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Starter. A cheetah moves from 17 m/s to 28 m/s while chasing a fawn. If this acceleration takes place over 2.25 seconds, what is the acceleration of the cheetah?

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  • A cheetah moves from 17 m/s to 28 m/s while chasing a fawn. If this acceleration takes place over 2.25 seconds, what is the acceleration of the cheetah?
  • Seeing a police officer monitoring traffic, Mr. Smith slows his car from 78 km/hr to 58 km/hr in 2.14 seconds.  What is the acceleration of the car?
what is force1
What is Force ?
  • A force (F) is a push, pull or twist. It cannot be seen but you can see how a force affects an object.
measuring force
Measuring Force
  • The strength of a force

is measured in newtons (N).

  • 1 Newton is the amount of force needed to make a 1kg object accelerate to 1m/s2 in the absence of an opposing force.
  • Measured using

a force meter.

types of forces
Types of Forces
  • Thrust – the force that is intended to make an object move.
  • Friction – the force that opposes the motion of an object.
  • Weight– the downward pull of gravity on the object.
  • Support – the upward push of the surface on which the object rests.
types of forces1
Types of Forces





Support/ Lift

net force
Net Force
  • The resulting force (net force) on an object can be found by combining forces.
  • If two forces are acting in the same direction they are added.
  • If two forces are acting in opposite directions they are subtracted.
balanced forces
Balanced Forces
  • If two equal size forces act in opposite directions on an object the net force is...


  • Balanced forces do not change the motion of an object.

F = 15 N

F = 15 N

Net Force = 15N - 15N

= 0N

unbalanced forces
Unbalanced Forces
  • If a pair of opposing forces are unequal the net force is not Zero.
  • Unbalanced forces will cause an object to accelerate, decelerate or change directions.

F = 15 N

F = 25 N

Net Force = 25N - 15N

= 10N right

net forces

50 km/h

500 N

500 N

Net Forces
  • Think of a car travelling at a constant speed of 50 km/h.
  • The engine provides sufficient force to just overcome all the frictional forces that are acting to decrease the speed.
net forces1

500 N

Net Forces
  • The air resistance will decrease because the car has a “tail wind” (it is being blown from behind).
  • This means the forces acting on the car are no longer balanced.



50 km/h

500 N

400 N

500 N

balanced or unbalanced
Balanced or Unbalanced ?

The ship is floating on the water. Which forces are acting on the ship?


Because Gravity and Support are equal the ship stays afloat.


balanced or unbalanced1
Balanced or Unbalanced ?

Gravity is acting on the elephant and there is much less air resistance.


Because the forces are unbalanced the elephant accelerates downward.


changing forces
Changing Forces

What happens to the weight of the ship?

What happens to the upthrust on the ship?

Which is the largest force - weight or upthrust?




net forces summary
Net Forces - Summary
  • If the forces on an object are balanced:
    • If the object is stationary, it will remain stationary.
    • If the object is moving, it will continue to move at the same speed and in a straight line.
  • The object will continue to do what it is already doing without any change.
net forces summary1
Net Forces - Summary
  • If the forces are unbalanced, two things can happen:
    • The speed can change. The object will accelerate or decelerate.
    • The direction of motion can change.
calculating force
Calculating Force

Force = Mass × Acceleration

  • Mass must be in kg
  • Acceleration in m/s2
  • Force in N




try this 1
Try This ! #1
  • You and your bicycle have a mass of 81kg. You accelerate at a rate of 1.8 m/s2. Calculate the net force that is accelerating the bicycle.

F = mxa

= 81kgx 1.8m/s2

= 145.8 N




try this 2
Try This ! #2
  • The Space Shuttle has a mass of 2,041,000 kg. Calculate the shuttles acceleration if it can produce a force of 32,656,000 N.

a =


= 16 m/s2






32,656,000 N

2,041,000 kg

  • The speed-time graph represents the motion of a car from its starting point to when it stops.
  • With reference to the information in the graph, calculate the net force acting on the car between 0 and 3 seconds.
  • The mass of the car is 400g.
weight force
Weight Force

Weight = Mass ×

  • Mass must be in kg
  • Acceleration in m/s2
  • Weight Force in N
  • Acceleration due to

gravity is 10 m/s2


due to Gravity




mass vs weight
Mass Vs. Weight
  • Mass is the amount of matter in

an object. Measured in kg.

  • Mass can be measured using a

balance or a scale.

  • Weight is the force of gravity (Fg) pulling downwards on an object. Measured in N.
try this 3
Try This ! #3
  • If your mass was 60 kg what would your weight be?

Fg = mxg

= 60kgx 10m/s2

= 600 N




mass vs weight1
Mass Vs. Weight
  • If your mass was 60 kg then your weight force would be 600 N.
  • Mass = 79 kg Weight =
  • Mass = 115 kg Weight =
  • Mass = Weight = 432 N

790 N

1150 N

43.2 kg

mass vs weight2
Mass Vs. Weight
  • Where is weight the greatest?
  • Where is mass the greatest?
try this ncea exam 2005
Try This ! NCEA Exam 2005
  • Ariana wins a competition for a Tandem Skydive. The plane flies to a height of 5 000 m above sea level. Ariana is strapped to her jumpmaster. Ariana and the jumpmaster have a combined mass of 150 kg.
    • Using the equation Fg = mg find the combined weight of Ariana and the jumpmaster.

Weight = __________ N

1 500


try this ncea exam 20051
Try This ! NCEA Exam 2005
  • One force acting on Ariana and the jumpmaster is gravity. Name the other force acting on them, and state the direction in which it acts.
  • Force
  • Direction ______________

Friction (air resistance)



try this ncea exam 20052
Try This ! NCEA Exam 2005
  • During the first 10 seconds the net force acting on Ariana and the jumpmaster is 825 N.
  • Calculate the net acceleration of Ariana and the jumpmaster. Include an appropriate unit.

Net acceleration = __________

5.5 m/s2