Forces

A force is when an object is pushed, pulled, stretched or squashed.

Forces can change the:

(a) speed of an object

(b) shape of an object

(c) direction an object is travelling.

A force can cause an object to accelerate, decelerate or remain at a steady speed.

Measuring Force

Force is measured in units called Newtons (N).

A Newton Balance is the instrument used to measure force.

Action Force (N)

lifting a 0.5 kg mass

lifting a schoolbag

opening a door

The Newton Balance

When a force is applied to a Newton Balance:

1. the spring stretches becoming longer

2. the pointer indicates on the scale how big the force is.

The bigger the force, the more the spring stretches.

Mass and Weight

Mass is the amount of matter (atoms and molecules) that an object has.

Mass is measured in kilograms (kg).

Weight is the gravitational force that the Earth exerts on an object.

Weight is measured in Newtons (N).

10

Mass (kg) Weight (N)

5

10

15

20

0.5

1.0

1.5

2.0

2.5 25

On Earth we found that: m W

x

÷

The weight of an object is given by:

W

m g

Quantity Unit

Weight ( W )

Mass ( m )

Gravitational Field Strength

( g )

Newtons ( N )

Kilograms ( kg )

Newtons per Kilogram ( N kg-1 )

g mW

The gravitational field strength is the force acting on each kilogram of mass.

On earth, the gravitational field strength (g) is 10 N kg-1.

My Mass and Weight

Measure your mass in kilograms (kg) by standing on bathroom scales.

scales

person

Result

My mass = kg

Now calculate your weight using the equation:

g mW

Example 1

A persons mass is measured to be 78kg. Calculate the weight of the person on Earth.

-1kg N 10g

kg 78m

???W

g mW

10 78

N 780W

Gravitational Field Strength

The gravitational field strength is the force acting on each kilogram of mass.

The value of g depends on where you are in the universe.

Location Gravitational Field Strength (N kg-

1)

EarthMoonJupiterMars

Deep Space

101.62640

Example 1

Calculate the weight a 23 kg package

(a) on the Moon

(b) on Mars

(c) in deep space

(a)

-1kg N 1.6g

kg 23m

???W

g mW

1.6 23

N 36.8W

(b)

-1kg N 4g

kg 23m

???W

g mW

4 23

N 92W

(c)

-1kg N 0g

kg 23m

???W

g mW

0 23

N 0W weightless

Friction

Friction is a force which acts against moving objects.

Air friction is called air resistance.

kicking forceair resistance

As the speed of an object increases, the force of friction opposing it increases.

Decreasing Friction

Friction can be reduced by:

• oiling moving parts (engine).

• waxing the surface in contact with the ground (ski’s).

• streamlining to lower air resistance (cyclist).

Increasing Friction

Friction can be increased by:

• applying brakes.

• increasing the tread of a tyre.

• using a parachute to increase air resistance.

Balanced Forces

Balanced forces are when forces are equal and opposite.

They look like this:

12 N 12 N 3 kg

weight = 3 x 10 = 30 N

tension in string = 30 N

Newton’s First Law

Newton’s first law states:

“ an object will remain at rest or continue in the same direction at the same speed, unless acted upon by an

unbalanced force.”

This means if a car is travelling at a steady speed of 10 ms-1, the engine force equals the force of friction.

engine force: 1000

N

force of friction: 1000 N

Spaceship

A rocket in space can switch off its motors, and it will travel at a constant speed.

This is because no forces are acting on it.

(no weight or resistance)

Maximum Speed

When a car reaches its maximum speed, the force of friction equals the maximum force of the engine.

It can therefore go no faster.

Unbalanced Force

An unbalanced force is a single force that can replace all other forces.

The unbalanced force is sometimes called the resultant force.

Example 1

State the resultant force for the following:

3 kg12 N4 N

3 kg

4 N3 N

5 N

unbalanced force:

8 N to the right

unbalanced force:

6 N to the right

Seat Belts

A seat belt in a car provides a backwards force to prevent the passenger from moving forward if the car stops suddenly.

If there was no such backward force present the passenger would continue moving forward at a steady speed (from Newton’s first law).

The seatbelt provides an unbalanced force to decelerate the driver (from Newton’s second law).

..\Seatbelt.mpg

Yellow Book

Resultant Force – Page 66

Q46, 47, 48, 49

Newton’s Second Law

Newton’s second law states:

“ an object acted upon by a constant unbalanced force, moves with constant acceleration in the direction of the

unbalanced force.”

This law relates the unbalanced force, mass and acceleration.

a mF

x

÷F

m a

Quantity Unit

Unbalanced Force ( F )

Mass ( m )

Acceleration ( a )

Newtons ( N )

Kilograms ( kg )

Metres per second per second ( ms-2 )

Example 1

Calculate the acceleration of the following 3 kg mass.

3 kg14 N2 N

2-14F N 12

kg 3m?a

mF

a

312

-2ms 4a

Example 2

The engine of a car exerts a force of 1,700 N, causing the car to accelerate at 1.5 ms-2.

The frictional force is 800 N.

Calculate the mass of the car.

800-1,700F

N 90021.5msa

?m

aF

m

1.5900

kg 600m

1,700 N800 N

Yellow Book

Newton’s Second Law – Pages 66 - 69

Q50, 51, 55 (a) (b), 57, 61, 63*