1. 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.

2. Action Force (N) 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

3. The Newton Balance When a force is applied to a Newton Balance:
the spring stretches becoming longer
the pointer indicates on the scale how big the force is.
The bigger the force, the more the spring stretches.

4. 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).

5. Mass (kg)
Weight (N)
0.5
1.0
1.5
2.0
2.5 5 10 15 20 25 10
On Earth we found that:

6. Gravitational Field Strength
The weight of an object is given by:
Quantity
Unit
Weight ( W )
Mass ( m )
Gravitational Field Strength
( g )
Newtons ( N )
Kilograms ( kg )
Newtons per Kilogram
( N kg-1 ) x ÷ W m g
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.

7. 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:

8. Example 1
A persons mass is measured to be 78kg. Calculate the weight of the person on Earth.

9. 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)
Earth
Moon
Jupiter
Mars
Deep Space 10
1.6 26 4

10. Example 1
Calculate the weight a 23 kg package
(a) on the Moon
(b) on Mars
(c) in deep space
(a)

11. (b)
(c)
weightless

12. Friction Friction is a force which acts against moving objects.
Air friction is called air resistance.
air resistance
kicking force
As the speed of an object increases, the force of friction opposing it increases.

13. 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.

14. Balanced Forces
Balanced forces are when forces are equal and opposite.
They look like this:
3 kg
tension in string = 30 N
12 N
12 N
weight = 3 x 10 = 30 N

15. 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.
force of friction: 1000 N
engine force: 1000 N

16. 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.

17. 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 kg
4 N
3 N
5 N
3 kg
12 N
4 N
unbalanced force:
8 N to the right
unbalanced force:
6 N to the right

18. 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

19. Yellow Book
Resultant Force – Page 66
Q46, 47, 48, 49

20. Newton’s Second Law F m a 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. x ÷ F m a

21. Metres per second per second ( ms-2 )
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.
2 N
3 kg
14 N

22. The frictional force is 800 N. Calculate the mass of the car.
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 N
1,700 N

23. Newton’s Second Law – Pages 66 - 69
Yellow Book
Newton’s Second Law – Pages
Q50, 51, 55 (a) (b), 57, 61, 63*