope will undergo the same acceleration.KNOW HOW TO DRAW FREE BODY DIAGRAMS INCLUDING SCALE.Explain why field lines can never touch or cross:Field lines give direction of force on a mass or charge, if they were to touch or cross, the particle would move in two directions at the same time and this is impossible.Pulley scenario: Tension in both strings is the same, both masses must move with the same acceleration. Hence for the one moving up, , and for the one moving down, .Lift scenario:Lift can accelerate up/down, or move up/down with constant speed.If moving downwards at decreasing speed, T>W, or Fn>FwWhen lift accelerates upwards, mass stays the same but weight is greater.Balls colliding with wall: Since , average force on wall = (no. of ball hitting per unit time)(change in momentum of each ball)Inertia: The resistance of all bodies to change their initial states of rest or uniform velocity, increases with massWeight: The force of gravity acting on a bodyNormal contact force: The force that is perpendicular to the surface of contact. It is equal to the weight if there is no resultant force. It is less than the weight when the body is pushed down, and greater than the weight when the body is pulled up; hence gives the feeling of weight (elevator situation)Friction: Force that resists and opposes motionTension: Bidirectional force in a stringExplain why a gun will recoil when a bullet is fired: Newton’s 3rd Law states that action and reaction forces of two bodies are equal and opposite. The force on the gun and the force on the bullet are an action and reaction pair. Thus force on gun in opposite direction to that on bullet, in the backwards direction, thus gun recoils.A block is at rest on a horizontal surface. The magnitude of the minimum force to start the block moving is Fm. The magnitude of the force required to keep the block moving at constant speed is Fc.Fm will always be greater than Fc. (Fm, net force needed to accelerate the block from rest at that instant, meanwhile Fc = frictional force.)

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