Chapter 1 Motion, Part I
Chapter 2 Motion, Part II
E4. Because inertia causes the water to continue to move even though the brush stops abruptly.
E8. Because your direction is changing. Velocity is speed & direction.
E20. She experiences a net force of zero because her velocity is not changing (newton's 1st lat). The force pulling her from the rope is balancing the drag force on her skies from the water.
E27. The are equal and opposite forces from newton's 3rd law.
E28. Because in either case the decelerate instantly and therefore have the same large force (F=ma).
E31. The person does more work. in each case the force is the same (force = weight = mg). But, for the person the force is exerted over a larger distance since they are much taller. So, the work = fxd is much larger. (You could also argue this from potential ebergy standpoint (i.e. work = change in energy = U = mgh)).
E35. The ball is always accelerating downhill, because the force of gravity is always downhill.
E38. gravity slows you down. The force of gravity is downaward pulling on you. Another way to look at it is that the kinetic energy of your motion gets converted intop gravitational potential energy as you pull against gravity going up the hill.
P1. m = 800kg , a = 4m/s^2 ; F=ma = 800kg x 4 m/s^2 = 3,200 kg m/s^2 = 3,200 N
P2. V = V0 + at ; V0=0, --> V=at so, then t = V/a = 24.6 m/s / 4 m/s^2 t = 6.15 s
E8. when the weight is hanging straight down the line of force goes through the pivot point (shoulder) so the torque is zero. in the other case the torque is give by the weight times the length of the arm which is larger then zero.
E19. by using mechanical advantage and torque. Torque is force x lever arm. A small force times a large distance (lever arm) creates a large torque. This large torque then is applied using a pivot point on a short lever arm to create a large force.
E22. The energy is going into friction, which then gets dissipated as heat energy.
E28. It is equal in size and opposite in direction. the net force is zero and there is no acceleration.
E37. The angular momentum of a rotating object is L = I x w. since the moment of inertia I is much smaller and since the total angular momentum L is conserved the angular velocity w must be much higher.
P8. p = mv = .0001 kg x 1 m/s = .0001 kg m/s
P9. p = mv = 800 kg x 3 m/s = 2,400 kg m/s