# P5.1 - P5.4

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• Created by: GraceLong
• Created on: 14-04-16 18:43
• P5.1 - P5.4
• Projectile motion
• Newton said that if a ball could be kicked hard enough from a peak it would never hit Earth because it would stay in orbit. Gravity would make the ball fall to Earth but it wouldn't because of the curvature of the Earth.
• To use 'suvat' equations, the equations need to be separately for horizontal and vertical motion
• If a ball is thrown horizontally from the top of a tower, it will reach the ground at the same time as a ball dropped from the top of the tower. The trajectory of the ball thrown horizontally is parabolic - there is no horizontal force on the ball so it has a constant horizontal velocity.
• The resultant velocity is the vector sum of the constant horizontal velocity and the vertical velocity - they are at right angles so the resultant is represented by the diagonal of the rectangle
• Horizontal speed stays constant, the ball slows down as it goes up and the ball speeds up as it falls. The only force on the ball is a vertical force due to gravity
• Action and reaction
• The momentum of an object depends on mass and velocity. In any collision, the total momentum before the collision is equal to the total momentum after the collision. An explosion is the opposite of a collision
• A boy has a weight because he is attracted to the Earth due to gravity - the Earth is also attracted to the boy which is the reaction force. If the boy jumps in the air he is attracted to Earth and moves down. The Earth is also attracted to the boy but because it has such a large mass, the movement is too small for us to notice
• Every material is made of tiny moving particles with kinetic energy - the higher the temperature, the higher the KE. In a gas the particles are far apart and free to move.
• Forces always occur in pairs. Newton's third law says action and reaction pairs of forces: are equal in magnitude; are opposite directions; and act on different objects.
• Particles collide with walls of containers, creating a force and therefore a pressure - if there is a smaller container there is a higher pressure because they hit the wall more frequently thereofore more FORCE; if there is more temperature there is more KE and more collisions with the walls therefore more force
• Force = rate of change in momentum   If a moving particle was in a sealed box it would have a mass, a moving velocity and momentum. Each time the particle collides perpendicularly with the wall it rebounds with the same speed in the opposite direction. Force on wall = 2 mv/t
• As a rocket moves up, the hot fases releases move down and because momentum is conserved, the high momentum of the large massed rocket moving up is balanced by the high velocity of the exhaust gases
• Vectors and equations of motion
• All bodies fall to Earth under the effect of gravity, they accelerate at 10m/s^2. An object thrown into the air slows down with an acceleration of -10m/s^2. At the top of its ascent, its velocity is 0
• s = distance  u = initial velocity          v = final velocity          a = acceleration     t = time taken
• The resultant of two forces that are not in the same straight line is found using parallelogram of forces - if they are a right angle it is a rectangle (Pythagoras). The resultant is the diagonal line of the parallelogram
• A scaler has magnitude only e.g. speed. A vector has both magnitude and direction e.g. velocity. Vectors are often represented by arrows in the correct direction and the length represents the magnitude
• Satellites, gravity and circular motion
• Planet's orbit faster around the Sun because they travel a shorter distance and they travel quicker
• The larger and the mass and the closer the object, the greater the gravitational force. Force is inversely proportional to the square of distance.
• Satellites move at a tangent and gravity makes it accelerate towards Earth - therefore it orbits around Earth. Comets travel very quickly when close to the Sun
• Every object in the Universe attracts every other object which is important on an astronomical scale. Planets orbit because of gravitational attracting and the force towards the center is called the CENTRIPETAL FORCE
• Polar orbits - above North and South pole taking about 90 minutes and they can see the whole of the Earth's surface so can be used for short-range weather forecasts
• Geostationary orbits- they orbit above the equator and take 24 hours (so they always appear at the same place on the equator) and they are used for communication. If the satellites get too close together, their signals would overlap due to diffraction
• The nearer the satellite to Earth, the shorter the orbit time . The distance travelled is the radius from the centre of Earth to the satellite times two pi. If the radius is small, speed will be large