St Leonard's College · Web viewA force is a push or a pull on an object resulting from the objects’ interaction with another object. Forces can be contact forces (e.g. due to friction)

Year 10 Physics

Forces

Name: _________________

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Motion and forces formulas

Use the following formulae where they are relevant to questions:

v =

a =

v = u + at

F = m a (also: Weight = m g)

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Forces - Introduction

A force is a push or a pull on an object resulting from the objects’ interaction with another object. Forces can be contact forces (e.g. due to friction) or non-contact forces (e.g. due to gravity). Usually there is more than one force acting on an object.

For example as you stand on the ground there are two main forces acting on you – the force of gravity pulling you towards the earth and normal reaction force of the earth pushing back up on you. These forces are balanced and the net force on you is zero, thus your motion does not change.

The sum of all the forces acting on an object is called the net force. When the forces on an object are unbalanced the net force will be non-zero and the shape of the object or the motion of the object will change. The size of the net force can be determined by how much the shape or the motion of the object changes. Changing the motion of an object implies accelerating it.

The net force is found by multiplying the mass of the object by the acceleration that the net force causes. Force is a vector. F = ma

This is much better thought of as The acceleration, a, is in m/s2, the mass, m, is in kilograms and the SI unit of force, F, is the NEWTON (N). A force of 1 Newton will accelerate a 1 kg mass at 1m/s2.

Newton’s First Law

Question 1Imagine sitting in a moving car. What happens to you when the car suddenly stops?

This is the reason that you need to wear a seatbelt in the car. You could be a body in motion that will continue in motion through the windscreen unless a net force acts on you (your seatbelt).

Question 2 (continued from Question 1)Explain what happens to you when the car accelerates rapidly from rest?

Question 3Explain what happens to you when the car turns a corner to the left?

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An object at rest will remain at rest unless acted on by a net force.An object in motion will remain in motion with uniform velocity unless acted on by a net force.In other words, objects like to keep doing what they’re doing.

Inertia

Inertia is an object’s tendency to remain in its current state. An object at rest will tend to remain at rest; an object in motion will tend to remain in motion. The inertia of an object is related to the mass of the object. The more mass an object has, the more inertia it has, which is why a greater force is needed to start or stop a big object moving, compared to a little object. Newton’s First Law is sometimes called the Law of Inertia.

Question 4State Newton’s first law of motion in your own words.

Force diagramsThe forces acting on an object can be represented using a force diagram. A convention is to draw longer or thicker arrows to represent a larger force.

Determining the net forceTo find the net force (Fnet) acting in a particular plane you can add the forces. A force that acts in the opposite direction should be subtracted.

ExampleA force of 10N to the right is applied to an object. The object also experiences a frictional force of 2N that resists the object’s motion. What is the net force on the object? Describe the object’s motion.

The net force on the object is:

Fnet = 10N – 2N to the right= 8N to the right (as force is a vector it must have a direction)

The object accelerates to the right.

Example 2An object is falling through the air and is experiencing a downwards force due to gravity (weight) of 20N. It is also experiencing a force due to air resistance of 20N. What is the net force acting on the object? What is the object’s motion?

The net force on the object is:

Fnet = 20N – 20N downwards= 0N

The object is falling at a constant velocity as the net force is zero.

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10N2N

20N

20N

Inertia and motion worksheet

1. Consider the following examples of forces acting on a body. In each case find the net (unbalanced) force and describe the resulting motion of the object. Assume north is up the page.(a)

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(b)..........................................................................................................................................

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2. A 500 kg box is placed on the tray of a Utility truck but it is not secured by ropes. The Ute drives along a highway, then the driver has to brake suddenly to avoid a wombat on the road. Use the concept of inertia to explain the danger to the driver..................................................................................................................................................

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3. A 1 kg pendulum is mounted on the roof of a vehicle as shown in the diagram.Describe the motion of the pendulum in each example.(a) The vehicle accelerates from rest.

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.....................................................................................................................................................(b) The vehicle brakes from a speed of

60 km/h...........................................................................................................................................................................................................................................................................................................

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Forces on a vehicle

For a vehicle (or any object) on a flat road the force due to gravity will always equal the normal force and the car will not move in the vertical plane. Therefore, on a flat road the net force on a car will be the sum of the forces in the horizontal plane:

Different types of motion of a car

Complete the table below by adding appropriately sized force vectors to each diagram and by describing the net force in the box below for each stage.

Stationary Accelerating Constant velocity Decelerating

Drawing force diagrams

To construct force diagrams, it is important to know the types of forces. If given a description of a physical situation, begin by using your understanding of the force types to identify which forces are present. Then determine the direction in which each force is acting. Finally, draw a basic diagram and add arrows for each existing force in the appropriate direction; label each force arrow according to its type.

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Normal force – The upwards push of the ground

Resistance forces – The forces that resist the forwards motion (air resistance and friction)

Force due to gravity – The downward pull on the car due to the gravitational field of the earth

Thrust – The driving force applied to the driving wheels

Net Force, Fnet = thrust from the engine – resistance forces (air resistance plus friction)

Drawing force diagrams worksheet

Use the method described on the previous page to construct force diagrams for the situations described below. If you’re not feeling artistic you can represent each object as a filled-in circle. If the object is on a surface remember to draw in the surface.

1. A book is at rest on a table top.

2. A gymnast is suspended motionless from a trapeze.

3. A skydiver is at terminal velocity.

4. A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces.

5. A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces.

6. A car is stopped at a red light.

7. A hot air balloon is accelerating upward.

8. A car is coasting to the right and slowing down.

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The greater the mass the slower the acceleration when the same force is applied. Therefore, a larger force is needed to give a larger mass the same acceleration.

ExampleIf a car has a driving force (thrust) of 1750N, a frictional force of 1000N and a mass of 750kg. Find the acceleration of the car. Take to the right to be positive.

Fnet = Fdriving - Ffriction = maFnet = 1750N – 1000N = 750N750 = ma750 = 750aa = 1 m/s2

Question 1Determine the magnitude of the net force acting on the object shown below.

Question 2If the block had a mass of 4 kg, what would be its acceleration?

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Driving force1750N

Friction force1000N

Driving force1750N

Frictionforce1000N

Net force750N

The net force is obtained from the addition of the forces acting on the object

Mass and Weight

Mass and weight are different quantities. Mass is defined as the amount of matter in a body and it is measured in kilograms.

Weight is not the same as mass. Weight is the force of gravity on an object. The acceleration due to gravity is 'g', so the weight of a mass 'm' is given by F = mg (Newtons). The amount of matter in an object does not change, hence the mass of an object is always the same. However, 'g' varies depending on how far an object is from the centre of the earth. And ‘g’ will be different on other planets and the moon – the greater the mass of an object, the greater the gravitational pull of that object.

Gravitational field strength

On earth we usually take acceleration due to gravity to be 9.8 m/s2. However, when we are standing on earth we are clearly not accelerating as the force due to gravity is balanced by the reaction force of the ground.

Another way to express ‘g’ is in Newton’s per kilogram (N/kg). We can say on earth the strength of the gravitational field is 9.8N/kg. That is at the surface of the earth, the earth pulls on objects with a force equal to 9.8N per kilo of mass of that object.

Question 3What is the weight of a 50 kg student on the surface of the Earth (g = 9.8 m/s2)?

Question 4What is the weight of a 50 kg student on the surface of the Moon (g = 1.6 m/s2)?

Question 5The weight of an object:A. tells how much matter is in the objectB. is the same everywhere in the universeC. depends only on the volume of the objectD. is equal to the gravitational force acting on the object.

Question 6Describe the difference between mass and weight and explain why weight can change depending on location but mass does not.

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Weight = mass x acceleration due to gravity

W = mg

Weight is a force and is measured in Newton’s (N).

Newton’s Second Law worksheet

1. State Newton’s Second Law of Motion....................................................................................................................................................................................................................................................................................................................................................................................................................................................

2. Use the formula: F ma to complete the following table. Be sure to convert to SI units.Force (F) (Newtons) Mass (m) Acceleration (a)

100 40 kg2 200 g

70 3.5 m/s2

100 10 cm/s2

3. Consider the forces acting on a body of mass 40 kg resting on a frictionless surface.

(a) Calculate the net force acting on the body.....................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

(b) Calculate the acceleration of the body.........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

4. The photo shows a drag car coming to a stop at the end of a race.

(a) To come to a complete stop the drag car negatively accelerates. Identify another term

for ‘negative acceleration’. ...................................................................

(b) Identify the force that brings the drag car to a stop. ......................................................

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Pearson Science 10, Section 9.3, Review questions, Qs 1, 6, 12, 13, 14, 15 and 16. (p 390 -391)

Net force questions

InstructionsWhere possible show a diagram and definitely show your working.Use g = 9.8 m/s/s when needed.

Many of these problems require two steps, not just one, and may require some deep thinking!

1) During a tennis serve, a 0.057kg tennis ball was accelerated from rest to 30 m/s in 0.07s. a) Determine the acceleration of the ball in m/s2.

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b) Determine the size of the average net force acting on the ball.

2) A 1200kg car has to come to rest from 16.7m/s in 18s.a) Determine the deceleration of the car.

b) Determine the average braking force required to achieve this deceleration.

3) Use Newton’s Laws to explain why a 1.0 kg shot-put can be thrown further than a 1.5 kg shot-put thrown by the same person.

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4) (Harder question! Hint: Draw a diagram and solve with two calculations). When travelling at 100 km/h a car has to overcome a drag force due to air resistance of 800N. If the car has a mass of 900kg, determine the average force that the motor needs to apply if it is to accelerate at 2.0 m/s2.

Diagram:

5) Mary is paddling a canoe in Albert Park Lake. The paddles are providing a constant driving force of 45 N south and the frictional forces total 25 N north. These forces are shown in the diagram below. The mass of the canoe is 15 kg and Mary has a mass of 50 kg.

a) What is Mary’s mass?

b) Calculate Mary’s weight.

c) Calculate the net horizontal force acting on the canoe.

d) Calculate the acceleration of the canoe.

6) On the surface of the Earth a geological hammer has a mass of 1.5 kg. Determine its mass

and weight on Mars where g = 3.6 m/s2.

Extension Question

7) A 0.50 kg metal block is attached by a piece of string to a dynamics cart as shown below. The block is allowed to fall from rest, dragging the cart along. The mass of the cart is 2.5 kg.

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a) If friction is ignored, what is the acceleration of the block as it falls?

b) How fast will the block be travelling after 0.5 seconds?

c) If a frictional force of 4.3 N acts on the cart, what will its acceleration be?

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Force on the wagon from the donkey. Force on the donkey from the wagon.

Newton’s Third Law

When an object applies a force to a second object, the second object applies an equal and opposite force to the first object. By this we mean that forces always exist in pairs, an object cannot push on a second object without the second object pushing back on it. An example of this could be a girl pushing on a wall, and the wall pushing back on the girl.

Example 1

Action force: The donkey pulls on the wagon to the right.Reaction force: The wagon pulls on the donkey, with the same sized force, in the opposite direction.

As per Newton’s Second Law, If the net force on either object is non-zero the object will accelerate in the direction of the net force.

Example 2A soccer player kicks a ball to the right.

Action force: Soccer player’s foot pushes on the ball to the right.

Acceleration as a result of the action force: The ball accelerates to the right away from the foot.

Reaction force: The ball pushes on the player’s foot to the left.

Acceleration as a result of the reaction force:

The foot accelerates away from the ball (i.e. the player’s foot decelerates).

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For every action there is an equal but opposite reaction.

If object A exerts a force on object B, then object B exerts the same kind of force on object A, with the same magnitude and opposite direction.

Action-Reaction Force Pairs

An action force and the corresponding reaction force are called an action-reaction force pair.

Question 1

(a) Fully describe the missing half of the following action–reaction pair.

A bat pushes a ball to the right.

(b) Describe the acceleration of either object in the above action-reaction pair. The first one is done for you.

The ball accelerates to the right.

Question 2

(a) Fully describe the missing half of the following action–reaction pair.

The Earth pulls down on an apple.

(b) Describe the acceleration of either object in the above action-reaction pair.

If the stick holding the apple to the tree were to suddenly break then…

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Steps for determining action-reaction pairs and the resulting acceleration1: Identify the two objects involved (objects A and B).

e.g. foot and ball

2: Identify the type of force (push or pull).e.g. push

3: Write a sentence to describe the force object A applies to object B and add a direction.e.g. “foot pushes ball to the right”

4: Write a sentence to describe the force object B applies to object A and reverse the direction (remember it is always the same type of force)

e.g. “ball pushes foot to the left”

5: Write a sentence to describe the acceleration of object B as a result of the force applied by object A and include a direction (hint: acceleration will always be in the same direction as the force applied)

e.g. “ball accelerates to the right”

6: Write a sentence to describe the acceleration of object A as a result of the force applied by object B and include a direction (remember: acceleration will always be in the same direction as the force applied)

e.g. “foot accelerates to the left” (this means the foot decelerates to the right).

Action-reaction force pairs worksheet

Event Diagram Action force Acceleration as a result of action force

Reaction force Acceleration as a result of reaction force

Soccer player kicks ball

Soccer player’s foot pushes the ball to the right

The ball accelerates away from the foot to the right

The ball pushes on the player’s foot to the left

The foot accelerates away from the ball to the left (i.e. the player’s foot decelerates)

Swimmer pushes water with his hand

Bus hits fly while driving down the highway

A gymnast drops straight down from the rings

Rocket accelerates in space

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Forces and Newton’s Third Law of Motion worksheet

1. What does a net force cause an object to do?

2. If a 10N force is applied to an object with a mass of 50kg, what is its acceleration?

3. If a similar 10N force is applied to an object of 100kg, what is its acceleration?

4. When we double the mass from 50kg to 100kg, how does this affect the object’s acceleration?

5. My drag car is speeding along, having left the starting line and approaching the finishing line.

I’m travelling at 20m/s before I apply the brakes and within 2 seconds I’m travelling at 10m/s.

What is my acceleration, and in which direction is it occurring? [Towards the starting line or the finishing line?]

Lisa has a mass of 60kg. She is standing on rollerblades in a school corridor. Lisa starts skating and before long she’s travelling at 5m/s. Looking ahead, Lisa sees Steven (who has a mass of 120kg) who also is wearing roller-blades, at rest. She goes for it. She heads towards Steven and crashes into him (ouch).

[PHYSICS LAB] LISA STEVEN [LIBRARY]

During the impact, Lisa exerted a force on Steven of 100N.

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6. What force did Steven exert back on Lisa?

7. Was the force she exerted on Steven directed towards the library or the physics lab?

8. Was the force Steven exerted on Lisa directed towards the library or the physics lab?

9. Draw in the forces.

Lisa (60kg) Steve (120kg)

Figure 1: Lisa’s on the left. Steven is on the right.

10. In which direction will Steven accelerate? (Towards the Library or the Physics Lab?)

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11. In which direction will Lisa accelerate?

12. Who will experience the greater acceleration, and why (remember that the forces are equal and opposite)? (Hint: you could do some calculations to help)

13. Is it possible that Lisa might maintain her motion towards the library? Why / why not?

Further questions to consider

14. If forces are always equal and opposite in action and reaction, how is it possible for an object to accelerate?

15. Explain, in detail, using the third law of motion, how a person is able to walk forward.

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