Question

A girl pushes her little brother on his sled with a force of 300 N for 750 m. a. How much work is this? b. How much energy is used? c. How much power does she use if it takes her 25s? d. How much power does she use if she runs and complete the 750m in 10s? A 75.0 kg man pushes on a 500,000 kg wall for 250 s but it does not move. a. How much work does he do on the wall? b. How much energy is used? c. How much power is exerted?

Answer 1

7) Given that F= 300 N & d= 750 meters cos$\Theta$=0⁰

a) We know that work done(W)=(F$\times$d cos$\Theta$)=(300$\times$750$\times$1)=(225000 Joules)=(225 KJ) (since cos0⁰=1)

b) Energy spent on an object is transferred as a work done on that object.Hence enegy used = (225 KJ)

c) Here given that t= 25 sec, hence power =(Rate of doing work) = (W/t) = (225 KJ/25) =(9 KJ/sec)

d) Here 'd' remains same but t=10 sec, power = (W/t) = (225 KJ/10) =(22.5 KJ/sec)

8) Here m=75 kg and wall does not moved

Work is said to be done only if the object is moved to a distance(d) from respective initial position.

a) Here wall does not moved,hence work done on the wall by him=(0) Joules

b) But,Energy is spent to move there may exist internal energy but physically no energy.

c) Power =(W/t) = (0/t) = (0 Joules/sec)

Answer 2

W=Fd =(300N)(750m) =225,000 N. m. Or 225,000 Joules

Answer 3

SOLUTION :

7.

a.

Work done 

= Force * distance moved

= 300 * 750

= 225000 J 

= 225 kJ (ANSWER)

b.

Energy used

= Work done 

= 225000 J 

= 225 kJ(ANSWER)

c.

Power used 

= energy used per sec

= 225000 / 25

= 9000  W

= 9 kW (ANSWER).

d.

Power used 

= energy used per sec

= 225000 / 10

= 22500 W

= 22.5 kW (ANSWER).

8.

a.

There is no movement of the object,

So, 

Work done = 0  J

b.

Energy spent 

= work done 

= 0  J

(However impulse is created there = Force applied * time)

c.

Power used = 0 W