Chapter 5: Exploring Forces (Physics)

• (i) Muscular force → (b) a child lifting a school bag
• (ii) Magnetic force → (e) a compass needle pointing north
• (iii) Frictional force → (a) a cricket ball stopping on its own just before the boundary
• (iv) Gravitational force → (c) a fruit falling from a tree
• (v) Electrostatic force → (d) a balloon rubbed on woollen cloth attracting hair

(i)–(b), (ii)–(e), (iii)–(a), (iv)–(c), (v)–(d)

1. True.
2. False. Friction acts against the motion, so the ball slows down.
3. False. Charged objects attract or repel each other even without touching (electrostatic force).

(i) True (ii) False (iii) False

They move apart (repel). Rubbing gives both balloons the same kind of charge, and like charges repel each other through the electrostatic force.

They repel each other, because both carry the same kind of charge.

Water pushes up on anything placed in it (the upthrust or buoyant force). An object floats when this upthrust can balance its weight.

• The coin is small but heavy for its size (its metal is much denser than water). The water it displaces weighs less than the coin, so the upthrust is too small and it sinks.
• Wood is lighter than the same volume of water. The block floats once it has displaced its own weight of water, and that happens before it is fully under water.

Floating depends on how heavy an object is for its size (its density), not just on how big it is.

The coin is denser than water (its upthrust is less than its weight), so it sinks; wood is less dense than water, so the upthrust balances its weight while it is only partly under water.

1. Up: gravity acts downwards, and air resistance (friction) also acts downwards, against the motion. Both slow the ball.
2. Down: gravity acts downwards; air resistance now acts upwards, against the motion.
3. At the top: the ball is still for an instant, so there is no air resistance. Only gravity acts, still downwards, which is why it starts to fall.

The force of the throw stops acting as soon as the ball leaves the hand.

Gravity always acts downwards. Air resistance opposes the motion (down while rising, up while falling). At the top only gravity acts.

(i) To stop before A, increase friction on the flat part: cover it with a rough cloth, a towel, sand or a carpet.

(ii) To go past A, reduce friction: make the surface smoother (polish it, or use glass or a smooth sheet) or sprinkle fine powder. Making the slope steeper also works.

(i) Make the flat surface rougher (cloth, sand) to increase friction. (ii) Make it smoother (glass, powder, polish) to reduce friction.

When we walk, our foot pushes backwards on the ground, and friction pushes us forwards. Ice and polished floors are very smooth, so there is very little friction. The foot slides instead of gripping, and we slip.

Water or oil on a floor makes it even more slippery.

Smooth surfaces give very little friction, so our feet cannot grip and they slide.

Yes. In non-uniform motion the speed (or direction) keeps changing, and a change in speed or direction needs an unbalanced force. For example, a car speeding up or braking, or a ball rolling to a stop because of friction.

Yes: a changing speed or direction always means an unbalanced force is acting.

The Moon is much smaller and lighter than the Earth, so its gravitational pull at its surface is only about $\frac{1}{6}$ of the Earth's. Since weight is the force of gravity on the object, the weight becomes $\frac{1}{6}$.

The mass does not change. Mass is the amount of matter in the object, and that is the same anywhere.

The Moon's weaker gravity (about 1/6) reduces the weight; the mass stays the same.

A floating object sinks until the water it displaces weighs as much as the object itself. The deeper it sinks, the more water it displaces, so the heavier it is. Object 1 sinks deepest and object 3 the least.

(ii) w₁ > w₂ > w₃