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# Chapter summary

• Projectiles are objects that move through the air.

• Objects that move up and down (vertical projectiles) on the earth accelerate with a constant acceleration g which is approximately equal to 9,8 m·s−2 directed downwards towards the centre of the earth.

• The equations of motion can be used to solve vertical projectile problems.

$vf=vi+gtΔx=(vi+vf)2tΔx=vit+12gt2vf2=vi2+2gΔx$(1)
• Graphs for vertical projectile motion are similar to graphs for motion at constant acceleration. If upwards is taken as positive the $Δx$ vs t, v vs t ans a vs t graphs for an object being thrown upwards look like this:

• Momentum is conserved in one and two dimensions.

$p=mvΔp=mΔvΔp=FΔt$(2)
• An elastic collision is a collision where both momentum and kinetic energy is conserved.

$pbefore=pafterKEbefore=KEafter$(3)
• An inelastic collision is where momentum is conserved but kinetic energy is not conserved.

$pbefore=pafterKEbefore≠KEafter$(4)
• The frame of reference is the point of view from which a system is observed.