1. Newton’s Laws
We have already looked at Newton’s laws. Now we are going to look at them in more depth. You may have to revise this work if you can not fully answer the following questions.
What apparatus do we use to measure Forces?
Newton balance
What three things can a Force do to an object?
Change an objects shape, speed or direction
How do you know the Forces acting on an object are balanced?
Remains at rest or continues to move with a constant speed in a straight line
Write down Newton's first Law
A body will remain at rest or move with steady speed in a straight line unless acted upon by an unbalance force
Write down Newton's second Law
Fun = ma

2. Dynamics and Space Learning Intention You will be able to:
Use Newton’s laws and balanced forces to explain constant velocity, making reference to frictional forces.
Give practical examples of balanced forces
Carry out calculations involving the relationship between unbalanced force, mass and acceleration for situations where more than one force is acting.
Apply of Newton’s second law to space travel, including rocket launch, landing and terminal velocity.

3. We are going to look again at frictional forces
We are going to look again at frictional forces. We have done this before:
When one surface slides over another surface a force called friction acts between them.
Friction forces always act against the direction of movement.
When a solid object moves through a gas the force of friction acts against it.
The faster the solid object moves through the gas, the greater the friction force.
This can be shown in speed time graphs
speed time graphs

4. All forces by definition must be vectors.
Forces are always applied in a particular direction.
Friction
Friction is a force that opposes motion.
This can mean it causes an object to slow down or it may mean that it stops an object moving.
engine force
friction
There are situations where we want to decrease friction. An example would be between the skis and the snow for a skier.
There are situations where we want to increase friction. An example would be between the brakes and wheel on a bike.

5. Newton’s Second Law of Motion
Force, mass and Acceleration 
The three quantities: unbalanced force, mass and acceleration can be combined to produce the following formula.
Unbalanced force = mass x acceleration
Fun = m a
Fun is unbalanced force measured in N
m is mass measured in kg
a is acceleration measure in m/s2
Cyclist
The cyclist in the video wants to increase his speed. He must therefore increase his peddling force to be greater than the friction force.
The forces acting on the bike
will be unbalanced.
The cyclist will accelerate
to a greater speed
until once again the
forces are balanced.

6. More than one force
In most situations there will be more than one force acting on an object. If we want to calculate the acceleration of the object we must first find the unbalanced force. The combination of forces is known as the resultant.
Example: The engine of the car provides a driving force of newtons. The frictional force acting on the car is 100 newtons. The mass of the car is 800 kg. Calculate the acceleration of the car.
Step 1: Find unbalanced force.
Fun = – 100
Fun = 2 400N
Step 2: Calculate acceleration
a = Fun/m
a = 2 400/800
a = 3 m/s2.
100 N
2 500 N
800 kg

7. 1 A car of mass 1200 kg experiences friction equal to 500 N when travelling at a certain
speed. If the engine force is 1400 N, what will be the car’s acceleration?

8. 2 A car of mass 2000 kg has a total engine force of 4500 N
2 A car of mass 2000 kg has a total engine force of 4500 N. The frictional drag force
acting against the car is 1700 N. What is the acceleration of the car?

9. 3 Two girls push a car of mass 1000 kg
3 Two girls push a car of mass 1000 kg. Each pushes with a force of 100 N and the force of friction is 120 N.
Calculate the acceleration of the car.

10. 4 A boat engine produces a force of N and the friction and water resistance total 3500 N. If the mass of the boat is 2000 kg, what will be its acceleration?

11. 5 A careless driver tries to start his car with the hand brake still on. The engine exerts a force of 2500 N and the hand brake exerts a force of 1300 N. The car moves off with an acceleration of 1.2 m/s2. What is the mass of the car?

12. 6 A car of mass 1200 kg can accelerate at 2 m/s2 with an engine force of 3000 N.
What must be the total friction force acting on the car?

13. 7 A helicopter winches an injured climber up from a mountainside.
The climber’s mass is 65 kg.
a) What is the weight of the climber?
b) If he is accelerated upwards at 1.0 m/s2, what unbalanced force is required?
c) What total upwards force must be produced by the helicopter?

14. 8 An 800 kg car is accelerated from 0 to 18 m/s in 12 seconds.
a) What is the resultant force acting on the car?
b) At the end of the 12 s period the brakes are operated and the car comes to rest in a time of 5 s. What is the average braking force acting on the car?

15. 9 Give two reasons why a car will have a smaller acceleration in similar conditions when a roof rack is added.

16. When a rocket blasts off into space then there are two forces acting in the rocket. Apollo lift off
If the thrust is larger than the weight.
We have an unbalanced force.
An unbalanced force gives rise to
an acceleration according to Newton’s
Second law: Fun = ma
Then the rocket will accelerate off
into space.
Thrust
Weight

17. 10 Draw a picture in your jotter of a rocket taking off.
Ignoring air resistance, there are two forces acting on the rocket.
Draw arrows to show the directions that these forces act.
Name the forces on your diagram

18. 11 A space shuttle rocket produces a thrust of 10 000 N when fired to reposition the shuttle.
If the total mass of the shuttle is kg, what will be its acceleration.

19. 12 A space ship is moving at a steady speed in deep space.
Explain whether you think that its rocket motor will need to be running for this to happen.

20. 2010 Q21.
A balloon of mass 400 kg rises vertically from the ground.
The graph shows how the vertical speed of the balloon changes during the first 100s of its upward flight.
(a) Calculate the acceleration of the balloon during the first 60 s.
(b) Calculate the distance travelled by the balloon in 100 s.
(c) Calculate the average speed of the balloon during the first 100 s.
(d) Calculate the weight of the balloon.
(e) Calculate the total upward force acting on the balloon during the first 60 s of its flight.