FORCES

1
FORCES
2
Vectors

• Vectors have magnitude and direction.
• Scalar quantities have magnitude only.
• A force is an action that causes a change
• Forces are measured in units known as Newtons (N).

3
Contact and Non-contact Forces

• These two force types exist in nature and affect
  the way that humans move.
• The most important non-contact force is gravity.
  Others include magnetic and electrical.
• Examples of Contact Forces
• Ground Reaction Force
• Joint Reaction
• Friction
• Fluid resistance
• Muscle Force
• Elastic Force

4
Contact and Non-contact Forces

• Free body diagrams are vector diagrams showing
  all the contact and non contact forces acting on
  an object.

5
Contact and Non-contact Forces

• Draw and label a free body diagram for
• a.) person running
• b.) high jumper
• c.) cyclist
• d.) a discus that is in flight

6
Newtons Laws of Motion
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Isaac Newton

• Isaac Newton was a mid 1600 American scientist.
• As a scientist he was responsible for many
  advances including the reflecting telescope, the
  calculus branch of mathematics and most
  importantly for us the effect of gravity and the
  laws of motion.
• The unit of force is named after him.

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First Law of Motion

• Often referred to as the Law of Inertia. The law
  states that
• objects will continue to either stay at rest or
  move with the same speed and direction unless
  another force acts upon the object.
• In other words objects will tend to keep doing
  what they were already doing.

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Example

• Suppose that you filled a baking dish to the rim
  with water and walked around an oval track making
  an attempt to complete a lap in the least amount
  of time. The water would have a tendency to spill
  from the container during specific locations on
  the track. In general the water spilled when
• the container was at rest and you attempted
  to move it
• the container was in motion and you
  attempted to stop it
• the container was moving in one direction
  and you attempted to change its direction.

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Activity

• Using Newtons First Law of Motion describe
• a.) how a cars seat belt will protect you in a
  front-on car crash
• b.) To dislodge sauce from the bottom of a sauce
  bottle, by turning it upside down and thrusting
  downward at high speeds and then abruptly
  stopping
• c.) Blood rushes from your head to your feet
  while quickly stopping when riding on a
  descending elevator

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Newtons Second Law of Motion

• The Law of Acceleration

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Newtons Second Law of Motion

• This law is also referred to as the Law of
  Acceleration and states that
• when a body is acted upon by a constant force
  its acceleration is proportional to the force
  and inversely proportional to the mass of the
  object
• Force can be represented using the formula
• F ma

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Newtons Second Law of Motion

• Momentum refers to the magnitude of movement of
  an object and is represented using the equation
• p m v
• Impulse is the force required to change the
  momentum of an object.
• Impulse F t

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Newtons Second Law of Motion

• Activity
• Complete the worksheet looking at Newtons Second
  Law of Motion.

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Newtons Second Law of Motion

• Activity Answers
• Force
• Determine F when a 40kg object is accelerating at
  10ms-2
• F 40 x 10 400N
• Determine F when a 2 kg object is accelerating at
  2ms-2
• F 2 x 2 4N
• Determine the acceleration of an object when a
  force of 10N is applied to a mass of 4kg
• A F/m 10 / 4 2.5ms-2
• Determine the mass of an object has a 10N force
  applied causing the object to accelerate at
  10ms-2
• m F/a 10/10 1kg

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Newtons Second Law of Motion

• Activity Answers
• Momentum
• Find the momentum of a 30kg object that has a
  velocity of 10ms-1
• p m v 30 x 10 300Ns
• Find the velocity of a 20kg object that has 40N
  of momentum
• v p / m 40 / 20 2ms-1
• Find the mass of an object that has 400N of
  momentum and is traveling a 10ms-1
• m p / v 400 / 10 40 kg

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Newtons Second Law of Motion

• Activity Answers
• Impulse
• A golf ball of mass 65.0g is struck such that it
  is reaches a velocity of 45ms-1. The force is
  applied for a period of 0.40 seconds. Calculate
• The force that is initially applied to the golf
  ball
• The impulse that is applied to change the balls
  momentum
•  

F m (v u)/t 0.065 ( 45 0) / 0.4
2.925/0.4 7.3125N
Impulse F t 7.3125 x 0.4 2.925Ns
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Newtons Second Law of Motion

• Activity Answers
• Impulse
• A tennis player returns a serve during a game of
  tennis. The 60g ball struck his racquet at
  50ms-1 and was returned with 40ms-1. The ball was
  in contact with the racquet for 0.5 second.
  Calculate
• The force that is applied to the tennis ball
• The impulse that is applied to change the balls
  momentum
•  

F m (v u)/t 0.06 ( -40 50)/0.5 0.06
(-90)/0.5 -5.4/0.5 -10.8N
Impulse F t -10.8 x 0.5 - 5.4Ns
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Newtons Second Law of Motion

• Activity Answers
• Impulse
• Two rugby players are running at the same speed.
  Player 1 has a mass of 70 kg, whilst Player 2 has
  a mass of 90kg. Which player would have the
  greatest momentum? How would this affect
  tackling?
•  

Player 1 p m v p 70 v
Player 2 p m v p 90 v
Player 2 has greater momentum. This will allow
him to have a greater effect on an opponents
momentum.
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Newtons Second Law of Motion

• Activity Answers
• Impulse
• A soccer play kicks a ball with mass of 0.45kg
  with a velocity of 5ms-1. What is the resultant
  momentum?
• p m v
• 0.45 x 5
• 2.25 Ns
•  

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Newtons Third Law of Motion

• The Law of Action-Reaction
•  

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Newtons Third Law of Motion

• The third law of motion is often referred to as
  the law of action-reaction. It states
• To every action there is an equal or opposite
  reaction
• In simple terms this means that forces cannot act
  in isolation. When a long jumper lands not only
  is he applying a force to the ground on impact
  but the earth exerts an equal and opposite force.

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Newtons Third Law of Motion

• Try these questions to test your understanding of
  the Third Law of Motion.
• While driving down the road, a firefly strikes
  the windshield of a bus and makes a quite obvious
  mess in front of the face of the driver. The
  firefly hit the bus and the bus hits the firefly.
  Which of the two forces is greater the force on
  the firefly or the force on the bus?
• Trick Question! Each force is the same size. For
  every action, there is an equal ...

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Newtons Third Law of Motion

• Many people are familiar with the fact that a
  rifle recoils when fired. This recoil is the
  result of action-reaction force pairs. A
  gunpowder explosion creates hot gases which
  expand outward allowing the rifle to push forward
  on the bullet. Consistent with Newton's third law
  of motion, the bullet pushes backwards upon the
  rifle. The acceleration of the recoiling rifle is
  ...
• a. greater than the acceleration of the
  bullet.
• b. smaller than the acceleration of the
  bullet.
• c. the same size as the acceleration of the
  bullet.
• The force on the rifle equals the force on the
  bullet. Yet, acceleration depends on both force
  and mass. The bullet has a greater acceleration
  due to the fact that it has a smaller mass.