Forces Newton’s Laws

Forces- Newton’s LawsForces- Newton’s Laws

GCSE Year 11GCSE Year 11

Learning IntentionsLearning Intentions

By the end of the lesson we will be able By the end of the lesson we will be able to…to…

- Identify and Identify and label forceslabel forces acting on an acting on an objectobject

- Recall the effect of Recall the effect of applying balancedapplying balanced forcesforces

- Recall the effect of Recall the effect of applying unbalancedapplying unbalanced forcesforces

The Effect of ForcesThe Effect of Forces The effect of a force is only shown when there The effect of a force is only shown when there

is an ‘unbalanced force’ acting (also known as is an ‘unbalanced force’ acting (also known as a a netnet force or force or resultantresultant force). force).

Balanced forcesBalanced forces

If there is no net force then the forces on an If there is no net force then the forces on an object are said to be balancedobject are said to be balanced

Ricky’s force due to weight isRicky’s force due to weight is equalequal to the to the reaction force of the chair but they act in reaction force of the chair but they act in oppositeopposite directions directions

Ricky Tomlinson is at rest…

Chair’s reaction

force

Weight

Frictional

ForceThrust

Continues at constant speed in a straight line…

Balanced Forces and MotionBalanced Forces and Motion

When an object has balanced forces acting on it When an object has balanced forces acting on it willwill

remain at restremain at rest

or or move with move with constant speedconstant speed in a in a straight linestraight line

(uniform velocity / no acceleration)(uniform velocity / no acceleration)

Learning IntentionsLearning Intentions

By the end of the lesson we will be able By the end of the lesson we will be able to…to…

- Recall that an Recall that an unbalanced forceunbalanced force acting acting on an object will cause its on an object will cause its motion to motion to changechange

- State State Newton’s LawsNewton’s Laws of motion of motion

Newton and his LawsNewton and his Laws

Sir Isaac Newton is a dead famous Sir Isaac Newton is a dead famous physicist who studied forces.physicist who studied forces.

Newton’s First Law of motion-Newton’s First Law of motion- An object will remain An object will remain at restat rest or travel at a or travel at a

constant velocityconstant velocity unless an unless an unbalanced unbalanced force actsforce acts on it on it

(Constant velocity means the object will not (Constant velocity means the object will not change its change its speedspeed or or directiondirection))

Page 35

InvestigationInvestigation

How does the change in velocity of an How does the change in velocity of an object vary as the unbalanced force is object vary as the unbalanced force is increased?increased?

Acceleration is Acceleration is directly directly proportional to the proportional to the forceforce

Interpreting the GraphsInterpreting the Graphs

acceleration

For

ce

a k F

a

1 F

Graph and Results Table for Girder One

Calculate the Gradient of the middle line using m = (y2-y1) / (x2-x1) and two points on the line of best fit.

Learning IntentionsLearning Intentions

By the end of the lesson we will be able By the end of the lesson we will be able to…to…

- State State Newton’s 2Newton’s 2ndnd Law Law of motion and of motion and use it to solve simple problemsuse it to solve simple problems

- Recognise the connection between Recognise the connection between circular motioncircular motion, acceleration and an , acceleration and an unbalanced forceunbalanced force

Newton’s Second Law of MotionNewton’s Second Law of Motion

The size of the acceleration due to an The size of the acceleration due to an unbalanced force acting on an object can unbalanced force acting on an object can be calculated using the formula-be calculated using the formula-

Unbalanced Force = Mass x AccelerationUnbalanced Force = Mass x Acceleration

FF == mm xx aa

NewtonNewton == kgkg xx m/sm/s22

AccelerationAcceleration and change in velocity have similar meaning and change in velocity have similar meaning (increase in speed, decrease in speed or change of direction)(increase in speed, decrease in speed or change of direction)

F

am

Unbalanced Force = Mass x AccelerationUnbalanced Force = Mass x Acceleration

FF == mm xx aa

NewtonNewton == kgkg xx m/sm/s22

F

am

Newton’s 2Newton’s 2ndnd Law Law

Newton’s 2Newton’s 2ndnd Law- Example Law- Example QuestionQuestion

What is the force needed to make a rock What is the force needed to make a rock of 2 kg accelerate at 0.25 m/sof 2 kg accelerate at 0.25 m/s22? What is ? What is the acceleration of the rock when an the acceleration of the rock when an unbalanced force of 100 N is applied to it?unbalanced force of 100 N is applied to it?

F = m x F = m x aa

100 = 2 x 100 = 2 x aa

100 / 2 = 100 / 2 = aa

50 m/s50 m/s22 = a = a

FF = m x a = m x a= 2 x 0.25 = 2 x 0.25 = 0.5 N= 0.5 N

Your Turn…Your Turn…

Try question 1 and 4 page 37Try question 1 and 4 page 37

Bit more complicated…Bit more complicated…

Darren Clarke strikes a ball on a level Darren Clarke strikes a ball on a level putting green.putting green.

Name the force opposing the motion of the Name the force opposing the motion of the ball across the greenball across the green

If the ball has a mass of 0.05 kg then what If the ball has a mass of 0.05 kg then what is the value of the opposing force causing is the value of the opposing force causing a deceleration of 0.3 m/sa deceleration of 0.3 m/s22??

The AnswerThe Answer

FRICTIONFRICTION F = m x aF = m x a

F = 0.05 kg x (-0.3) m/sF = 0.05 kg x (-0.3) m/s22

F = -0.015 N (acting against the F = -0.015 N (acting against the motion of the ball)motion of the ball)

0.015 N

Motion of the ball

Causing the ball to slow

down

Sinks it…Sinks it…

One step further…One step further…

Padraig Harrington has an identical shot Padraig Harrington has an identical shot the same distance from the pin (hole). the same distance from the pin (hole).

A wind has started to blow from behind A wind has started to blow from behind Padraig with a constant force of 0.01 N. If Padraig with a constant force of 0.01 N. If he plays the same shot as Darren, what he plays the same shot as Darren, what will the acceleration of the ball be?will the acceleration of the ball be?

The AnswerThe Answer

Forces Acting on the ballForces Acting on the ball

Unbalanced forceUnbalanced force

0.01 N - 0.015 N = -0.005 N0.01 N - 0.015 N = -0.005 N

This force causes a decelerationThis force causes a deceleration F = m x aF = m x a

- 0.005 = 0.05 x a- 0.005 = 0.05 x a

a = (-0.005 / 0.05) = - 0.1 m/sa = (-0.005 / 0.05) = - 0.1 m/s22

0.015 N0.01 N

Motion of the ball

The ball will not slow down as quickly

Missed it…Missed it…

Circular MotionCircular Motion

Centripetal ForceCentripetal Force

The unbalanced force is The unbalanced force is provided by a force acting provided by a force acting towards the centre of the circle towards the centre of the circle called the called the centripetal force.

An unbalanced force causes a An unbalanced force causes a change in motion, an , an accelerationacceleration (speed steady but (speed steady but direction always changing)direction always changing)

F

v

+veBucket on a rope

Electron around a nucleus

Planetary Motion

Car turning a corner

Tension

Electrostatic

Gravity

Friction

Add to the diagrams the direction and name

of the force causing the object’s motion

Circular Motion FactsCircular Motion Facts

The The smaller/greatersmaller/greater the mass, the bigger the mass, the bigger the force needed for circular motionthe force needed for circular motion

The The smaller/greatersmaller/greater the radius of the the radius of the circle, the bigger the force needed for circle, the bigger the force needed for circular motioncircular motion

The The slower/fasterslower/faster the speed of the object, the speed of the object, the bigger the force needed for circular the bigger the force needed for circular motionmotion

Gravity On

Gravity Off

I’m so lonely right

now…

No resultant force…No resultant force…

When the centripetal force is removed When the centripetal force is removed the object will the object will continuecontinue to move along the to move along the tangenttangent to the circle to the circle

Earth

Moon

If gravity was removed the

moon’s motion would be in a

straight line as shown