Kinetic energy exercises

Test your knowledge with questions about kinetic energy and solve your doubts with the commented resolution.

question 1

Calculate the kinetic energy of a ball of mass 0.6 kg as it is thrown and reaches a velocity of 5 m/s.

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Correct answer: 7.5 J.

Kinetic energy is associated with the movement of a body and can be calculated using the following formula:

Substituting the question data in the formula above, we find the kinetic energy.

Therefore, the kinetic energy acquired by the body during movement is 7.5 J.

question 2

A doll with a mass of 0.5 kg was dropped from a window on the 3rd floor, at a height of 10 m from the ground. What is the kinetic energy of the doll when it hits the ground and how fast did it fall? Consider the acceleration of gravity to be 10 m/s².

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Correct answer: kinetic energy of 50 J and speed of 14.14 m/s.

When playing the doll, work was done to move it and energy was transferred to it through movement.

The kinetic energy acquired by the doll during launch can be calculated by the following formula:

Replacing the utterance values, the kinetic energy resulting from the movement is:

Using the other formula for kinetic energy, we calculate how fast the doll fell.

Thus, the kinetic energy of the doll is 50 J and the speed it reaches is 14.14 m/s.

question 3

Determine the work done by a body of mass 30 kg so that its kinetic energy increases as its velocity increases from 5 m/s to 25 m/s?

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Correct answer: 9000 J.

Work can be calculated by varying kinetic energy.

Replacing the values ​​of the statement in the formula, we have:

Therefore, the work required to change the body's speed will be equal to 9000 J.

See too: Work

question 4

A motorcyclist is riding his motorcycle on a road with radar at a speed of 72 km/h. After passing through the radar, it accelerates and its speed reaches 108 km/h. Knowing that the mass of the motorcycle and rider combination is 400 kg, determine the variation in kinetic energy suffered by the rider.

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Correct answer: 100 kJ.

We must first perform the conversion of the given speeds from km/h to m/s.

The change in kinetic energy is calculated using the formula below.

Substituting the problem values ​​in the formula, we have:

Thus, the kinetic energy variation in the path was 100 kJ.

question 5

(UFSM) A mass bus m travels along a mountain road and descends a height h. The driver keeps the brakes on so that the speed is kept constant in module throughout the journey. Consider the following statements, check whether they are true (T) or false (F).

( ) The kinetic energy variation of the bus is null.
( ) The mechanical energy of the bus-Earth system is conserved, as the speed of the bus is constant.
( ) The total energy of the bus-Earth system is conserved, although part of the mechanical energy is transformed into internal energy. The correct sequence is

a) V – F – F.
b) V – F – V.
c) F – F – V.
d) F – V – V.
e) F - V - F

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Correct alternative: b) V – F – V.

(TRUE) The bus's kinetic energy variation is zero, as the speed is constant and the kinetic energy variation depends on changes in this magnitude.

(FALSE) The mechanical energy of the system decreases, because as the driver keeps the brakes on, the potential energy gravitational decreases when converted to thermal energy by friction, while kinetic energy remains constant.

(TRUE) Considering the system as a whole, energy is conserved, however, due to the friction of the brakes, part of the mechanical energy is transformed into thermal energy.

See too: Thermal energy

question 6

(UCB) A given athlete uses 25% of the kinetic energy obtained in running to perform a poleless high jump. If it reached a speed of 10 m/s, considering g = 10 m/s², the height reached due to the conversion of kinetic energy into gravitational potential is as follows:

a) 1.12 m.
b) 1.25 m.
c) 2.5 m.
d) 3.75 m.
e) 5 m.

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Correct alternative: b) 1.25 m.

Kinetic energy is equal to gravitational potential energy. If only 25% of the kinetic energy was used for a jump, then the quantities are related as follows:

Replacing the values ​​of the statement in the formula, we have:

Therefore, the height reached due to the conversion of kinetic energy into gravitational potential is 1.25 m.

See too: Potential energy

question 7

(UFRGS) For a given observer, two objects A and B, of equal masses, move with constant speeds of 20 km/h and 30 km/h, respectively. For the same observer, what is the reason?_THE/AND_B between the kinetic energies of these objects?

a) 1/3.
b) 4/9.
c) 2/3.
d) 3/2.
e) 9/4.

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Correct alternative: b) 4/9.

1st step: calculate the kinetic energy of object A.

2nd step: calculate the kinetic energy of object B.

3rd step: calculate the ratio between the kinetic energies of objects A and B.

Therefore, reason E_THE/AND_B between the kinetic energies of objects A and B is 4/9.

See too: Kinetic energy

question 8

(PUC-RJ) Knowing that an 80 kg cybernetic runner, starting from rest, performs the 200 m test in 20 s maintaining a constant acceleration of a = 1.0 m/s², it can be said that the kinetic energy reached by the corridor at the end of 200 m, in joules, is:

a) 12000
b) 13000
c) 14000
d) 15000
e) 16000

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Correct alternative: e) 16000.

1st step: determine the final speed.

As the runner starts from rest, its initial velocity (V₀) has a value of zero.

2nd step: calculate the runner's kinetic energy.

Thus, it can be said that the kinetic energy reached by the corridor at the end of the 200 m is 16 000 J.

question 9

(UNIFESP) A child weighing 40 kg travels in his parents' car, sitting in the back seat, fastened by the seat belt. At a given moment, the car reaches a speed of 72 km/h. Right now, the kinetic energy of this child is:

a) 3000 J
b) 5000 J
c) 6000 J
d) 8000 J
e) 9000 J

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Correct alternative: d) 8000 J.

1st step: convert speed from km/h to m/s.

2nd step: calculate the child's kinetic energy.

Therefore, the child's kinetic energy is 8000 J.

question 10

(PUC-RS) In a pole vault, an athlete reaches a speed of 11 m/s just before planting the pole in the ground to climb. Considering that the athlete can convert 80% of his kinetic energy into gravitational potential energy and that the gravity acceleration at the location is 10 m/s², the maximum height that its center of mass can reach is, in meters, about,

a) 6.2
b) 6.0
c) 5.6
d) 5.2
e) 4.8

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Correct alternative: e) 4.8.

Kinetic energy is equal to gravitational potential energy. If 80% of the kinetic energy was used for a jump, then the quantities are related as follows:

Replacing the values ​​of the statement in the formula, we have:

Therefore, the maximum height that its center of mass can reach is approximately 4.8 m.

See too: Gravitational Potential Energy