1. Dynamics and Space Learning Intention You will be able to:
Name the three possible observations that indicate a force is acting on an object.
Use Newton's first law and balanced forces to explain constant speed.
Define frictional forces and how they affect an object’s movement.

2. Forces
In the next activities you will investigate forces. Put simply – forces are pushes, pulls or twists.
When a force acts on an object it can alter the speed, the direction or the shape of the object.
Complete the table below using the words speed, direction or shape after watching this video clip Forces
We use newton balances to measure forces. Please take care of them – they are measuring instruments, not toys.
The symbol used for force is F.
The unit of force is the newton (N).
Scene
Change of speed, direction or shape
Car crash into wall
Roller coaster
Rocket lift off

3. If 2 forces acting on an object are equal and opposite we say that the forces are balanced.
eg N 5 N eg 20 N N
If 2 forces acting on an object are not equal and opposite we say that the forces are unbalanced.
eg 6 N N eg
10 N
10 N

4. Questions
In the following examples, say whether the forces on the objects are balanced or unbalanced. A B C
8 N
8 N
12 N
11 N
6 N
6 N
5 N D E
7 N
7 N
9 N
14 N

5. Watch this NASA Scientists To Summarise
When there are no forces acting on an object or when the forces acting on the object are balanced, then the object will remain stationary or continue travelling at a steady speed in a straight line. or
When the forces acting on an object are unbalanced then the object will accelerate (in the direction of the unbalanced force).
20 N
20 N
Unbalanced force = 15 N to the right.
Acceleration will be to the right.
5 N
20 N

6. 1 Calculate the size of the unbalanced force in each of the following examples, and in each case give its direction.
Draw each diagram in your jotter
16 N A B C
24 N
18 N
16 N
16 N
19 N
24 N D E
35 N
35 N
64 N
36 N

7. 2 These objects are acted on by the forces shown
2 These objects are acted on by the forces shown. In each case say whether the object will be stationary, moving at a steady speed or accelerating. In the cases of an accelerating object, give the direction of the acceleration. Draw each diagram in your jotter. Reminder video Rugby Scrum
6 N
12 N A B C
4 N
4 N
36 N
42 N
12 N
5 N D E
248 N
264 N
18 N
18 N

8. Friction Forces
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.

9. Friction is a force which tries to stop objects from moving
Friction is a force which tries to stop objects from moving. If is a force which can be useful to us when we wish to stop objects from sliding or when we wish to slow them down.
Air resistance (friction between a solid and the air it is travelling through) is used by a parachutist to decrease speed. The same principle is used by a plane landing on a very short runway.
After watching Friction complete the table below:
Scene
Friction Force (High or Low)
Parachute stopping car
Duck on ice
Sledging
Parachutist landing

10. NEWTON’S FIRST LAW OF MOTION  
You saw previously that if an object had balanced forces acting on it then it remained at rest of moved at constant speed. If the forces acting on it were unbalanced then the object accelerated. The first time that these ideas were fully explained was more than 300 years ago by Sir Isaac Newton – he left us a set of physics laws that bear his name.   
Newton’s first law says: A body will remain at rest or move with steady speed in a straight line unless acted upon by an unbalance force. Cyclist
The cyclist in the video is peddling flat out.
He is going at a steady speed.
The forces acting on the bike
must be balanced.
The force forwards of the cyclist equals
the backwards force of friction.

11. 1 By considering Newton’s First Law of Motion, draw simple diagrams to show the balanced pairs of forces acting on the following:
a) A car moving at a steady speed along a level road.
b) A speed boat moving at a steady speed through the water.
c) A helicopter hovering in the air.

12. 2 When you sit on a stool you are at rest
2 When you sit on a stool you are at rest. Earth exerts a gravity force which tries to pull you towards it, and yet you do not move because there is a second force acting on you.
In which direction does this force act?
Draw a sketch diagram showing you on the stool and the two forces acting on you.
How big is this second force compared with the gravity force?

13. a At which point did she open her parachute?
3 This video talk about the forces acting on a parachutist. Parashutist This graph shows how the speed of a free-fall parachutist varied during her fall to the ground, starting from the moment she jumped from the plane.
Speed in
a At which point did she open her parachute?
b At which point did she hit the ground?
c State and explain which part of the graph shows Newton’s First Law of Motion.
d Explain why her steady speed during EF was less than her steady speed during CD. C D B F A E

14. 2003 SQA General Q15.
During a football match, one player heads the ball towards the goal.
(a) When the ball is headed, the player applies a force to the ball.
This force has three effects on the ball.
Complete the sentence below to describe the three effects.
“The force changes the The and the of the ball.”
(b) Later in the match, another player takes a penalty kick.
The player kicks the stationary ball with a force of 27 newtons.
The mass of the ball is 0.6 kilogram.
Calculate the acceleration of the ball.

15. 2005 SQA General Q5.
The diagrams below show the forces acting on a number of moving objects.
Which object is moving at constant speed?

16. 2006 Q14.
A rowing crew takes part in a race.
The time for their boat at each stage of the race is shown.
(a) Describe how to find the average speed of the boat from the start of the race to the finish.
(b) Calculate the average speed of the boat during the first 500 metres of the race.
(c) The crew supplies a force to move the boat forward.
When the boat is moving a force opposes the motion of the boat.
(i) Name the force that opposes the motion of the boat.
(ii) During the first 500 metres, there is a constant unbalanced force acting on the boat.
Describe the motion of the boat during this section of the race.
(iii) During one stage of the race, the speed of the boat is constant.
What can be said about the forces acting on the boat during this stage?