1 of 43

Starter1. 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?

2. 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?

2 of 43

Forces

3 of 43

What is Force ?

4 of 43

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.

5 of 43

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.

6 of 43

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.

7 of 43

Types of Forces

ThrustFriction

Weight

Support/ Lift

Object

8 of 43

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.

9 of 43

Balanced Forces If two equal size forces act in

opposite directions on an object the net force is...

F = 15 N

F = 15 N

Net Force = 15N - 15N = 0N

Balanced forces do not change the motion of an object.

Zero.

10 of 43

Unbalanced Forces If a pair of opposing forces are

unequal the net force is not Zero.

F = 15 N

F = 25 N

Net Force = 25N - 15N = 10N right

Unbalanced forces will cause an object to accelerate, decelerate or change directions.

11 of 43

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.

50 km/h 500 N500 N

12 of 43

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 N500 N 400 N+50 km/h 500 N60 km/h

13 of 43

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

Weight

Support

Because Gravity and Support are equal the ship stays afloat.

Balanced or Unbalanced ?

14 of 43

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

Weight

Because the forces are

unbalanced the elephant

accelerates downward.

Balanced or Unbalanced ?

Support

15 of 43

What happens to the weight of the ship?What happens to the upthrust on the ship?Which is the largest force - weight or upthrust?

increases

equal

increases

Changing Forces

16 of 43

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.

17 of 43

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.

18 of 43

ForcesCalculating

19 of 43

Force = Mass × Acceleration

Mass must be in kg Acceleration in m/s2

Force in N

Calculating Force

F

m a

20 of 43

F

m a

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 = m x a = 81kg x

1.8m/s2

= 145.8 N

21 of 43

F

m a

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

Fm32,656,000

N2,041,000 kg

22 of 43

Starter The speed-time graph represents

the motion of a car from its starting point to when it stops.

23 of 43

Starter

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.

24 of 43

Weight = Mass ×

Mass must be in kg Acceleration in m/s2

Weight Force in N Acceleration due to

gravity is 10 m/s2

Weight Force

Fgm g

Accelerationdue to Gravity

25 of 43

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.

26 of 43

Fgm g

Try This ! #3 If your mass was 60 kg what

would your weight be?

Fg = m x g = 60kg x

10m/s2

= 600 N

27 of 43

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 N43.2 kg

790 N1150 N

28 of 43

Mass Vs. Weight Where is

weight the greatest?

Where is mass the greatest?

29 of 43

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.

a) Using the equation Fg = mg find the combined weight of Ariana and the jumpmaster.

Weight = __________ N1 500 A

30 of 43

Try This ! NCEA Exam 2005

b) 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 ______________

Upwards A

Friction (air resistance)

31 of 43

Try This ! NCEA Exam 2005 During the first 10 seconds the

net force acting on Ariana and the jumpmaster is 825 N.

c) Calculate the net acceleration of Ariana and the jumpmaster. Include an appropriate unit.

Net acceleration = __________ M5.5 m/s2