SEHS 4.3..The Fundamentals of Biomechanics III

Define Newton’s three laws of motion• Newton's first law states that a body at rest will

remain at rest, and a body in motion will remain in motion with a constant velocity, unless acted upon by a force. This law is also called the law of inertia

• Newton's second law states that a force acting on a body is equal to the acceleration of that body times its mass. Expressed mathematically, F = ma, where F is the force in Newtons, m is the mass of the body in kilograms, and a is the acceleration in meters per second per second.

Cont’d• Newton's third law states that for every action there

is an equal and opposite reaction. Thus, if one body exerts a force F on a second body, the first body also undergoes a force of the same strength but in the opposite direction.

• Might want to know this one too…. the law of conservation of energy states that the total energy of an isolated system cannot change—it is said to be conserved over time. Energy can be neither created nor destroyed, but can change form; for instance, chemical energy can be converted to kinetic energy.

Newton’s First LawAn object at rest remains at rest and

an object in motion remains in motion with the same speed and

direction unless acted on by a force.

aka – The Law of InertiaInertia is the tendency of an object to

resist a change in its motion. A soccer ball will remain resting on the grass until a force acts on it (a kick).

Once it is kicked, the soccer ball’s inertia will keep it going because the ball RESISTS

changing its motion.

If the ball doesn’t hit anything, the forces of gravity and friction will eventually stop the ball.

On Earth, gravity and friction are unbalanced forces that often change an object’s motion.

•Inertia explains many common events, such as why you move forward in your seat when a car stops suddenly. •When the car stops, inertia keeps you moving forward.•A force, such as the pull of a seat belt, is required to change your motion.

Things tend to keep on doing whatever they’re doing until something else acts on them.

Objects with a lot of mass have a lot of inertia.

It’s hard to change the motion of objects with lots of inertia…

…But easy to change the

motion of objects with little inertia.

FORCE – TIME GRAPHS

NEWTON’S 2ND LAW OF MOTION

The Second Law of Motion• Suppose you are baby-sitting two children

who love wagon rides. Their favorite part is when you accelerate quickly. When you get tired and sit in the wagon, one of the children pulls you. He soon finds he cannot accelerate the wagon nearly as fast as you can.

• How is the wagon’s acceleration related to the force pulling it?

How is the wagon’s acceleration related to the force pulling it?•According to Newton’s second law of motion, acceleration depends on the object’s mass and on the net force acting on the object.

Newton’s Second LawAn unbalanced force causes an object to

accelerate. The acceleration of the object is equal to the net force acting on it divided by the object’s mass.

Newton’s Second LawWhen a pitcher throws a baseball, the harder he

throws, the more the ball accelerates. The mass of the ball stays the same, but the

force increases.

What is Acceleration? The speeding up, slowing down, or change in direction of an object. Acceleration is affected by the forces applied to

objects as well as the mass of the objects in question.

Acceleration = Force / Mass (If you double the mass of an object you cut the acceleration in half)

Objects with lots of inertia (ability to resist a change in motion) have a large mass and objects with little inertia have a smaller mass

•Look at the pictures on the right. •Which vehicle do you think would require a greater force to push?•Why do you think this?

Weight v. MassWeight = the force of gravity acting on an

object. You stand on a scale, gravity pulls you down, and the needle measures your weight

Mass = how much matter makes up an object.

***A person will have the same mass no matter where in the universe it is

measured. However, a person’s weight will be different depending on the force

of gravity where it is being measured. ***

MOMENTUM AND IMPULSENewton’s Second Law

STARTER: Pairs discussion • What makes an object hard to stop?

• Is it harder to stop a bullet, or a truck travelling along the

highway?

• Are they both as difficult to stop as each other?

Learning Objectives

• Define linear momentum and impulse • Explain the relationship between linear

momentum and linear impulse • Analyze force-time graphs

Momentum

• The bullet is hard to stop because it is travelling very fast, whereas the truck is hard to stop because it has a very large mass.

Momentum

• It makes sense to assume that a bullet travelling twice as fast would be twice as hard to stop, and a truck twice the mass would also be twice as hard to stop.

Momentum

Momentum is a measure of the “oomph”(quantity of

motion) that an object has due to its motion. The more mass an object has and the more speed it has the more momentum it has.

The formula for momentum is _______________

(p is momentum, m is mass, and v is velocity)

Momentum is a conserved quantity.

- The momentum of a system will not change unless an outside impulse (strike with time ) is applied to it. (Newton’s 1st Law)

- If the system remains isolated, its total momentum will not change.

- That does not mean that individual parts of a system cannot interact with each other and exchange momentums.

The unit of momentum is a kg•m/s

Impulse

The only way to change momentum is through impulse.

Impulse is an outside force applied for aspecific time.

The harder you push and the longer you push the more the momentum will be changed.

How hard is it to stop a moving object?

To stop an object, we have to apply a force over a period of time.

This is called Impulse Impulse = F * Δt

J = impulse (N∙s) F = force (N) Δt = time elapsed (s)

Impulse is expressed as N.s (The Newton Second)

J

FORCE TIME GRAPHS

Impulse and Force-time graphs

Horizontal Force (N)

F = ma +force +acceleration

F = ma -force - acceleration

Time (s)

At each instant in time during a contact, a force acts to produce an acceleration.

The Impulse is the net effect of all those instantaneous forces.

In other words, it is the average force multiplied by the total time over which the forces have acted.

Running Contact • During a single running contact, your body

undergoes both positive and negative forces that produce positive and negative accelerations.

• A force acting for a period of time produces

an impulse. • If the positive and negative impulses cancel each

other out (equal areas), then the net impulse is zero and the runner is moving at a constant speed.

Force Time Graph Question

Newton’s Third LawWhenever one object exerts a force on

a second object, the second object exerts an equal and opposite force on the first object. These are known

as: Action/Reaction Forces.

GROUP THOUGHT

Push the block down into the water

THINK!

What do you see happening? Why is it happening?

Newton’s 3rd law I f a body A exerts a f orce on body B, body B will exert an equal but opposite f orce on body A.

Hand exerts force on table

ACT I ON Together these arrows are known as a FORCE PAIR

Table exerts force on hand

REACTION

• You constantly use action- reaction force pairs as you move about.

• When you jump, you push

down on the ground.

• The ground then pushes up on you. It is this upward force that pushes you into the air. This is unbelievably cool!

• When you walk forward, you push backward on the ground.

• Your shoe pushes Earth backward, and Earth pushes your shoe forward.

• Do the action/reaction forces cancel each other out?

• NO!!!• Look at the volleyball player

on the left • She exerts an upward force

on the ball.• In return, the ball exerts an

equal but opposite downward reaction force back on her wrists.

• The action and reaction forces act on different objects.

• On the other hand, the volleyball players are both exerting a force on the same object – the volleyball.

• When they hit the ball from opposite directions, each of their hands exerts a force on the ball equal in strength but opposite in direction.

• The forces on the volleyball are balanced and the ball does not move either to the left or to the right.

Action-reaction pairs explain how a gymnast can flip over a vaulting horse, how a kayaker can move through the water, and how a dog can leap off the ground.

•In a similar way, a kayaker moves forward by exerting an action force on the water with a paddle.•The water pushes back on the paddle with an equal reaction force that propels the kayak forward.

Draw the FORCE PAIRS onto the diagrams below

A: C at on the tab le B: P encil on the paper

J r

C : Board on the wall D: Fuelon the rocket

A :C at on the table B : Pencil on the paper

Table on the cat Paper on the pencil

C :Board on the w all D: Fuelon the rocket

VVall on the board Kocket on the 1ue1

,,.

f , ,_

GROUP THOUGHT

• Why use starting blocks in a sprint?

Law of Conservation of Momentum

• Complete on worksheets

Explain how Newton’s three laws of motion apply to sporting activities

• 1st law--Basically, if an object is in motion, it keeps going unless something stops it. What are examples of outside forces that affect inertia? Most anything in the real world--gravity, the surface of the playing field, a defensive player, or the braking action of an athlete's body to stop.

Cont’d• 2nd Law -- If a baseball player hits a ball with double

the force or with a bat of double the mass, the rate at which the ball will accelerate (speed up) will be doubled. Football players can slow down, stop, or reverse the direction of other players depending upon how much force they can generate and in which direction.

Cont’d• A swimmer propels herself through the water because the

water offers enough counterforce to oppose the action of her hands pushing, allowing her to move. An athlete can jump higher off a solid surface because it opposes his body with as much force as he is able to generate, in contrast to sand or other unstable surface.