52 Unit 6 Practice and Assessment

Outcome 1

1) For each object below, draw a :

a) A car hanging from a crane (there are two forces).

b) A car skidding to a stop (there are three forces).

c) A car with the parking brake set being pushed on by a someone, but not moving (there are four forces here, but two of them are the same type).

 

2) A person stands on a scale.

a) What type of force is pulling them down?

b) What type of force is provided by the scale to hold them up?

c) Draw a free body diagram of this situation.

Outcome 2 

4) Consider the following items:

  • Pliers
  • Tweezers
  • Shovel

(a) For each case, draw a stick figure of the tool and label the , , , , and .

(b) State the for each item above.

Outcome 3

5) For each item in the list in Exercise 4), state whether the tool is providing or increasing . Explain how you know.

Outcome 4

6) When a person raises their heels off the ground, the foot acts like a lever.

(a) Typically we consider the foot as a , but if we treat the ankle bone as the , the in the calf muscle as the , and the from the floor as the , what class of lever is this system?

(b) Calculate the mechanical advantage of this system.

(c) Calculate the tension applied by the calf muscles (F_A) to lift a person with weight of 637 N.

(d)Calculate the force in the ankle joint between the foot and the lower leg bones (F_P). [Hint: Both the normal force from the floor and the calf tension point upward. In order for the foot to be in static equilibrium, the force of the lower leg pushing down on the foot must cancel out both of those upward forces.]

(e) Convert your previous two answers (calf tension and force on ankle) to pounds.

 

A foot with heel raised off the floor. The normal force acts on the ball of the toe and is equal and opposite to the weight. The tension in the Achilles' tendon acts upward on the heel, and the force of the leg bones on the foot acts downward at the ankle. The horizontal distance from the tension force the the ankle is 4 cm and the horizontal distance from normal force to ankle is 12 cm.
The foot acting as a lever arm. Image Credit: OpenStax College Physics

[1]

 

7) The head and neck are also a lever system.

(a) State the class of this lever system.

(b) Calculate the mechanical advantage of this system.

(c)  Calculate the force of tension in the neck muscles (F_M) to hold the head in the position shown in the diagram.

(d) Calculate the force on the head-neck joint (F_J).

(e) Convert your previous two answers to pounds.

A head and neck system as a lever. The force at the joint acts upward on the head, the force of the neck muscles acts on the back of the head 5 cm behind the joint. The weight of the head acts downward on the front of the head 2.5 cm in front of the joint.
The head and neck acting as a lever system.Image Credit: OpenStax College Physics

[2]

 

8) The structure in the following image remains at rest. What do you know about the force and net torque on the structure?

An inverted triangular structure at rest with a block weighting one side, an arm weighting the other,  and a rocket pushing up on the arm. Rocket images  from http://wpclipart.com are in the Public Domain.

 

9) An engineer performing an inspection on the structure from the previous exercise and measures 45 from beneath the center of gravity of the block to the point where the structure contacts the ground. The block weighs 1200 N. She then measures the distance to the beneath the center of gravity of the arm to be 95 m. The arm weighs 1200 N as well. Finally she measures the distance to beneath the rocket to be 150 m from the contact point. She then calculates the force being provided by the rocket.

(a) what value does the engineer get when calculating the force provided by the rocket?

 

(b) The engineer then calculates the normal force on the structure from the ground, what value does she get?


  1. OpenStax, College Physics. OpenStax CNX. Aug 3, 2018 http://cnx.org/contents/031da8d3-b525-429c-80cf-6c8ed997733a@11.42
  2. OpenStax, College Physics. OpenStax CNX. Aug 3, 2018 http://cnx.org/contents/031da8d3-b525-429c-80cf-6c8ed997733a@11.42

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