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Forces and Motion - Gravity and Friction

Grade 3IB

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

🔑Concepts

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A force is a push or a pull that can change the speed, direction, or shape of an object. Force is measured in Newtons (NN).

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Gravity is an invisible force that pulls all objects toward the center of the Earth. On Earth, gravity accelerates objects at approximately g≈9.8 m/s2g \approx 9.8 \text{ m/s}^2.

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Weight (WW) is the measure of the force of gravity pulling on an object's mass (mm).

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Friction is a force that opposes motion when two surfaces rub against each other. It acts in the opposite direction of the movement.

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Rough surfaces create more friction (FfF_f), while smooth surfaces create less friction. Lubricants like oil or water can reduce friction.

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Air resistance is a type of friction that occurs when an object moves through the air (FairF_{air}).

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Balanced forces result in no change in motion (Net Force =0 N= 0 \text{ N}), while unbalanced forces cause an object to speed up, slow down, or change direction.

📐Formulae

W=m×gW = m \times g

Fnet=Fpush−FfrictionF_{net} = F_{push} - F_{friction}

1 kg≈9.8 N (on Earth)1 \text{ kg} \approx 9.8 \text{ N (on Earth)}

💡Examples

Problem 1:

A student pushes a heavy box across a wooden floor with a force of 10 N10 \text{ N}. The friction between the floor and the box is 3 N3 \text{ N}. What is the total net force acting on the box?

Solution:

10 N−3 N=7 N10 \text{ N} - 3 \text{ N} = 7 \text{ N}

Explanation:

Because friction works in the opposite direction of the push, we subtract the friction force from the pushing force to find the net force (FnetF_{net}).

Problem 2:

If a small toy has a mass (mm) of 2 kg2 \text{ kg}, what is its approximate weight (WW) on Earth using g≈10 m/s2g \approx 10 \text{ m/s}^2?

Solution:

W=2 kg×10 m/s2=20 NW = 2 \text{ kg} \times 10 \text{ m/s}^2 = 20 \text{ N}

Explanation:

To find the weight in Newtons, we multiply the mass by the pull of gravity.

Problem 3:

Why does a ball stop rolling eventually on a grass field even if nobody touches it?

Solution:

The force of friction (FfF_f)

Explanation:

The blades of grass rub against the surface of the ball, creating friction that opposes the ball's motion and eventually brings it to a stop.