FORCES

VectorsVectors have magnitude and direction.Scalar quantities have magnitude only.A force is an action that causes a

changeForces are measured in units known as

Newtons (N).

Contact and Non-contact ForcesThese 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 ForceJoint ReactionFrictionFluid resistanceMuscle ForceElastic Force

Contact and Non-contact ForcesFree body diagrams are vector diagrams showing all the contact and non contact forces acting on an object.

Contact and Non-contact ForcesDraw and label a free body diagram for:a.) person runningb.) high jumperc.) cyclistd.) a discus that is in flight

Newton’s Laws of Motion

Isaac NewtonIsaac 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 law’s of motion.The unit of force is named after him.

First Law of MotionOften 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.

ExampleSuppose 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.

ActivityUsing Newton’s First Law of Motion describe:

a.) how a car’s 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

Newton’s Second Law of MotionThe Law of Acceleration

Newton’s Second Law of MotionThis law is also referred to as the Law of Acceleration and states that

‘when a body is acted upon by a constant force it’s acceleration is proportional to the force and inversely proportional to the mass of the object’

Force can be represented using the formula

F = ma

Newton’s Second Law of MotionMomentum refers to the magnitude of movement of an object and is represented using the equation:

p = m vImpulse is the force required to change the momentum of an object.

Impulse = F t

Newton’s Second Law of MotionActivity

Complete the worksheet looking at Newton’s Second Law of Motion.

Newton’s Second Law of MotionActivity Answers

ForceDetermine 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 4kgA = 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

Newton’s Second Law of MotionActivity Answers

MomentumFind 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 momentumv = 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

Newton’s Second Law of MotionActivity AnswersImpulseA 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 ballThe impulse that is applied to change the ball’s 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

Newton’s Second Law of MotionActivity AnswersImpulseA 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 ball’s 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

Newton’s Second Law of MotionActivity AnswersImpulse

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 1p = m vp = 70 v

Player 2p = m vp = 90 v

Player 2 has greater momentum. This will allow him to have a greater effect on an opponent’s momentum.

Newton’s Second Law of MotionActivity AnswersImpulse

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

 

Newton’s Third Law of Motion

The Law of Action-Reaction

 

Newton’s Third Law of MotionThe 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.

Newton’s Third Law of MotionTry 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 ...

Newton’s Third Law of MotionMany 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.