FORCE AND MOTION

In simplest terms forces can be described as a push or pull. They result from the interaction between two or more objects. Force is not a property of an object but a force is always applied by one object on another when the objects interact. As a consequence of this forces always act in pairs. Remember Newton’s 3rd Law which states that: “If an object A exerts a force on object B, then object B will exert an equal but opposite force on object A.” That is FAB = -FBA. Forces can be classified into two main groups: contact forces and non-contact forces. Contact forces occur when the pushing (or pulling) object is in contact with the object being pushed or pulled. Non-contact forces occur between objects that are not in contact with each other. So far scientists have only discovered five different types of forces. Three of these: gravitational, electric and magnetic forces cause most of the observable interactions we see. The other two forces are the nuclear and weak interaction forces. They are rarely observed and occur only in the nuclei of atoms. Interestingly all contact forces are electrostatic in nature! Can you think why this is so?

If the forces on an object are balanced, then the net force acting on the object Σ𝑭 = 0𝑁, and the object’s motion will be unchanged. This means that it will continue to move with a constant velocity (speed and direction), or that it will remain stationary. If the forces acting on an object are not balanced, then the net force acting on the Σ𝑭 > 0𝑁, and the object’s motion will change. In what ways could the object’s motion change as a result of the Σ𝑭 > 0𝑁?

Free-Body Force Diagrams Arrows are used to represent the forces acting on an object when drawing a free-body force diagram. • The arrow is drawn with its tail starting at the object’s centre of mass, or point of application, and the

arrowhead pointing away from the centre of mass, or point of application. • The length of the arrows drawn should represent the relative sizes of the forces acting on the object. Unless you are asked to draw the forces acting at their point of application it is easier, and less confusing, to draw them acting on the object at its centre of mass. For neatness and clarity they should also be drawn with the aid of a ruler! • When analysing the motion of an object it is often necessary to break up one, or more, of the forces

acting on it into components that are parallel to and perpendicular to its direction of motion. Consider the following example. A speed skier of mass 70 kg is travelling down a ski slope at a constant speed of 140 km/hr as shown in the picture below. The angle that the slope makes with the horizontal is 25o. (You can use trigonometry to verify this.)

• Draw and label each of the forces acting on the skier.

• What is the net frictional force acting on the

skier? • What is the resultant force acting on the

skier? Explain your answer.

Force and Motion – Experiment 1 – Balloon Powered Car Use your understanding of Newton’s Laws and forces to describe the motion of a balloon powered car from the time of its release until it comes to a stop. You will need to consider the following aspects for the different “phases” of the car’s motion in order to fully answer the task. • Are the forces acting on the car in equilibrium? • Is the thrust force greater than the sum of the resistive forces? • What are the relative sizes of the thrust and resistive forces? Acceleration Phase

Coast Phase

Stationary

Force and Motion – Experiment 2 – Balloon Helicopter

Assemble and have a brief play with your helicopter. Carefully examine the balloon helicopter and its construction. Use your understanding of Newton’s Laws and forces to explain how the balloon helicopter works. You should include appropriately labelled diagrams as part of your explanation. It is also suggested that you have a quick read about torques and moments on page 40 to help with your explanations. The rest of this page and the next have been left blank for you to write your explanation.

AIR, ITS PROPERTIES & BEHAVIOUR

Before we investigate how aeroplanes fly we need to look at the properties and behaviour of the air through which they move. Air is a mixture of gases, 78% Nitrogen, 21% Oxygen with small amounts of water vapour, carbon dioxide, argon and other Nobel gases. Although the Earth’s atmosphere extends above its surface to an altitude of approximately 120 km most of its atmosphere is found in a layer called the Troposphere that extends to an altitude of about 11km. If the average pressure exerted by the Earth’s atmosphere is 101325 Pascals (N/m2) at sea level, what is the weight of the air above us equal to?

Fluids

Gases and liquids consist of particles (molecules) that can freely move past one another and have the tendency to assume the shape of the container they occupy. Substances that exhibit these properties are said to be fluids. In the next few experiments you will be looking at the behaviour of fluids as they move over the surface of other objects, and the effect that this movement has on the objects themselves.

Fluid Flow & Pressure Effects – Experiment 1 Purpose: To investigate the behaviour of air as it moves over an object and the effect that

this movement has on the object.

What You Need: 1 sheet A4 paper, scissors.

1 desert or teaspoon.

What To Do: Part A. From the A4 sheet cut a strip that is 5 cm wide and 25 cm long. Hold one end just underneath your bottom lip and blow across the top of the strip.

Part B. Gently hold a teaspoon vertically by the tip of its handle and place its convex side into a stream of water from the tap.

(A)

(B)

Prediction A

What do you think will happen to the paper when you blow across the top of it?

Observation A

Prediction B

What do you think will to the spoon when it is placed into the stream of water?

Observation B

Explanation(s):

Diagram

Conclusion:

Fluid Flow & Pressure Effects – Experiment 2

Purpose: To investigate the behaviour of air as it moves over an object and the effect that this movement has on the object.

What You Need: 2 full water bottles, retort stand and clamps and

cotton thread. What To Do: Suspend the two water bottles so that they hang

and are separated by about 2-3 cm as shown in the diagram. Blow a steady stream of air between them.

Prediction: What do you think will happen to the bottles when you blow between them?

Observation(s):

Explanation:

Diagram

Conclusion(s):

Fluid Flow & Pressure Effects – Experiment 3 Purpose: To investigate the behaviour of air as it moves over an object and the effect that this movement has on the object. What You Need: 1 polystyrene ball, 1 beach ball, 1 air blower. What To Do & Results: If you are using a vacuum cleaner for this experiment you will need to ensure that it is blowing air out, not sucking it in. Ask an adult to help you do this. Hold the nozzle so that it is pointing straight up into the air and turn it on. Prediction(s): What do you think will happen when you place the polystyrene ball into the air stream? What do you think would happen to the ball if you gently tilted the air stream until it was at an angle of about 20o – 30o from the vertical?

Observation(s): (As part of your answer draw a diagram showing what happened)

Explanation:

Return the air stream to the vertical and allow the polystyrene ball to stabilise. Prediction: What do you think will happen to the ball when you gently try and push it out of the air

stream? Try it!

Observation:

Explanation:

Repeat this experiment for the balloon and beach ball. Which ball performed the best? Can you offer any suggestions as to why?

Conclusion: In terms of air movement around the ball try and explain your observations. You may find it useful to include a labelled diagram as part of your answer.

Fluid Flow & Pressure Effects – Experiment 4 Purpose: To investigate the behaviour of air as it moves through a confined space and the

effect that this has on the object making the confined space.

What You Need: 1 sheet A4 paper, sticky tape, scissors and ruler.

What To Do: From the A4 sheet cut a strip that each measures 8 cm x 21 cm. Make the paper bridge as shown in the diagram and tape it to the desk.

Diagram of Observations

Prediction What do you think will happen to the paper when you blow through the tunnel?

Observations:

Explanation:

Conclusion:

Which answer, a), b) or c) is correct? Based on your observations to the above experiment explain your answer.

Whose principle is this?

7.0 cm 3.5 cm

tape

blow

Fluid Flow & Pressure Effects – Experiment 5 Purpose: To investigate the effect that air moving over an aerofoil has on the wing. What You Need: The equipment shown in the diagram will be set up for you. The shape of the

model wing is called an aerofoil. Prediction: What do you think will happen to the wing when air from a fan is directed

towards it?

What To Do: Use a fan to blow a stream of air towards the model wing.

Observation:

Explanation:

Fan!

Conclusion: In terms of the lift produced try and explain your observations. You include a

labelled diagram that shows how the air moves over the aerofoil and the forces acting as part of your answer.

Side view

Aerofoil