Forces and Energy
Section: Physics | Syllabus: Cambridge Primary Science (0846)
Mass vs Weight
- Mass: the amount of "stuff" in an object
- measured in kilograms (kg) or grams (g), using a balance
- mass never changes
- Weight: the force of gravity pulling on an object
- measured in Newtons (N), using a force meter (newton meter)
- Gravity is different on different planets
- on the Moon, you would have the same mass but a lower weight, because the Moon's gravity is weaker
Force Diagrams
- Scientists use force diagrams to show the forces acting on an object - each arrow shows:
- Name: what type of force it is (written as a label)
- Size: a bigger arrow means a stronger force
- Direction: the arrow points the way the force pushes or pulls
Common Forces to Know
| Force Name | Description |
|---|---|
| Gravity (Weight) | Pulls objects DOWN towards Earth |
| Normal Force | A surface pushing UP on an object resting on it |
| Applied Force | A push or pull from a person or machine |
| Friction | Opposes movement between two surfaces |
| Air Resistance | Friction from air on a moving object |
Balanced forces on a book resting on a table - the book stays still
Unbalanced forces on a ball being pushed - the ball moves in the direction of the larger force
Springs
- Hanging a mass on a spring applies a pulling force, which makes the spring stretch (get longer)
- the bigger the force (more mass), the more the spring stretches
- Safety: when hanging masses on a spring, stand back and keep hands clear in case the spring or mass falls or springs back suddenly
Drawing Force Arrows
- To add a missing force arrow to a diagram: work out which force is missing (e.g. friction opposing an applied force), then draw an arrow starting at the object, pointing in the correct direction, with a label naming the force
Effects of Forces
Forces can cause objects to:
- Accelerate: start moving or speed up (unbalanced force in the direction of motion)
- Decelerate: slow down (unbalanced force opposite to motion)
- Change direction: a force applied from the side
- Change shape: stretch, squash, bend or twist
- Heat up: friction between two surfaces (like a bat hitting a ball) can make them hotter
- Balanced forces: forces that are equal in size but opposite in direction
- the object stays still, or keeps moving at the same speed
- Unbalanced forces: when one force is larger than the other
- the object will accelerate, decelerate, or change direction, moving in the direction of the larger force
- the net (resultant) force is found by subtracting the smaller force from the larger one - e.g. a 150N push against a 75N friction force gives a net force of 150 - 75 = 75N
Common Mistakes
- ❌ Thinking that if an object is not moving, there are no forces acting on it
- ✓ a book on a table has gravity pulling it down AND the table pushing it up - the forces are balanced, so the book stays still
Worked Example: Balanced or Unbalanced?
- Question: a car is moving at a steady speed on a straight road. Are the forces acting on the car balanced or unbalanced? Explain your answer. [2 marks]
- Answer: the forces are balanced [1]. If the car is moving at a steady speed (not speeding up or slowing down), the forward force from the engine equals the backward forces (friction and air resistance) [1]
Floating and Sinking
- Whether an object floats or sinks depends on TWO things: its mass and its shape
The Forces Involved
- Weight: gravity pulls the object DOWN
- Buoyant force (upthrust): the water pushes the object UP
- If the buoyant force equals the weight, the object floats
- If the weight is greater than the buoyant force, the object sinks
How Shape Affects Floating
- A metal ship is very heavy, yet it floats - this is because of its shape
- Ships are designed with a wide, hollow shape so that lots of water pushes up against the hull
- this creates a large buoyant force that equals the ship's weight
The Aluminium Foil Experiment
- A flat piece of aluminium foil floats on water, because it is spread out
- lots of its surface touches the water, so the buoyant force is large
- The same foil scrunched into a tight ball sinks
- the mass has not changed, but the shape has - the ball has less surface in contact with the water, so the buoyant force is smaller
The same mass of foil floats when flat, but sinks when scrunched into a ball
Worked Example: The Clay Boat
- Question: a student has a piece of modelling clay. When rolled into a ball, it sinks. The student then shapes it into a boat shape and it floats. Explain why. [3 marks]
- Answer: the mass of the clay stays the same [1]. When shaped as a boat, more surface area is in contact with the water [1]. This increases the buoyant force until it equals the weight, so the clay floats [1]
Worked Example: Factors Affecting Floating
- Question: name TWO factors that affect whether an object floats or sinks
- Answer: mass and shape
Forces and Gases
- Like all matter, a gas (such as air) has mass
- this can be shown by weighing two identical balloons on a balance, one empty and one full of air - the full balloon is heavier
- Squashing (compressing) a gas is an example of a force changing its volume
- gas particles are normally spread far apart, because they are not touching
- when a gas is compressed, the particles are pushed closer together, but they still are not touching
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