Deck 12: Linear Kinetics of Human Movement

Question 1

Identify three practical examples of each of Newton's laws of motion, and clearly explain how each example illustrates the law.

Answer 1

Newton's first law , otherwise known as the law of inertia, states that a body will maintain a state of rest or constant velocity unless acted upon by an external force that changes that state. This is fairly obvious.
Example:
When placed upon a tee, a golf ball will stay there until the club hits it. The ball flying with a constant velocity is not as obvious, as forces such as gravity and air resistance are at work, and changing the velocity of the ball as it travels through the air.
Newton's second law of motion deals with the relationships between force, mass, and acceleration. Known as the law of acceleration, it states that a force applied to a body causes an acceleration of that body of a magnitude proportional to the force, in the direction of the force, and inversely proportional to the body's mass. The second law is where the equation F = ma is derived from.
Example:
Returning to the golf ball, when the golf ball is struck with a club, it travels in a line of action that correlates with the way the club was swung, and travels with an acceleration imparted to it by the club.
Newton's third and final law states that for every action, there is an equal and opposite reaction.
Example:
The golf ball example is not as useful here, so instead consider two football players. If a defensive back tackles a wide receiver with 450 N of force, the wide receiver also exerts a force of 450 N on the defensive back.

Question 2

How much force must be applied by a kicker to give a stationary 2.5 kg ball an acceleration of 40 m/s 2

Answer 2

Recall that the force of an object in motion is given by the equation F = ma, where m is the mass of the object in question (in kilograms), a is the object's acceleration (in m/s 2 ), and F is the force it has, measured in Newton units (kg m/s 2 ). Thus, to find the force of an object in motion, simply plug the variables into the formula.
Mass (m) = 2.5kg, acceleration = 40 m/s 2
So, to kick a 2.5 kg ball with an acceleration of 40 m/s 2 , the kicker must have to use 100 N worth of force.

Question 3

Select one sport or daily activity, and identify the ways in which the amount of friction present between surfaces in contact affects performance outcome.

Answer 3

Friction is ever-present and important in all aspects of life. Just walking down the street involves a great deal of friction. On dry and paved streets, there will be a great deal of friction between the shoe and the ground, leading to a relatively low likelihood of slipping. However, on unpaved dirt roads or in icy conditions, the coefficient of friction decreases drastically, leading to easier slippage.
This principle applies to cars on the road as well, and is one of the reasons that driving on icy roads or in poor weather conditions can be so dangerous. Friction is also a huge factor in the world of sports. Reducing friction between the field or track to increase speed, while at the same time providing enough friction to maintain a grip on the surface which a player is running on is essential for athletes in sports such as football, soccer, and track and field.
This is why cleats or spikes are used: the rough surface of the shoe allows for maximum gripping of the playing surface, while at the same time minimizing friction. On the other hand, basketball players require a maximal amount of contact with the court in order to allow them to slide from position to position.

Question 4

A high jumper with a body weight of 712 N exerts a force of 3 kN against the ground during takeoff. How much force is exerted by the ground on the high jumper

Question 5

A 2 kg block sitting on a horizontal surface is subjected to a horizontal force of 7.5 N. If the resulting acceleration of the block is 3 m/s 2 , what is the magnitude of the friction force opposing the motion of the block (1.5 N)

Question 6

What factors affect the magnitude of friction

Question 7

Explain the interrelationships among mechanical work, power, and energy within the context of a specific human motor skill.

Question 8

If s between a basketball shoe and a court is 0.56, and the normal reaction force acting on the shoe is 350 N, how much horizontal force is required to cause the shoe to slide

Question 9

Explain in what ways mechanical work is and is not related to caloric expenditure. Include in your answer the distinction between positive and negative work and the influence of anthropometric factors.

Question 10

A football player pushes a 670 N tackling sled. The coefficient of static friction between sled and grass is 0.73 and the coefficient of dynamic friction between sled and grass is 0.68.

Question 11

A 108 cm, 0.73 kg golf club is swung for 0.5 s with a constant acceleration of 10 rad/s 2. What is the linear momentum of the club head when it impacts the ball (3.9 kg. m/s)

Question 12

Lineman A has a mass of 100 kg and is traveling with a velocity of 4 m/s when he collides head-on with Lineman B, who has a mass of 90 kg and is traveling at 4.5 m/s. If both players remain on their feet, what will happen

Question 13

A 6.5 N ball is thrown with an initial velocity of 20 m/s at a 35° angle from a height of 1.5 m. (a) What is the velocity of the ball if it is caught at a height of 1.5 m (b) If the ball is caught at a height of 1.5 m, how much mechanical work is required (a. 20 m/s, b. 132.5 J)

Question 14

Two skaters gliding on ice run into each other head-on. If the two skaters hold onto each other and continue to move as a unit after the collision, what will be their resultant velocity Skater A has a velocity of 5 m/s and a mass of 65 kg. Skater B has a velocity of 6 m/s and a mass of 60 kg.

Question 15

A 50 kg person performs a maximal vertical jump with an initial velocity of 2 m/s. (a) What is the performer's maximum kinetic energy during the jump (b) What is the performer's maximum potential energy during the jump (c) What is the performer's minimum kinetic energy during the jump (d) How much is the performer's center of mass elevated during the jump (a. 100 J, b. 100 J, c. 0, d. 20 cm)

Question 16

A ball dropped on a surface from a 2 m height bounces to a height of 0.98 m. What is the coefficient of restitution between ball and surface

Question 17

Using the principle of conservation of mechanical energy, calculate the maximum height achieved by a 7 N ball tossed vertically upward with an initial velocity of 10 m/s. (5.1 m)

Question 18

A set of 20 stairs, each of 20 cm height, is ascended by a 700 N man in a period of 1.25 seconds. Calculate the mechanical work, power, and change in potential energy during the ascent.

Question 19

Select one of the following sport activities and speculate about the changes that take place between kinetic and potential forms of mechanical energy.
a. A single leg support during running
b. A tennis serve
c. A pole vault performance
d. A springboard dive

Question 20

A pitched ball with a mass of 1 kg reaches a catcher's glove traveling at a velocity of 28 m/s.
a. How much momentum does the ball have
b. How much impulse is required to stop the ball
c. If the ball is in contact with the catcher's glove for 0.5 s during the catch, how much average force is applied by the glove