Question

Draw a sketch and also a good FBD to identify the relevant forces doing work.Pay attention to the units and positive or negative value of the quantities.1. A football player pushes a tackling dummy forwards at a constant 0.85 m/s for 11 m using7150 J of energy. The tackling dummy has a mass of 120 kg.a) Find the force the football player exerts.b) What is the frictional force the dummy exerts?c) What work is done by friction as the dummy is slid along the field for 11 m?d) Is the mass necessary for this question? If not, what force question could be asked that would require the mass of the dummy?

Answer 1

ANSWERS

(a) 650 N

(b) 650 N

(c) 7150 J

(d) An example would be "what is the coefficient of kinetic friction between the dummy and the ground?"

Step-by-step explanation

Given:

• The constant velocity of the football player and dummy, v = 0.85 m/s

,

• The distance the player pushes the dummy for, d = 11 m

,

• The energy used by the player to push the dummy, E = 7150 J

,

• The mass of the tackling dummy, m = 120 kg

First, we have to draw a free-body diagram of this situation,

(a) The energy that the player uses to push the dummy is the work done,

`E=W=F\cdot d`

So the force is,

`F=(E)/(d)=(7150J)/(11m)=650N`

Hence, the football player exerts a force of 650 N on the dummy.

(b) We know that the football player and the dummy are moving at a constant velocity. This means that the acceleration is 0, so, by Newton's second law of motion,

`F_(player)-F_(f,dummy)=0`

So the force of friction the dummy exerts is equal to the force exerted by the football player,

`F_(f,dummy)=F_(player)=650N`

Hence, the frictional force the dummy exerts is 650 N.

(c) Since the frictional force the dummy exerts is equal to the force that the football player exerts, and the distance it is applied for is the same, then the work done is the same as well.

Hence, the work done by friction as the dummy is slid along the field for 11 m is 7150 J.

(d) No, mass is not necessary for this question. A force question that would involve the mass of the dummy would be a question involving the weight of the dummy and, therefore, its mass.

A question like this may be "what is the coefficient of kinetic friction between the dummy and the ground?". In this case, we would need to use the frictional force, found in part (b), and the normal force of the dummy, which is equal to its weight - this is because it is not moving in the vertical direction.