20 Physics -- Work, Energy and Power

A car is moving with a constant speed in a straight line. Is there any net work done by external forces on the car?

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There is no external force as car is having constant speed

A man cycles up a hill, whose slope is 1 in 20 with a velocity of 6.4km/hr along the hill. The weight of the man and cycle is 98kg. What work per minute is he doing? What is his horsepower?

The blocks A and B have equal masses. The surface of A is smooth but that of B has a friction coefficient of 0.1 with the floor. Block A is moving at a speed of 10 m/s towards B which is kept at re...

a) In a lab frame of reference,

Velocity of center of mass is given by,

v'=(mv_OA+ mv_OB)/2m

Now, in center of mass frame of reference,

Since, there will be perfectly elastic collision. After collision, they exchange their velocity.

In lab frame of reference,

Deceleration due to the friction =µg

From kinematics we have,

V_F=V_B-µgt

Since the final velocity of B is zero, we get t=10s.

Distance travelled d is given by,

d = V_Bt - (µgt²)/2

Inserting given values, the distance travelled by block is 50m.

b)...

A cord is used to lower vertically a block of mass M, through a distance "d" at a constant downward acceleration of g/8. The work done by the cord on the block isa. Mgd b. Mgd/8c. 3Mgd/8d. -7Mgd/8

Here, Ma=Mg-T

or, M(g/8)=Mg-T

or, T= M(g-g/8)

or, T= 7Mg/8

so, work done = T*d cos180⁰

=7Mg/8 * (-d)

= -7Mgd/8

So, correct option is d

A 2 kg ball moving straight down strikes the floor at 8 m/s. It rebounds upward at 6 m/s. What is the magnitude of the change in momentum of the ball?

The change in magnitude of the ball is 28 kgm/s.

Solution:

Here,

mass of the ball (m) = 2kg

initial velocity of the ball (u) = 8 m/s (say downward direction is positive)

final velocity of the ball (v) = -6 m/s (because in upward direction)

Hence,

change in linear momentum of the ball (delta P) = m(v-u)

= 2 (-6 - 8)

= 2 (-14)

= -28 kgm/s

The magnitude of change in linear momentum of the ball is 28 kgm/s.

Conservation and the related theory; CONSERVATION OF: THEORY ENERGY Kirchoffs Voltage Law Einsteins Photoelectric EffectBernouli PrincipleInterference ...

A body kept moving with uniform speed in a circle by a centripetal force acting on it. However, the work done by this force is zero. Is it true? Explain.

Work done is equal to Force (acting in the direction of displacement) times the displacement i.e. W=F.S where (.) is dot product.For the work done we need component of force along the direction of displacement but displacement and force are 90, at each and every point on the circular track and therefore the component of force along the direction of displacement will always be zero in case of a body moving in a circular track or a body performing circular motion. We know W=F.S or F.Scos $(θ)$ ,...

If energy is neither created nor destroyed, what happens to the so much energy spent against friction?

The energy is dissipated in the form of heat. The heat energy so produced is not available for work.

A moving bus suddently stops. Is its momentum destroyed?

The energy of the bus has to go somewhere, and the momentum too, as they are independently conserved.

A meteorite burns in the atmosphere before it reaches to the earth's surface. What happens to its momentum?

The meteorite moves under the force of gravity therefore , its momentum goes on changing . But total momentum of meteorite and earth is always conserved

Comets move around the sun in highly elliptical orbits. The gravitational force on the comet due to the sun is not normal to the comets velocity in general. Yet the workdone by the gravitational fo...

The gravitational force on the comet due to the sun is not normal to the comet's velocity in general. Yet the work done by the gravitational force over every complete orbit of the comet is zero. Why? The gravitational force is conservative force.

Two protons are brought towards each other. Will the potential energy of the system increases or decreases?

Since the protons are positively charged particles, they will repal each other. To bring them nearer, work has to be done against the force of repulsion and this will be stored up in the system as potential energy. So the potential energy of the system will increase.

While inflating a rubber tube of a bicycle, the air pump gets heated, why ?

While inflating the rubber tube of a bicycle, a piston moves in a barrel There exists a friction between the barrel and the piston as the piston moves in the barrel. Some work is to be done against the friction. This work done against the friction is converted in to the heat which heats up the pump.

A 10 kg block placed on a rough horizontal floor is being pulled by a constant force 50 N. Coefficient of kinetic friction between the block and the floor is 0.4. Find work done by each individual ...

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