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Forces - Momentum

This GCSE Physics quiz on forces looks at momentum. All moving objects possess the property of momentum which is the tendency to keep moving in the same direction. The more momentum an object has, the more difficult it is to stop and the more difficult it is to change its direction. Momentum is a vector quantity ~ it has direction as well as a size. To change the momentum of an object requires the application of a force. It is calculated by multiplying the mass of an object by its velocity and is represented by the letter p in equations - p = m x v, so a small mass with a high velocity can have a greater momentum than a larger mass moving with a slower velocity.

When a resultant force acts on an object, it will cause a change in momentum that acts in the direction of the force. The longer this force is applied, the greater will be the change in momentum.

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An example of this in everyday life is the way that you catch things, particularly with hard objects. As you catch them, you move your hands in the direction of the object's momentum. This means that the change in momentum takes place over a longer time period, so less force is required to stop the object in flight and it hurts your hands less. Accelerating or decelerating is another example - a smaller force acting over a longer time can produce the same momentum change.

The idea of momentum is also used in vehicle safety. Seat belts stretch and 'give' slightly during a collision in order to increase the length of time taken to stop you moving. This means that a lower force is exerted on your body resulting in less damage. Crumple zones of vehicles achieve the same, as do airbags which compress as your body hits them. Outside of the vehicle, crash barriers are also designed to be flexible for the same reason, less force is required to stop a vehicle than if they were rigid so there should be less damage to the occupants.

If there are no external forces acting on a system, the total momentum remains the same during a collision. This is known as the conservation of momentum and a favourite of the examiners is to describe two objects colliding, sticking together and then asking you to work out the velocity after the collision. At first, answering such questions may seem difficult but if you keep in mind the conservation of momentum and the equation for momentum, they are not too bad, as you will hopefully realise by the end of this quiz.

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  1. A 0.5 kg trolley is pushed at a velocity of 1.2 m/s into a stationary trolley of mass 1.5 kg. The two trolleys stick to each other after the impact. Calculate the momentum of the 0.5 kg trolley before the collision.
    p = m x v, so p = 0.5 x 1.2 = 0.6 kg m/s
  2. What is the correct formula for momentum?
    Some questions will demand that you use it in this form, but to answer other questions, you may have to rearrange it, so be prepared!
  3. What is the momentum of an object which has a mass of 5 kg travelling at a speed of 2.5 m/s?
    Straighforward use of the momentum equation. In your exam, unless the units are printed at the end of the answer space, you are expected to specify them
  4. An object of mass 1000 kg moving at a velocity of 5 m/s on a level surface, collides with a stationary object of mass 1500 kg. Calculate the momentum of the 1000 kg object before the collision.
    Numerically, options 3 and 4 were both correct, it all comes down to units!
  5. A 0.5 kg trolley is pushed at a velocity of 1.2 m/s into a stationary trolley of mass 1.5 kg. The two trolleys stick to each other after the impact. Calculate the velocity of the two trolleys straight after the impact assuming a closed system.
    Work out the value for the total momentum before the impact. The total momentum after the impact is the same (closed system). You know the combined mass of the two trolleys when they are stuck together, so a simple rearrangement of the momentum equation will allow you to calculate the new velocity
  6. What is momentum measured in?
    Abbreviated to kg m s-1 or kg m/s. Can also be expressed as newton seconds, abbreviated to N s
  7. In a closed system, the total momentum before an event is equal to what?
    A closed system means that none of the energy within the system is lost to the environment. A closed system is often referred to as an ideal system
  8. Momentum is a property of what?
    Only moving objects have momentum, a stationary object will have no momentum
  9. An object of mass 1000 kg moving at a velocity of 5 m/s on a level surface, collides with, and becomes attached to, a stationary object of mass 1500 kg. Calculate the velocity of the two moving objects after the collision.
    At GCSE, you can assume conservation of momentum applies in collision calculations
  10. A car and a truck are both travelling along a motorway at 30 m/s and 20 m/s respectively. The car has a mass of 1,200 kg and the truck has a mass of 4,500 kg. Which one requires the greater force to stop?
    The missing information is the time to bring the car and the truck to a halt. Even though the truck has a greater momentum, if it is allowed more time for braking, it could be brought to a halt with a lower braking force. To stop the truck in the same time as the car would require a greater change in momentum and therefore a greater force

Author: Martin Moore

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