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Exercises in Physics for the lecture "Physics 1", Electronics and Telecommunications, regular studies, 1 year,
winter semester 2010/2011
Set 5. Conservation of Energy
Work and kinetic energy. Work done by the gravitational force. Work done by a spring force.
Work done by a variable force. Work and potential energy. Conservative and non
conservative forces. Conservation of energy.
Ex.1
A small block of mass m=0.032 kg can slide along the frictionless loop-the-loop with loop radius
R=12 cm. The block is released from rest at point , at height h=5R above the bottom of the loop.
1. How much work does the gravitational force do on the block as the block travels from point P
to point Q and the top of the loop.
2. If the gravitational potential energy of the block-Earth system is taken to be zero at the bottom
of the loop, what is that potential energy when the block is at point P, at point Q, and at the top
of the loop.
Now, instead of being released , the block is given some initial speed downward along the track.
Do the answers to 1) and 2) increase, decrease or remain the same?
Ex.2
4
A truck of mass 1.2 · 10 km/h with failed brakes is moving downward at 130 km/h just before the
0
driver steers the truck up a frictionless escape ramp with an inclination of α = 15 .
1. What minimum length L must the ramp have if the truck is to stop along it?
2. Does the minimum length L increase, decrease, or remain the same if the truck mass is
decreased?
3. Does the minimum length L increase, decrease, or remain the same if the truck speed is
decreased?
Ex.3
A block of mass m=2 kg is dropped from height h=40 cm onto a spring of spring constant
k=1960 N/m.
Find the maximum distance the spring is compressed.
Ex.4
A conservative force :
where x[m] acts on a particle moving along an x axis. The potential energy associated with this
force has a value of 27J at x=0.
1. Write an expression for U as a function of x .
2. What is the maximum positive potential energy?
3. At what negative value of x is the potential energy equal to zero?
4. At what positive value of x is the potential energy equal to zero?
Ex.5
A skier of mass m=60kg starts from rest at Height H=20m above the end of a ski-jump ramp. As
the skier leaves the ramp, his velocity makes an angle of φ=28º with the horizontal. Neglect the
effect of air resistance and assume the ramp is frictionless.
1. What is the maximum height h of his jump above the end of the ramp?
2. If he increased his weight by putting on a backpack, would h then be greater, less or the same?
Ex.6
A boy is initially seated on the top of a hemispherical ice mound of radius R=13.8m. He
begins to slide down the ice with a negligible initial speed. Approximate the ice as being
frictionless. At what height does the boy lose contact with the ice?
Ex.7
A block of mass m=3.5 kg is accelerated from rest by a compressed spring of spring constant
k=640 N/m. The block leaves the spring at the spring relaxed length and then travels over a
horizontal floor with a coefficient of kinetic friction µ = 0.25. The frictional force stops the block
k
in distance D = 7.8 m.
Find:
1. the increase in the thermal energy of the block-floor system,
2. the maximum kinetic energy of the block,
3. the original compression distance of the spring.
Ex.8
A child whose weight is F =267 N slides down a playground slide of length l = 6.1m. The
g
playground slide makes an angle of α=20º with the horizontal. The coefficient of kinetic friction
between slide and child is µ =0.10.
k
1. How much energy is transferred to thermal energy?
2. If the child starts at the top at speed of v =0.457 m/s, what is his speed at the bottom?
0
Ex.9
A spring (k=200 N/m) is fixed at the top of a frictionless plane inclined at angle α=40º . A block
of mass m=1 kg is projected up the plane, from an initial position that is distance d=0.60m from
the end of the relaxed spring with an initial kinetic energy of E =16J.
k0
1. What is the kinetic energy of the block at the instant it has compressed the spring
∆x=0.20m?
2. With what kinetic energy must the block be projected up the plane if it is to stop
momentarily when it has compressed the spring by 0.40m.
Ex.10
A swimmer moves through the water at an average speed of 0.22 m/s. The average drag force is 110
N. What average power is required of the swimmer?