Force
What is the acceleration of 2000kg truck if a force of 4200N is used to make it start moving forward?
F=ma f/m = a 4200/2000 = 2.1m/s
What is the work required to lift a 10kg box 15meters?
W=f*d = 10kg*9.81 * 15m = 1470 N*m
What is the potential energy of a 50kg diver on a 100m cliff?
PE = mgh = 50*9.81*100 = 49050 J
What is the momentum of a 100kg fullback carrying a football on a play at 3.5m/sec
p=mv = 100kg * 3.5m/s = 350kg*m/s
A 2,000-kilogram railroad car moving at 5 m/sec collides with a 6,000kilogram railroad car at rest. If the cars stick together, what is their velocity after the collision?
M1v1+m2v2=(m1+m2)v
(2000*5)/(2000+6000) = vf = 1.25 m/s
What is the force on a 1000 kg elevelator that is falling due to gravity?
F=ma f=1000kg * 9.81m/s2 = 9810
You did 150J of work lifting a 120 Newton backpack. How High did you lift it?
W=fd d=W/f = 150J/120N = 1.25m
What is the Kinetic Energy of a 6kg bowling ball going 10m/s?
1/2 mv2 = 1/2 * 6 * 100 = 300J
A 2500 kg car traveling to the north is slowed down uniformly from an initial
velocity of 20.0 m/s by a 6250 N braking force acting opposite the car’s motion.
Use the impulse-momentum theorem to find out. What is the car’s velocity after 2.50 s?
ft=mvf-mvi vf=[ft+mvi]/m
(-6250N)(2.50s)+(2500kg)(20m/s) / 2500kg = 14 m/s north
A dry cleaner throws a 22 kg bag of laundry onto a stationary 9.0 kg cart. The cart and laundry bag begin moving at 3.0 m/s to the right. Find the velocity of the laundry bag before the collision.
M1v1 + m2v2 = (m1 + m2) v2
[(m1+m2)v2 - m1v1i]/m2 = v2
v2= (22+9)*3 /22kg = 4.27 m/s to right
Work in carrying a 10kg weight 10 meters is.
0 - Force must be in same direction as displacment.
A 50.0 kg student climbs up a 5.00 m rope at a constant speed. The student has a power output of 200.0 W. How long does it take the student to climb the rope? How much work does the student do
P=W/t w=fd = mgd = 50kg*9.81*5= 2452.5J
t= W/p = mgd/P = 12.26s
A box at 10m high has 1500J of energy. What will it's Kinetic Energy be when it drops 10m?
PE at 0 is 0. SO KE = PE 1500J
The Grinch throws his 7kg dog into the snowbank at 1m/2. How much Kinetic energy is lost?
KEf-kei= 1/2mv^2-1/2mv^2 = (1/2)(7kg)(1) = 3.5J
An ice hockey puck with a mass of 0.17 kg collides inelastically with a0.75 kg
snowball that is sliding to the left with a speed of 0.50 m/s. The combined puck and snowball slide along the ice with a velocity of 4.2 m/s to the right.
What is the velocity of the hockey puck before the collision?
25 m/s to the right v1i=[(m1+m2)vf-m2v2i]/m1
A 75 kg skier accelerates from rest to a speed of 12 m/s in 3.0 seconds on a frictionless hill. What is the slope of the hill?
Draw a FreeBodyDiagram
F=ma in the x
a=(vf-vi)/t = 12/3=4m/2
F=ma=mgcos(theta)
theta = invcos a/g = invcos (4/9.81) = 66 degrees
A motor does 5000 J of work in 20seconds. What is the power of the motor?
P=W/t = 5000/20 = 250 watts
A 4kg monkey is on a 5m vine. He starts from rest at an angle of 30 degrees with the vertical . When will he be going the fastest and what will be his speed?
PE top = Ke bottom mgh=1/2mv^2 v=sqrt(2gh) = sqrt(2*9.81*(5-5cos30) = 3.6
A net force of 200 N acts on a 100-kg boulder, and a force of the same magnitude acts on a 130-g pebble for the same amount of time. How does the rate of change of the boulder’s momentum compare to the rate of change of the pebble’s momentum?
a) Greater than b) Less than c) Equal to
p=mv=FΔt
since F is the same momentum is the same
Ball A of mass 5.0 kilograms moving at 20. m/s collides with ball B of unknown mass moving at 10 m/s in the same direction. After the collision, ball A moves at 10 meters per second and ball B at 15 meters per second, both still in the same direction.
(A) What is the mass of ball B? (B) Was this an elastic collision or inelastic collision?
m1v1+m2v2 = m1v1f + m2v2f m2=10kg
KE=1/2m1v1^2 +1/2m2v2^2 = 1500
KEf = 1375 inelastic
An 8-kg box is released on a 30° incline and accelerates down the incline at .3 m/s^2. Find how large the coefficient of kinetic friction is.
Draw a FreeBodyDiagram
sum fx =ma=8*.3=2.5
sum fx =fk-mgsin = ma fk=ma +mgsin = 41.64
sum fy=0 fn=mgcos = 68.0
uk = Fk/Fn = 41.64/68 = 0.61
A 15 kg cart is moving with a velocity of 7.50 m/s down a level hallway. A constant force of 10 N acts on the cart, and its velocity becomes 3.20 m/s.
How much work is done on the cart?
How far did the cart move while the force acted?
W=delta KE = 1/2mvf2-1/2mvi2 = (.5)(15kg)(3.2)^2-(.5)(15kg)(7.5)^2 = -345J
W=f*d = d=w/f = 345/10= 34.5N*m
A 70kg stuntman is attached to a bungee cord the is 15.0m at rest. He jumps off a 50m high bridge. When he stops moving, the bungee cord is at 44m. If the spring constant is 71.8 N/m what is the total PE relative to the water when he's at rest?
3.43x10^4 J p. 165 book
A hockey puck has a mass of 0.115-kg and strikes the pole of the net at (37 m/s). It bounces off in the opposite direction at (25 m/s).
(A) What is the impulse on the puck?
(B) If the collision takes .005s what is the force acted on the puck?
Mv2-mv2= .115(-25-37)=7.13kg*m/2
I=fdelta t 7.13/.005)=1426N
A 16.0 kg canoe moving to the left at 12.5 m/s makes an elastic head-on collision with a 14.0 kg raft moving to the right at 16.0 m/s. After the collision, the raft moves to the left at 14.4 m/s. Disregard any effects of the water.
a. Find the velocity of the canoe after the collision.
b. Verify your answer by calculating the total kinetic energy before and after the collision
14.1 m/2 to right.
m1v1+m2v2=m1v1 +m2v2