Forces and their effects

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Apuntes de la lección

Big idea: forces change things

Big idea. Key concept: Relationships. The size and direction of the forces on an object decide how it moves.
Related concept: Interaction. A force is always an interaction between two objects: one pushes or pulls on the other.
Global context: Scientific and technical innovation. Engineers use forces to design cars, bridges, springs and spacecraft.
A force is a push or a pull. Forces are measured in newtons (N) using a newtonmeter.
A force can make an object speed up, slow down, change direction or change shape. A car's thrust changes its speed, the Sun's gravity bends the path of a comet, and squeezing a spring changes its shape.
A force cannot change how much matter an object contains, so it cannot change an object's mass.

Three effects of a force

Contact and non-contact forces, and force arrows

A contact force only acts when two objects touch. Examples are friction, air resistance, tension in a rope and the normal contact force (the push of a surface on an object resting on it).
A non-contact force acts across a gap. Examples are gravity, magnetic force and the electrostatic force between charged objects.
We draw a force as an arrow. The arrow points in the direction of the force, and a longer arrow means a bigger force.
A free-body diagram shows one object on its own with arrows for every force acting on it. A book on a table has its weight pulling down and the normal contact force from the table pushing up.
These two arrows for the book are the same length. The upward force and the downward force are equal, so the book stays still.

Forces on a book resting on a table

Balanced and unbalanced forces

When several forces act on an object we can add them up. The resultant force is the single force that has the same effect as all of them together.
Forces in opposite directions are subtracted. Forces in the same direction are added. For example, 400 N to the right and 150 N to the left give a resultant of 250 N to the right.
If the forces are balanced, the resultant force is 0 N. A stationary object stays at rest, and a moving object keeps moving at the same speed in a straight line.
If the forces are unbalanced, there is a resultant force. The object speeds up, slows down or changes direction. The bigger the resultant force, the bigger the change.
A car moving at a steady speed has balanced forces: the thrust forwards equals the friction and air resistance backwards. An object does not need a resultant force to keep moving.

Balanced and unbalanced forces in a tug of war

Gravity, mass and weight

Gravity is a non-contact force that pulls objects with mass towards each other. Near the Earth it pulls everything towards the centre of the Earth.
Mass is the amount of matter in an object. It is measured in kilograms (kg) and does not change when you move to a different place.
Weight is the force of gravity on an object. It is measured in newtons (N), so it is a force and it has a direction (downwards).
Weight = mass × gravitational field strength, or W = m × g. On Earth g is about 10 N/kg (9.81 N/kg more exactly). On the Moon g is about 1.6 N/kg.
A 3 kg bag has a weight of 3 kg × 10 N/kg = 30 N on Earth. Its mass is still 3 kg on the Moon, but its weight is only 3 × 1.6 = 4.8 N.
This is why astronauts can bounce along on the Moon: their mass is the same, but their weight is about six times smaller.

Same mass, different weight on the Moon

Friction and air resistance

Friction is a contact force between two surfaces that are rubbing or trying to slide past each other. It acts in the direction opposite to the motion.
Friction happens because even smooth-looking surfaces have tiny bumps that catch on each other. Rougher surfaces give more friction.
Air resistance (drag) is friction from the air or any fluid. It acts against the motion, and it gets bigger as speed increases and as the object's front area gets bigger.
Friction can be useful (brakes, shoe grips) and a nuisance (it wastes energy as heat and wears parts out). A lubricant such as oil reduces friction between moving parts.
Streamlined shapes, such as a racing cyclist crouching low, reduce air resistance.
A skydiver speeds up at first because weight is bigger than air resistance. As speed rises, air resistance grows until it equals weight. The forces are then balanced and she falls at a steady speed.

Friction acts against the motion of a sledge

Think like a scientist: springs and Hooke's law

A spring stretches when a force pulls on it. The extension is the stretched length minus the original length.
In a safe school investigation, hang a spring from a clamp stand, add masses to a hanger one at a time and measure the length each time with a ruler. Keep feet clear of falling masses and wear eye protection.
The independent variable is the force (the load on the spring). The dependent variable is the extension. Control variables include the same spring and the same ruler.
If the line on a force-extension graph is straight through the origin, extension is directly proportional to force. This is Hooke's law. It stops working at the limit of proportionality.
Example: a spring stretches 4 cm when pulled by 2 N. If it obeys Hooke's law, 4 N gives 8 cm and 5 N gives 10 cm.
Inquiry task: plan a fair test to find out whether two different springs stretch by the same amount for the same load. List your variables, say how you will make the measurements reliable and say how you will decide which spring is stiffer.

Equipment for stretching a spring

Diapositivas

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Preguntas de práctica

Vista previa gratis — 8 de 53 preguntas. Regístrate para verlas todas.

1.Which unit is used to measure force?
Easy
- AKilogram (kg)
- BNewton (N)
- CMetre (m)
- DSecond (s)
2.Which of these is a contact force?
Easy
- AGravity
- BMagnetic force
- CFriction
- DStatic electric force
3.Which of these is a non-contact force?
Easy
- AGravity
- BFriction
- CAir resistance
- DTension in a rope
4.Which of these can a force NOT do to an object?
Easy
- AMake it speed up
- BChange its direction
- CChange its shape
- DChange its mass
5.A newtonmeter is used to measure which quantity?
Easy
- ATime
- BTemperature
- CForce
- DVolume
6.In a force diagram, what does a longer arrow mean?
Easy
- AA heavier object
- BA force that acts for longer
- CA force that is moving faster
- DA larger force
7.A book rests on a table. Which force from the table pushes up on the book?
Easy
- ANormal contact force
- BFriction
- CWeight
- DAir resistance
8.A car travels along a straight, level road at a steady speed. The engine's thrust is 1500 N. What is the total of friction and air resistance acting on the car?
Medium
- A0 N
- B1500 N forwards
- C3000 N backwards
- D1500 N backwards

Más preguntas

9.Team A pulls a rope to the left with 250 N. Team B pulls it to the right with 400 N. What is the resultant force on the rope?
Medium
- A150 N to the left
- B650 N to the right
- C150 N to the right
- D0 N
10.An object is at rest and the forces on it are balanced. What happens to it?
Medium
- AIt begins to speed up
- BIt starts to move slowly
- CIt changes direction
- DIt stays at rest
11.Which statement about weight is correct?
Medium
- AWeight is the amount of matter in an object, measured in kg
- BWeight is the force of gravity on an object, measured in N
- CWeight is how big an object is, measured in m
- DWeight is the force of friction on an object, measured in N
12.An astronaut has a mass of 80 kg on Earth. What is her mass on the Moon?
Medium
- A0 kg
- BAbout 13 kg
- CAbout 480 kg
- D80 kg
13.Why would a person weigh less on the Moon than on Earth?
Medium
- AThe Moon's gravity is weaker
- BThe person has less mass on the Moon
- CThere is no air on the Moon
- DThe Moon is further from the Sun
14.A box slides across the floor towards the right. In which direction does friction act on the box?
Medium
- AUpwards
- BTo the left
- CTo the right
- DDownwards
15.Which change would REDUCE the friction between a sledge and the snow?
Medium
- AAdding a layer of wax to the runners
- BPutting a heavy load on the sledge
- CRoughening the runners with sandpaper
- DUsing runners made of rubber
16.Why does a racing cyclist crouch low over the handlebars?
Medium
- ATo increase the friction with the road
- BTo reduce air resistance
- CTo increase the weight on the wheels
- DTo make gravity pull less
17.A parachutist is falling at a steady speed. Which statement about her weight (W) and the air resistance (R) on her is correct?
Hard
- AR is bigger than W
- BR is zero
- CR is smaller than W
- DR is equal to W
18.A box is pushed to the right with a force of 80 N. Friction on the box is 50 N to the left. What happens to the box?
Hard
- AIt speeds up to the right
- BIt moves at a constant speed
- CIt slows down and stops
- DIt stays where it is
19.What does it mean if a spring obeys Hooke's law?
Medium
- AIts length stays the same whatever the force
- BIt never goes back to its original length
- CIts extension is directly proportional to the force applied
- DIts length doubles every time more force is added
20.
In one experiment a student hung masses on a spring.
Load (N) Extension (cm)
0 0
1 2.0
2 4.0
3 6.0
4 8.0
5 12.0
Between which two loads does the spring stop obeying Hooke's law?
Hard
- A4 N and 5 N
- B0 N and 1 N
- C1 N and 2 N
- D3 N and 4 N
21.A student hangs different masses on a spring and measures the extension each time. What is the independent variable?
Medium
- AThe extension of the spring
- BThe mass hung on the spring
- CThe ruler used to measure
- DThe spring used
22.A student drags a wooden block across carpet, wood and plastic, each time pulling at a steady speed with a newtonmeter. She is comparing the friction on each surface. Which is a control variable?
Medium
- AThe type of surface
- BThe reading on the newtonmeter
- CThe mass of the block
- DThe order of the surfaces tested
23.A student drops a heavy ball and a flat sheet of paper from the same height. The ball lands first, so she concludes that heavier objects always fall faster. What is wrong with her conclusion?
Hard
- AAir resistance affects the paper far more, so the test is not fair
- BPaper is not pulled by gravity at all
- CShe should have dropped them from a lower height
- DGravity pulls only on heavy objects
24.Which of these are contact forces? (select all that apply)
Easy
- AFriction
- BAir resistance
- CGravity
- DTension in a rope
- EMagnetic force
25.Which of these are non-contact forces? (select all that apply)
Medium
- AGravity
- BA push on a door
- CMagnetic force
- DStatic electric force
- EFriction
26.The forces on a car are balanced. Which statements must be true? (select all that apply)
Medium
- AIf it is at rest, it stays at rest
- BIf it is moving, it keeps the same speed in a straight line
- CIt must be speeding up
- DIt must be slowing down
27.Which are sensible safety steps when stretching springs with hanging masses? (select all that apply)
Medium
- AWear eye protection
- BKeep feet clear from under the masses
- CStretch the spring as far as it will go to see it snap
- DThrow the masses onto the hanger
28.Mass is measured in newtons.
Easy
True or false?
29.Gravity is a non-contact force.
Easy
True or false?
30.A longer arrow on a force diagram stands for a bigger force.
Medium
True or false?
31.If an object is moving, there must be an unbalanced force acting on it.
Hard
True or false?
32.A person has the same weight on the Moon as on Earth.
Medium
True or false?
33.Friction always acts in the same direction as the motion.
Easy
True or false?
34.Match each force to what it is.
Easy
- Weight
- Friction
- Normal contact force
- Air resistance
- The pull of gravity on an object
- A force between rubbing surfaces that opposes sliding
- The push of a surface on an object resting on it
- Friction from the air on a moving object
35.Match each quantity to the unit it is measured in.
Medium
- Force
- Mass
- Gravitational field strength
- Spring extension
- Newton (N)
- Kilogram (kg)
- Newton per kilogram (N/kg)
- Centimetre (cm)
36.Match each pair of forces on an object to the resultant force.
Medium
- 5 N to the right and 5 N to the left
- 8 N to the right and 3 N to the left
- 2 N upwards and 6 N downwards
- 10 N to the right and 4 N to the right
- 0 N (balanced)
- 5 N to the right
- 4 N downwards
- 14 N to the right
37.Put the steps of a spring investigation in a sensible order.
Medium
- Work out each extension and plot a graph
- Measure the original length of the spring with no load
- Set up the spring on a clamp stand beside a ruler
- Add one mass at a time and measure the new length each time
38.Put these objects in order of weight on Earth, from smallest to largest (g = 10 N/kg).
Medium
- A 2 kg bag of sugar
- A 5 kg cat
- A 0.2 kg apple
- A 0.5 kg book
39.Complete the sentence about measuring force.
Easy
Force is measured in ____.
40.Complete the sentence about gravity.
Easy
The force that pulls everything towards the centre of the Earth is called ____.
41.Complete the equation for weight in words.
Medium
Weight = mass x gravitational field ____.
42.Complete the sentence about resultant force.
Medium
When the forces on an object are ____, the resultant force is zero.
43.Complete the sentence about springs.
Medium
A spring obeys Hooke's law if its ____ is directly proportional to the force.
44.Complete the sentence about reducing friction.
Hard
A substance such as oil that reduces friction between moving parts is called a ____.
45.A book rests on a table even though gravity pulls it down. Explain why it does not move.
Medium
46.Explain why a skydiver stops speeding up and falls at a steady speed.
Hard
47.Describe how you would investigate how the extension of a spring depends on the force pulling on it.
Medium
48.Calculate the weight of a 3 kg bag on Earth, where g = 10 N/kg. Give your answer in newtons (N).
Easy
49.A rock has a mass of 2.5 kg. The Moon's gravitational field strength is 1.6 N/kg. Calculate the weight of the rock on the Moon. Give your answer in newtons (N).
Medium
50.A trolley is pushed forwards with a force of 400 N and friction acts backwards with a force of 150 N. Calculate the resultant force in newtons (N).
Medium
51.A student weighs 450 N on Earth, where g = 10 N/kg. Calculate her mass in kilograms (kg).
Medium
52.A spring is 12 cm long with no load. With a 3 N load it is 18 cm long. Calculate the extension of the spring in centimetres (cm).
Medium
53.A spring obeys Hooke's law. A force of 2 N gives an extension of 4 cm. Calculate the force in newtons (N) needed to give an extension of 10 cm.
Hard

Apuntes de repaso

## Forces and their effects - A **force** is a push or a pull, in **newtons (N)**. It can change speed, direction or shape. - **Contact forces** need touching (friction, air resistance, tension, normal contact force). **Non-contact forces** act across a gap (gravity, magnetic, electrostatic). ## Resultant force - Add forces in the same direction and subtract forces in opposite directions. - **Balanced** forces: resultant 0 N, no change in motion. **Unbalanced** forces: speeds up, slows down or changes direction. ## Mass and weight - **Mass** in kg never changes. **Weight** in N is the pull of gravity. - **Weight = mass x gravitational field strength**. g is about 10 N/kg on Earth and 1.6 N/kg on the Moon. ## Friction and air resistance - They act against motion. Lubricants and streamlining reduce them. - A skydiver reaches a steady speed when air resistance equals weight. ## Springs - **Extension = stretched length - original length.** Straight line through the origin means Hooke's law. - Safe practice: eye protection, feet clear of masses. ## Inquiry - Name the independent, dependent and control variables, repeat measurements and evaluate your method.

Force: A push or a pull, measured in newtons (N).
Newton (N): The unit of force.
Contact force: A force that acts only when two objects touch, such as friction or tension.
Non-contact force: A force that acts across a gap, such as gravity or magnetic force.
Normal contact force: The push of a surface on an object resting on it.
Resultant force: The single force that has the same effect as all the forces on an object.
Balanced forces: Forces whose resultant is 0 N, so there is no change in motion.
Mass: The amount of matter in an object, measured in kilograms (kg).
Weight: The force of gravity on an object, measured in newtons (N).
Gravitational field strength: The force of gravity per kilogram, about 10 N/kg on Earth and 1.6 N/kg on the Moon.
Friction: A force between surfaces that opposes sliding.
Air resistance: Friction from the air on a moving object.
Extension: The stretched length of a spring minus its original length.
Hooke's law: For a spring within its limit, extension is directly proportional to force.

Tarjetas

What is a force?

A push or a pull, measured in newtons (N).

Name three effects a force can have on an object.

It can change its speed, its direction or its shape.

Give two examples of contact forces.

Friction and air resistance (also tension and the normal contact force).

Give two examples of non-contact forces.

Gravity and magnetic force (also electrostatic force).

What does a longer arrow mean on a force diagram?

A bigger force.

What forces act on a book resting on a table?

Weight downwards and the normal contact force upwards, which are equal.

What is the resultant of 400 N right and 150 N left?

250 N to the right.

What happens when forces are balanced?

The resultant force is 0 N, so there is no change in motion.

What happens when forces are unbalanced?

The object speeds up, slows down or changes direction.

What is the difference between mass and weight?

Mass is the amount of matter in kg. Weight is the pull of gravity in N.

How do you calculate weight?

Weight = mass x gravitational field strength (W = m x g).

What is g on Earth and on the Moon?

About 10 N/kg on Earth and about 1.6 N/kg on the Moon.

In which direction does friction act?

Against the motion.

How can friction be reduced?

Use a lubricant such as oil, or make the surfaces smoother.

Why does a skydiver fall at a steady speed?

Air resistance has grown until it equals her weight, so the forces are balanced.

How do you find the extension of a spring?

Stretched length minus original length.

What does Hooke's law say?

Within the limit, extension is directly proportional to the force.

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Load (N)	Extension (cm)
0	0
1	2.0
2	4.0
3	6.0
4	8.0
5	12.0