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    A car moving at a velocity of 20ms^-1 brakes to a standstill in a distance of 40m. A child of mass 15kg is sitting in a forward-facing child car seat fitted to the back seat of the car.

    (c) Road accidents involving children are being reduced by reducing the speed limit near schools to 20 mph (9ms^-1). Discuss the effect this has on road safety where the speed limit is currently 30 mph.

    This question is 3 marks and I don't know what to write D: help pls
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    (Original post by CookieHero)
    A car moving at a velocity of 20ms^-1 brakes to a standstill in a distance of 40m. A child of mass 15kg is sitting in a forward-facing child car seat fitted to the back seat of the car.

    (c) Road accidents involving children are being reduced by reducing the speed limit near schools to 20 mph (9ms^-1). Discuss the effect this has on road safety where the speed limit is currently 30 mph.

    This question is 3 marks and I don't know what to write D: help pls
    Think about what would happen if there was a crash. The deceleration would be greater the faster the initial velocity. So if you decrease the velocity, you decrease the deceleration (assuming same time for impact). Since F =ma, the slower the speed of the car, the smaller the force of the impact.
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    (Original post by CookieHero)
    A car moving at a velocity of 20ms^-1 brakes to a standstill in a distance of 40m. A child of mass 15kg is sitting in a forward-facing child car seat fitted to the back seat of the car.

    (c) Road accidents involving children are being reduced by reducing the speed limit near schools to 20 mph (9ms^-1). Discuss the effect this has on road safety where the speed limit is currently 30 mph.

    This question is 3 marks and I don't know what to write D: help pls
    Think about:

    The kinetic energy of the child inside the car.

    K.E. = \frac{1}{2}mv^2 and plug the values in to see that dropping the velocity by approximately half, reduces the energy involved by a factor of 4.

    i.e. the child is far more likely to survive the impact with less injury because he/she is hit with a substantially reduced force.

    Also consider:

    a) the stopping distance of the car (the questions hints at this by stating 40m). i.e. because the cars K.E. is one quarter at the lower speed, the car will stop in far less than half of 40m.

    b)consider also the reaction time of the driver to avoid the collision. The car will travel far less distance before the driver reacts to avoid the collision.
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    Thanks for your help I get what to write now
 
 
 
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