Detailed Explanation of Gravity and Weight 🌍⚖️
In Year 10 Physics, understanding the concepts of gravity and weight is very important. Gravity is a force that acts on all objects with mass, pulling them towards each other. On Earth, gravity pulls objects towards the centre of the planet. This is why when you drop something, it falls downwards.
What is Gravity? 🌌
Gravity is an invisible force that attracts one object to another. The Earth’s gravity is what keeps us on the ground instead of floating off into space. This force acts on every object no matter its size or mass, but the effect is most noticeable on objects with smaller mass compared to Earth’s massive size.
What is Weight? 🏋️♂️
Weight is the force due to gravity acting on an object’s mass. It is different from mass because:
- Mass is the amount of matter in an object and is measured in kilograms (kg).
- Weight is a force measured in newtons (N).
Weight tells us how strongly gravity is pulling on an object.
The Formula for Weight 🔢
The weight \( W \) of an object can be calculated by the formula:
Where:
- W is the weight in newtons (N)
- m is the mass of the object in kilograms (kg)
- g is the gravitational field strength in newtons per kilogram (N/kg)
On Earth, the gravitational field strength \( g \) is approximately 9.8 N/kg, but it can change slightly depending on location.
Relationship Between Mass, Weight, and Gravitational Field Strength ⚖️🌐
- Mass is constant and does not change no matter where you are.
- Weight depends on the gravitational field strength, so your weight on the Moon is less than your weight on Earth because the Moon’s gravity is weaker.
- If you know your mass and the gravitational field strength of a planet, you can calculate your weight there using the formula above.
Summary 📝
- Gravity pulls objects towards the Earth (or any massive body).
- Weight is the force gravity exerts on an object’s mass.
- Weight is given by multiplying mass by gravitational field strength (\( W = m \times g \)).
- Mass stays the same everywhere, but weight changes with gravity.
Understanding these concepts helps explain why objects fall and why we feel a force pressing us down when we stand on Earth. Always remember, weight is a force caused by gravity acting on an object’s mass! ⚖️🌍
10 Examination-Style 1-Mark Questions on Gravity and Weight 📝
- What force pulls objects towards the Earth?
Answer: Gravity - What is the unit of weight in the SI system?
Answer: Newton - What symbol is commonly used to represent gravitational acceleration?
Answer: g - What is the average value of gravitational acceleration on Earth in m/s²?
Answer: 9.8 - Weight is the force due to _______.
Answer: Gravity - What device measures weight?
Answer: Spring - Weight is a force, but mass is a measure of _______.
Answer: Matter - The weight of an object on the Moon is _______ its weight on Earth.
Answer: Less - An object with zero weight is said to be in _______.
Answer: Freefall - What does weight depend on besides mass?
Answer: Gravity
10 Examination-Style 2-Mark Questions on Gravity and Weight 📚
- Define gravity in terms of the force it exerts.
- Explain why objects fall towards the Earth when dropped.
- State the unit used to measure weight in the UK.
- Describe how weight differs from mass.
- What happens to an object’s weight if it is taken to the Moon? Explain briefly.
- How does the force of gravity affect the motion of planets around the Sun?
- Calculate the weight of a 5 kg object on Earth (gravitational field strength = 10 N/kg).
- Why do astronauts feel weightless in space even though gravity still acts on them?
- Describe what is meant by gravitational field strength and its unit.
- Explain why two objects of different mass fall at the same rate in a vacuum.
10 Examination-Style 4-Mark Questions on Gravity and Weight ✍️
Question 1
Explain the difference between mass and weight.
Model Answer:
Mass is the amount of matter in an object and does not change regardless of location. Weight is the force exerted on that mass by gravity. It depends on the gravitational field strength and can change if the object is on the Moon or another planet. Weight is calculated by multiplying mass by gravitational field strength (W = mg). Thus, while mass stays constant, weight can vary depending on the gravity. Weight is measured in newtons (N), and mass is measured in kilograms (kg).
Question 2
Describe how gravity affects objects on Earth, including the factors that influence weight.
Model Answer:
Gravity is the force that pulls objects towards the centre of the Earth. It gives objects weight by exerting a downward force on their mass. The size of this weight depends on both the mass of the object and the strength of the gravitational field. On Earth, the average gravitational field strength is about 9.8 N/kg. If an object’s mass remains the same but it is taken to a planet with weaker gravity, its weight would be less. Gravity is responsible for keeping everything on the ground instead of floating in the air.
Question 3
Calculate the weight of a person with a mass of 70 kg on Earth where gravitational field strength is 9.8 N/kg.
Model Answer:
Weight can be calculated using the formula: Weight = mass × gravitational field strength.
Here, mass = 70 kg and gravitational field strength = 9.8 N/kg.
Weight = 70 × 9.8 = 686 N.
This means the person’s weight is 686 newtons. Weight is a force, so it is measured in newtons. The mass remains 70 kg whether the person is on Earth or elsewhere.
Question 4
Explain why an astronaut’s mass stays the same in space but their weight changes.
Model Answer:
Mass is the amount of matter in an object and does not change regardless of location. Weight is the force of gravity acting on that mass. In space, the gravitational field strength is much weaker than on Earth, so the force pulling the astronaut down is less. This means their weight decreases. However, because mass is independent of gravity, it remains the same. So, astronauts feel almost weightless in space but still have the same mass.
Question 5
A 5 kg object is taken to the Moon where the gravitational field strength is 1.6 N/kg. Calculate the object’s weight there.
Model Answer:
Use the formula for weight: Weight = mass × gravitational field strength.
Mass = 5 kg; gravitational field strength on the Moon = 1.6 N/kg.
Weight = 5 × 1.6 = 8 N.
The object weighs 8 newtons on the Moon, which is much less than on Earth because the Moon’s gravity is weaker. The object’s mass remains 5 kg regardless of the location. This shows how weight depends on gravity but mass does not.
Question 6
What is meant by ‘gravitational field strength’ and how is it measured?
Model Answer:
Gravitational field strength is the force of gravity per unit mass experienced by an object at a point in a gravitational field. It tells us how much force gravity exerts on each kilogram of mass. The unit of gravitational field strength is newtons per kilogram (N/kg). On Earth’s surface, it is approximately 9.8 N/kg. This means each kilogram of mass experiences 9.8 newtons of gravitational force. It can vary depending on the location, such as on other planets or at different heights above Earth.
Question 7
Why does a person weigh less on a mountain top than at sea level?
Model Answer:
Weight depends on gravitational field strength, which decreases with distance from the Earth’s centre. At a mountain top, you are farther from the Earth’s centre than at sea level. This means the gravitational pull is slightly weaker on the mountain top. Because weight = mass × gravitational field strength, a weaker gravity results in less weight. The person’s mass stays the same but the weight decreases. So, you weigh slightly less on a mountain than at sea level.
Question 8
A ball has a mass of 0.2 kg. What force does gravity exert on it if the gravitational field strength is 9.8 N/kg?
Model Answer:
Weight = mass × gravitational field strength.
Mass = 0.2 kg, gravitational field strength = 9.8 N/kg.
Weight = 0.2 × 9.8 = 1.96 N.
So gravity exerts a force of 1.96 newtons on the ball. This force pulls the ball down towards Earth. The ball’s mass remains 0.2 kg regardless of the force.
Question 9
How does gravity affect the motion of objects when dropped from a height?
Model Answer:
Gravity pulls objects down towards the Earth when they are dropped. It causes them to accelerate, meaning their speed increases as they fall. This acceleration due to gravity is about 9.8 m/s² near the Earth’s surface. The greater the mass of the object, the greater the weight, but all objects accelerate at the same rate in free fall ignoring air resistance. Without gravity, objects would not fall but float away in space.
Question 10
Why is it important to understand the concept of weight when measuring forces in physics?
Model Answer:
Weight is a force caused by gravity acting on mass, so understanding it helps us measure and calculate forces accurately. Many physics problems involve forces, and weight is one of the most common forces we experience. Knowing that weight depends on both mass and gravity allows us to predict how objects behave on different planets or in different conditions. It helps in designing structures, vehicles, and equipment that must withstand forces due to weight. Also, understanding weight supports our learning about Newton’s laws and the effect of gravity on motion. This is why it is essential in physics to measure forces carefully.
10 Examination-Style 6-Mark Questions on Gravity and Weight with Model Answers 💡
Question 1
Explain how the gravitational force between two objects depends on their masses and the distance between them.
Model Answer:
The gravitational force between two objects is described by Newton’s law of universal gravitation. It states that the force is directly proportional to the product of the masses of the two objects. This means if either mass increases, the gravitational force increases. The force is inversely proportional to the square of the distance between the centres of the two masses. So, if the distance doubles, the force becomes one quarter. This relationship shows that gravity gets much weaker as objects move further apart. Gravitational force always acts along the line joining the centres of the masses and is an attractive force, pulling the two objects together. This law helps explain why planets orbit the Sun and why objects fall to Earth.
Question 2
Describe the difference between mass and weight, including the units for each.
Model Answer:
Mass is a measure of how much matter an object contains and is measured in kilograms (kg). It is a scalar quantity and does not change regardless of location. Weight, on the other hand, is the force exerted on an object due to gravity. It depends on both the mass of the object and the gravitational field strength where the object is located. Weight is measured in newtons (N) and is a vector quantity because it has both magnitude and direction, acting downwards towards the Earth. The formula for weight is weight = mass × gravitational field strength (W = mg). For example, on the Moon where gravity is weaker, an object’s weight is less, but its mass remains the same.
Question 3
Explain why astronauts feel weightless when they are in orbit around Earth.
Model Answer:
Astronauts feel weightless in orbit because they are in free fall towards Earth, but they are also moving forward fast enough to keep missing it. This creates a state called microgravity. Although gravity still acts on them, causing them to accelerate towards Earth, the spacecraft and astronauts are falling together, so there is no normal force pushing back against them. Weight is the force you feel when the ground pushes up on you, but in orbit, this support force is missing. As a result, astronauts float inside the spacecraft and feel weightless even though gravity is still acting on them.
Question 4
How does the gravitational field strength vary on different planets, and what effect does this have on weight?
Model Answer:
Gravitational field strength varies from planet to planet depending on their mass and radius. Larger and more massive planets have stronger gravitational fields. For example, the gravitational field strength on Earth is about 9.8 N/kg, while on the Moon it is about 1.6 N/kg. This means objects weigh less on the Moon because the force pulling them down is weaker. On a planet with a stronger gravitational field than Earth, objects would weigh more. The formula for weight shows this: weight = mass × gravitational field strength. Therefore, weight changes when gravity changes, but mass stays the same, regardless of location.
Question 5
Describe how a newtonmeter can be used to measure the weight of an object.
Model Answer:
A newtonmeter measures force, which includes the weight of an object. To measure weight, you attach the object to the hook of the newtonmeter. The object’s weight pulls down on the spring inside the metre, causing it to stretch. The amount the spring stretches is proportional to the force applied, which the newtonmeter scale converts to newtons. You then read the force value directly from the scale. This measurement gives the weight because it is the gravitational force acting on the object. To get an accurate reading, the newtonmeter must be held vertically so gravity acts straight down on the spring.
Question 6
Calculate the weight of a 5 kg object on Earth. (Gravitational field strength = 9.8 N/kg)
Model Answer:
Weight is calculated using the equation weight = mass × gravitational field strength. Here, the mass of the object is 5 kg, and the gravitational field strength on Earth is 9.8 N/kg. Multiplying these gives: 5 kg × 9.8 N/kg = 49 N. This means the object’s weight is 49 newtons. This force acts downwards towards the surface of the Earth. It is the force the object exerts on a surface or that a newtonmeter would read if the object was hanging from it.
Question 7
Explain the role of gravity in causing objects to have weight on Earth.
Model Answer:
Gravity is a force that pulls objects towards each other. On Earth, the planet’s mass creates a gravitational field that pulls objects towards its centre. This force is experienced as weight. Without gravity, objects would not be pulled down and would have no weight. Weight is the measure of the gravitational force acting on an object’s mass. This force is what causes objects to fall to the ground when dropped. It is also responsible for keeping the atmosphere and oceans on Earth. Gravity acts constantly and gives objects their weight depending on their mass and the gravitational field strength.
Question 8
How does the concept of weightlessness in space support the idea that weight is a force and not the same as mass?
Model Answer:
Weightlessness in space shows that weight depends on the force of gravity and is not the same as mass. Mass is an intrinsic property of matter and does not change, but weight is a force caused by gravity. Astronauts in a spacecraft orbiting the Earth feel weightless because they are in free fall, even though their mass remains the same. They do not feel their weight because the support force that usually acts on them is missing. This confirms that weight is the force due to gravity and can change depending on location, while mass stays constant.
Question 9
Discuss how the acceleration due to gravity affects objects during free fall.
Model Answer:
During free fall, objects accelerate downwards because of gravity. This acceleration is called the acceleration due to gravity and is about 9.8 m/s² near the Earth’s surface. It means the velocity of a freely falling object increases by 9.8 m/s every second. All objects, regardless of mass, fall with the same acceleration if air resistance is negligible. The force of gravity pulls the object down, and since it is unopposed in free fall, the object speeds up constantly. The acceleration due to gravity is a key factor to calculate how long it takes for an object to fall or its velocity after falling a certain distance.
Question 10
Explain why weight varies depending on the location but mass does not, using examples from Earth and the Moon.
Model Answer:
Weight varies with location because it depends on the gravitational field strength, which differs in various places. On Earth, the gravitational field strength is about 9.8 N/kg, giving objects their normal weight. On the Moon, the gravitational field strength is weaker, about 1.6 N/kg. An object with a mass of 10 kg weighs 98 N on Earth but only 16 N on the Moon. This difference exists because weight is the product of mass and gravitational field strength. Mass is the amount of matter and remains the same whether on Earth, the Moon, or anywhere else, so a 10 kg object has the same mass everywhere but a different weight depending on gravity’s strength.
