Why does the “Shoot the Monkey” demonstration work?

1. Why does the “Shoot the Monkey” demonstration work? • Only weight present as a force. • Newton says constant force implies constant acceleration. y D H Starting Velocity V0,y V0,x x

2. Gun fires and monkey drops at t=0 At some time T the dart has moved D in the x direction. Monkey and dart have the same horizontal position! But! Do they have the same vertical position? • What is the monkey’s • vertical position at T? • What is the dart’s • vertical position at T? We need to compare H to v0,yT. Our drawing will help!

3. Look at the green and dashed triangles. They are similar. If T times the base of the green triangle gives the base of the dashed triangle, what does T times the altitude of the green triangle give? So H=v0,yT. The dart and monkey are at the same place at the same time! D=v0,xT H Starting Velocity V0,y V0,x

4. And Zip the monkey met his fate.

5. 1.3 Ramps

6. Why are ramps so handy? Which is easier?

7. Observations About Ramps • Lifting an object straight up is often difficult • Pushing the object up a ramp is usually easier • The ease depends on the ramp’s steepness • Shallow ramps require only gentle pushes • You seem to get something for nothing

8. Clicker question A ramp makes lifting a box easier because… A) You can use less energy to lift the box B) The ramp exerts some force on the box C) A ramp lets you stop for a drink

9. Forces Present On the ball due to the support from the table (Fsupport) W=mg On the table (and the ball) due to the weight of the ball (mg) Fsupport These forces have thesame magnitudefor a ball that’s not accelerating

10. Newton’s Third Law For every force that one object exerts on a second object, there is an equal but oppositely directed force that the second object exerts on the first object. To every action there is an equal and opposite reaction

11. This can be hard to believe sometimes! But you can’t escape it

12. Forces on a Ramp ( No Friction ) Support Force or Fsupport Net Force along ramp Weight or mg

13. And you only have to overcome a smaller force when you use a ramp! You have to apply a force that’s smaller than the weight to accelerate the box Support Force Force along ramp from weight Weight or mg

14. What is wrong with this picture?

15. But you don’t get something for nothing! Introducing a new idea – WORK and ENERGY W=F d

16. Work is defined as the force parallel to the displacement times the displacement: W = Fll x d

17. W = Fll x d Work = Force(along direction of motion) x distance Straight Lifting: 300 lb. x 6 ft. = 1800 ft-lbs Up Ramp: 90 lb. x 20 ft. = 1800 ft-lbs (same work, less force)

18. Find WORK for lifting or sliding. • Lift – Big Force times Small Distance • Slide – Small Force times Big Distance Each product must be the same.

19. Energy and Work • Kinetic energy: energy of motion KE = ½ mv2 • Potential Energy: Stored energy gravitational PE = mgh • Energy: Capacity to do Work • Work: Transferring energy W = Fll x d ENERGY IS ALWAYS CONSERVED

20. Recap: clicker question When Homer exerts a force on hard, metallic Bender by punching Bender with his soft, fleshy hand… • Bender exerts a bigger force on Homer • Bender exerts a smaller force on Homer • Bender exerts the same force on Homer