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Forces and Motion - Simple Machines

Grade 3IB

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A Simple Machine is a mechanical device that changes the direction or magnitude of a force to make work easier.

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Work is defined as the application of a force FF over a distance dd. In scientific terms, work is only done if the object moves: W=FimesdW = F imes d.

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The Inclined Plane is a flat surface set at an angle (like a ramp). It allows us to move objects to a higher elevation using less force, but over a longer distance.

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A Lever consists of a rigid bar that pivots on a fixed point called a Fulcrum. It helps lift heavy loads by applying force at a different point.

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A Pulley uses a wheel and a rope to change the direction of a force. Using multiple pulleys can reduce the amount of force needed to lift an object.

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Friction is a force that acts in the opposite direction of motion. Simple machines like the Wheel and Axle help reduce friction to move objects more easily.

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Gravity is the force that pulls all objects toward the center of the Earth. Weight is the measure of gravity's pull on an object's mass: w=mimesgw = m imes g.

📐Formulae

W=F×dW = F \times d

Work=Force×Distance\text{Work} = \text{Force} \times \text{Distance}

w=m×gw = m \times g

Mechanical Advantage=Output ForceInput Force\text{Mechanical Advantage} = \frac{\text{Output Force}}{\text{Input Force}}

💡Examples

Problem 1:

A student pushes a heavy box across the floor with a force of 5 Newtons5\text{ Newtons} for a distance of 3 meters3\text{ meters}. How much work WW did the student perform?

Solution:

W=15 JoulesW = 15\text{ Joules}

Explanation:

Using the formula W=F×dW = F \times d, we multiply the force (5 N5\text{ N}) by the distance (3 m3\text{ m}). 5×3=155 \times 3 = 15. The unit for work is Joules (JJ).

Problem 2:

If you use a ramp (inclined plane) to move a 20 kg20\text{ kg} box, why does it feel easier than lifting it straight up?

Solution:

The force required is lower because the distance is increased.

Explanation:

Simple machines follow the trade-off principle. By increasing the distance dd the box travels along the ramp, the required input force FF decreases to perform the same amount of work WW.

Problem 3:

Identify the parts of a seesaw used to lift a friend.

Solution:

The board is the Lever and the center support is the Fulcrum.

Explanation:

A seesaw is a class 1 lever. The point where it balances and rotates is the fulcrum. When you push down on one side, the force travels through the lever to lift the person on the other side.

Simple Machines - Revision Notes & Key Formulas | IB Grade 3 Science