12) A 7.80 g bullet moving at 600 m/s penetrates a tree trunk to a depth of 6.0 cm. Using work-energy principle, find the average frictional force that stops the bullet.

A hurricane wind blows across a 8m x 17m flat roof at a speed of 135  km/h.  Calculate the force exerted on the roof.  Hint:  The thickness of the roof is small, so the change in potential energy between the inside of the house and the outside can be approximated to be zero.  ρair  =1. 28 kg/ m3 .

C7 17.) A 1 kg block (block A) is released from rest at the top of a 20 m long frictionless ramp that is 7 m high. At the same time, an identical block (block B) is released next to the ramp so that it drops straight down the same 7 m. Find the values for each of the following for the blocks just before they reach ground level. (a) gravitational potential energy Block A ______________ J Block B  ______________ J (b) kinetic energy Block A ____________________ J Block B ____________________ J (c) speed Block A ____________________ m/s Block B ____________________ m/s (d) momentum Block A _______________________ kg·m/s Block B _______________________ kg·m/s

1. A 23.0 kg child is playing on a swing with a length of 2.5 m. If the swing starts from rest and makes an angle of 40.0◦ with the vertical at the top of the swing, a) determine the child’s speed at the bottom of the swing neglecting friction. b) If the swing has a speed of 2.8 m/s at the bottom, determine the work done by friction during the downswing and, bonus) assuming the force of friction is constant, determine the maximum angle the swing attains on the upswing after passing through the bottom. Hint: Use the approximation that cos θ ≈ 1 − 1 2 θ 2 for angles given in radians.

19- As a 1,240 kg truck travels up a 15.0 m high hill, the nonconservative forces of friction and the force generated by the engine do work on the truck. If the work done by friction is −3.03  105 J and the work done by the engine is +6.36  105 J, determine the change in the truck's kinetic energy (in kJ) as it travels from the bottom of the hill to the top of the hill

Assume that before the accident happened, the car was going at a constant velocity of 60Km/h on a level highway for a distance of 10.0km. The car accelerated briefly to 70.0Km/h to pass another car and then returns to 60.0Km/h as it descends the other side of the hill, even though the driver has taken her foot off the accelerator. Describe the energy transformations taking place when the engine was not doing the work.

A 3500 lb SUV is traveling at 30 mph up a 6% slope when the engine quits. Use work and energy to determine the distance traveled until the vehicle comes to rest (at which point the parking brake is applied). Assume drag forces are negligible.