Physics Chp 5

• Work is done when a force acts to displace an object. Units are Nm which is also J

• W = Fd but the force and displacement have to be parallel to eachother.

• W = Fdcosθ the angle used is between F and d

• How much work does it take by your arm to lift a 0.5 kg book 1 m up off of the table?

• How much does your arm do walking 5 m to the door?

• W = Fd• W = (0.5kg)(9.8m/s2)(1m)• W = 4.9 Nm

• W = 0 the arm doesn’t do any force in the direction of the door.

• If you pull a 15 kg child on a sled using a force of 25 N at 30o above the horizontal, how much work do you do in the first 5 m?

• W = Fdcosθ

• W = (25N)(5m)cos(30o) =

• Kinetic Energy

• KE= 1/2mv2

• Potential Energy• PEg= mgh

• PEel= 1/2kx2 k is called the spring constant

• Mechanical Energy

• ME = KE + PE

• Conservation of Mechanical Energy• MEf = MEi

• If friction is present then Mechanical Energy is not conserved.

• If a 0.5 kg ball drops from a 30 m high building, how fast does it hit the ground?

• PE = mgh KE = ½ mv2 • ME = PE + KE and MEo = MEf

• PEo = KEf • (0.5kg)(9.8m/s2)(30m) = ½ (0.5kg)(v2) • v= 24 m/s

• Work – Kinetic Energy Theorem

• Wnet = ∆KE Wnet = Fnet d

• Power P= W/t Nm/s or J/s or W for watts• or P = Fv

• Ex. How much work could a 100 W light bulb do in 1 minute if it was all useable?

• P = W/t 100 W = W / 60s W = 6000 Nm