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1 of 28 © Boardworks Ltd 2007 2 of 28 © Boardworks Ltd 2007 What is momentum? All moving objects have momentum. This is a measure of how difficult it is to stop a moving object. If these two cars have the same mass but one is quicker than the other, which has the most momentum? The faster car. If both cars travel at the same velocity, but one is full with luggage and the other is empty, which will have the most momentum? The heavier car. The bigger an object is and the faster it moves, the more momentum it will have and the more difficult it will be to stop. 3 of 28 © Boardworks Ltd 2007 How is momentum calculated? The momentum of an object can be calculated using this equation: momentum = mass x velocity Mass is measured in kilograms (kg). Velocity is measured in metres per second (m/s). Momentum is measured in kilogram metres per second (kg m/s). 4 of 28 © Boardworks Ltd 2007 Scalar or vector? Velocity is a vector quantity – this means it has a magnitude (size) and direction. Scalar quantities, such as speed, only have a magnitude. As velocity is needed to calculate momentum, momentum must also be a vector quantity and it therefore has a direction. If two objects of the same mass are moving in opposite directions but at the same speed (i.e. their velocities are different), the momentum of each object will be of the same magnitude but a different direction. A ‘+’ and a ‘-’ are often used to indicate the direction of momentum of moving objects. 5 of 28 © Boardworks Ltd 2007 Momentum calculations 6 of 28 © Boardworks Ltd 2007 7 of 28 © Boardworks Ltd 2007 What is conservation of momentum? If two objects collide or interact, the forces acting on each one will be the same size but in opposite directions. The same is true for the change in momentum of each object. This means that the momentum lost by one of the objects will be gained by the other object. Therefore, whenever two objects collide or interact, momentum is conserved. 8 of 28 © Boardworks Ltd 2007 Using conservation of momentum 9 of 28 © Boardworks Ltd 2007 Conservation of momentum question Two trolleys collide and stick together. From the data below, calculate the velocity of the trolleys after the collision. trolley A mass = 3 kg velocity = 8 m/s momentum = 24 kg m/s (3 x 8) trolley B mass = 5 kg velocity = -4 m/s momentum = -20 kg m/s (5 x -4) total momentum before collision = 4 kg m/s (24 + -20) mass after collision = 8 kg (3 + 5) momentum after collision = 4 kg m/s velocity after collision = momentum / mass = 0.5 m/s 10 of 28 © Boardworks Ltd 2007 Investigating momentum 11 of 28 © Boardworks Ltd 2007 Momentum in explosions 12 of 28 © Boardworks Ltd 2007 Momentum: true or false? 13 of 28 © Boardworks Ltd 2007 14 of 28 © Boardworks Ltd 2007 Force and change in momentum When a force is applied to an object, the object’s velocity changes. This means that its momentum will also change. The change in momentum depends on the size of the force and the time for which it is applied. The relationship between these values is shown by this equation: force = change in momentum time Momentum is measured in kilogram meters per second (kg m/s). Time is measured in seconds (s). Force is measured in newtons (N). 15 of 28 © Boardworks Ltd 2007 Change in momentum example 1 A rugby ball of mass 0.5 kg is kicked from stationary to a velocity of 8 m/s. The kicker’s foot is in contact with ball for 0.1 seconds. What force does the kicker use? change in momentum force = time = (0.5 x 8) – ( 0.5 x 0) 0.1 = 4 0.1 = 40 N 16 of 28 © Boardworks Ltd 2007 Change in momentum example 2 A tennis ball is rolled at a toy car of mass 0.1 kg. The car is moved with a velocity of 0.5 m/s. If the ball and car are in contact for 0.05 seconds, with what force is the tennis ball rolled? change in momentum force = time = (0.1 x 0.5) – ( 0.1 x 0) 0.05 = 0.05 0.05 = 1N 17 of 28 © Boardworks Ltd 2007 Change in momentum calculations 18 of 28 © Boardworks Ltd 2007 Car crashes and momentum What happens if two cars travelling very quickly collide? Both cars come to a stop in a short space of time. This means that the cars and their occupants experience a large change of momentum very quickly. Why could this cause a very serious injury? A very large change of momentum in a short space of time means the car occupants will experience a very large force. Using this principle, how could you improve the safety of cars? 19 of 28 © Boardworks Ltd 2007 Reducing force in car crashes Many modern car safety features work by increasing the amount of time taken for the person to decelerate in a collision. How does this reduce the risk of serious injury? A longer deceleration means that change in momentum occurs over a longer time. There is therefore a smaller force acting on the person. What features of cars use this principle? seatbelts airbags crumple zones 20 of 28 © Boardworks Ltd 2007 How do car safety features work? 21 of 28 © Boardworks Ltd 2007 22 of 28 © Boardworks Ltd 2007 Multiple-choice quiz 23 of 28 © Boardworks Ltd 2007