What is acceleration due to gravity?

• If motion is vertical and no other forces are acting on a particle, the acceleration of the particle will be due to gravity
  • Acceleration due to gravity is denoted by the letter?g
  • The value for gravity on Earth?varies?depending on location (the average value is 9.80665)
  • Take?g = 9.8?m s-2?unless told otherwise
  • It is often easier to leave working in terms of?g?as it sometimes cancels out or it can be calculated at the end
  • Some questions may ask for the answer to be left in terms of?g
• Gravity will always act in the?downwards direction?towards the earth
  • If you decide?upwards?is the positive direction, then?a = -g
  • If you decide?downwards?is the positive direction then?a = g
  • It is important to be clear about which direction you have decided is positive

Displacement or Distance?

• ?There is a difference between?displacement?and?distance
• It is more pronounced in?vertical?motion problems
  • Particles typically travel in both directions (up and down)
  • What goes up must come down!
• Displacement,?s, is a vector
  • It is the?position?of a particle (at time?t) relative to its?starting?position
  • It only takes into account where the particle started and ended
  • If a particle has returned to its starting position then its displacement will be zero
• Distance could be …
  • … the distance from the?start

• • … the distance?travelled
  • … the distance from the?origin

(the origin is not necessarily the starting position)

• • … the distance from another particle

• Read the question carefully to be clear about which of these you are finding

How do I solve suvat problems involving gravity?

• ?Virtually the same as suvat in 1D for?horizontal?motion
  • Follow the same steps and be careful with negatives
• You might have to spot that gravity is the acceleration by seeing the phrases:
  • "falling freely"
  • "projected"
  • "thrown/dropped"
• A?diagram?is important to help make clear …
  • … the?positive direction?of motion
  • … the way in which acceleration relates to the positive direction
  • … differences between displacement and distance
• Particles moving upwards will reach a?maximum?height
  • At the maximum height, velocity is instantaneously zero,?v = 0
• If you use the value 9.8 for g then your?final answer?must be written as a?decimal?to?2?significant figures
  • In general the?accuracy of your final answer?should?match?the?accuracy of the value?used for?g?(1sf for g=10, 2sf for g=9.8 and 3sf for g=9.81)

(a)? Find the maximum height the toy rocket reaches.

(b)? Find the time taken from the instant the toy rocket is projected to the instant it returns to the ground.