Classical Mechanics Review 2: Units 1-11 Conservation of Energy E f k d W NC ( except Dynamics of the Center of Mass Perfectly Inelastic Collisions: a CM vf F Net , External friction ) 0 M Total m 1 v1 i m 2 v 2 i m1 m 2 Mechanics Review 2 , Slide 1 Example: Work and Inclined planes The block shown has a mass of 1.58 kg. The coefficient of friction is µk = 0.550 and θ = 20.0o. If the block is moved 2.25 m up the incline, calculate (including signs): (a) the work done by gravity on the block; (b) the work done on the block by the normal force; (c) the work done by friction on the block. For a constant Force: W F r F r cos q r2 W F dr M r1 q Mechanics Review 2 , Slide 2 Example: Block and spring A 2.5 kg box is released from rest 1.5 m above the ground and slides down a frictionless ramp. It slides across a floor that is frictionless, except for a small section 0.5 m wide that has a coefficient of kinetic friction of 0.2. At the left end, is a spring with spring constant 250 N/m. The box compresses the spring, and is accelerated back to the right. What is the speed of the box at the bottom of the ramp? What is the maximum distance the spring is compressed by the box? 2.5 kg k=250 N/m h=1.5 m mk = 0.4 d = 0.50 m E W f fk d Mechanics Review 2 , Slide 3 Example: Block slides down the hill A block of mass m starts from rest at the top of the frictionless, hemispherical hill of radius R. (a) Find an expression for the block's speed when it is at an angle ϕ. (b) Find the normal force N at that angle. (c) At what angle does the block "fly off" the hill? (d) With what speed will the block hit the ground? E 0 Mechanics Review 2 , Slide 4 Example: Pulley and Two Masses A block of mass m1 = 1 kg sits atop an inclined plane of angle θ = 20o with coefficient of kinetic friction 0.2 and is connected to mass m2 = 3 kg through a string that goes over a massless frictionless pulley. The system starts at rest and mass m2 falls through a height H = 2 m. Use energy methods to find the velocity of mass m2 just before it hits the ground? What is the acceleration of the blocks? E W f fk d θ m2 HH =2 m =2 2m kg v v 2 ad 2 f 2 i Mechanics Lecture 19, Slide 5 Example: Two Ice-Skaters Two ice-skaters of mass m1 and m2, each having an initial velocity of vi in the directions shown, collide and fall and slide across the ice together. The ice surface is horizontal & frictionless. 1. What is the speed of the skaters after the collision? 2. What is the angle θ relative to the x axis that the two skaters yy travel after the collision? xx m 1 v1 i m 2 v 2 i vf m1 m 2 v vx v y q tan 2 1 2 (v y / v x ) V Mechanics Review 3, Slide 6 Example: Collision with a vertical spring A vertical spring with k = 490 N/m is standing on the ground. A 1.0 kg block is placed at h = 20 cm directly above the spring and dropped with an initial speed vi = 5.0 m/s. (a) What is the maximum compression of the spring? (b) What is the position of the equilibrium of the block-spring system? 1kg E 0 vi E i mgh k a 0 Fy 0 1 2 2 mv i E f mg ( x ) 1 kx 2 2 Mechanics Review 2 , Slide 7 Example: Collision on a Vertical Spring A vertical spring with k is standing on the ground supporting a m2 block. A m1 block is placed at a height h directly above the m2 block and released from rest. After the collision the two blocks stick together. (a) What is the speed of the two blocks right after the collision? (b) What is the maximum compression of the spring? (c) What is the position of the equilibrium after the collision? E 0 v i 2 gh 2 vf m 1 v1 i m 2 v 2 i m1 m 2 E 0 K U g U sp 0 a 0 Fy 0 Mechanics Review 3, Slide 8 Example: Pendulum L v Conserve Energy from initial to final position mgL 1 mv 2 v 2 gL 2 Mechanics Review 2 , Slide 9 Example: Pendulum T L 2 v a L a v 2 L v 2 gL mg T mg mv L 2 T mg mv 2 L Mechanics Review 2 , Slide 10 Example: Pendulum v v 2 2 ghgh hh Conserve Energy from initial to final position. mgh 1 mv 2 v 2 gh 2 Mechanics Review 2 , Slide 11 Example: Pendulum v h 2 gh r a v 2 r T T mg mv r 2 T mv mg 2 mg r Mechanics Review 2 , Slide 12 Example: Block with Friction A 6.0 kg block initially at rest is pulled to the right along a horizontal surface by a constant horizontal force of 12 N. Find the speed of the block after it has moved 3.0 m if the surfaces in contact have μk = 0.15. What if the force F is applied at an angle θ = 40⁰? E f k d W NC ( except friction ) Mechanics Unit 9, Slide 13 Example: Popgun (Spring and Gravity) The launching mechanism of a popgun consists of a spring. when the spring is compressed 0.120 m, the gun when fired vertically is able to launch a 35.0 g projectile to a maximum height of 20.0 m above its position as it leaves the spring. (a) Determine the spring constant (b) Find the speed of the projectile as it moves through the equilibrium position of the spring. E 0 K U g U sp 0 Mechanics Review 2 , Slide 14 Example: Blocks, Pulley and Spring Two blocks m1 = 4 kg and m2 = 6 kg are connected by a string that passes over a pulley. m1 lies on an inclined surface of 20o and is connected to a spring of spring constant k = 120 N/m. The system is released from rest when the spring is unstretched. The coefficient of kinetic friction of the inclined surface and is 0.12. Find the work of friction when m2 has fallen a distance D = 0.2 m. Find the speed of the blocks at that time. At what distance d does the acceleration of the blocks becomes zero? E W f fk d k 20o m1 20o E i m 2 gd m1 gd sin q Ef m2 1 2 ( m1 m 2 ) v 2 1 kd 2 2 a 0 Fy 0 Mechanics Review 2 6, Slide 15 Example: Spring and Mass A spring is hung vertically and an object of mass m is attached to its lower end. Under the action of the load mg the spring stretches a distance d from its equilibrium position. 1. If a spring is stretched 2.0 cm by a suspended object of mass 0.55 kg what is the spring constant k? 2. How much work is done by the spring on the object as it stretches through this distance? F y kd mg 0 d W s F s dx 0 1 kd 2 2 Mechanics Unit 7, Slide 16 Example: Ballistic Pendulum m v M H A projectile of mass m moving horizontally with speed v strikes and sticks to a stationary mass M suspended by strings of length L. Subsequently, m + M rise to a height of H. Given H, what is the initial speed v of the projectile? Mechanics Unit 11, Slide 17 Ballistic Pendulum m v M H mv ( m M )V V 2 2 gH V m (m M ) v Conservation of momentum Conservation of energy after the collision Combine the two equations v mM 2 gH m Mechanics Unit 11, Slide 18