Q.Explain why

(a) Horse cannot pull a cart in empty space

Key idea: Newton’s third law — action and reaction act on different bodies.

The horse pushes the ground backward; the ground pushes the horse forward. In empty space, there is no ground to provide the reaction force, so the horse cannot accelerate itself or the cart.

(b) Passengers thrown forward when a bus stops suddenly

Key idea: Newton’s first law — inertia of motion.

When the bus brakes, the lower body stops with the bus, but the upper body continues moving forward due to inertia, causing the passenger to lurch ahead.

(c) Easier to pull a lawn mower than to push it

Key idea: Resolution of forces and normal reaction.

When pulling, the force has an upward component, reducing the normal reaction and thus friction. When pushing, the downward component increases normal reaction and friction, making it harder.

(d) Cricketer moves hands backwards while catching …

(a) A horse cannot pull a cart and run in empty space

Concept: Newton’s third law — every action has an equal and opposite reaction. For the horse to pull the cart forward, it must push backward against something. In empty space, there is no ground to push against.

1. When a horse pulls a cart on Earth, it pushes its hooves backward against the ground. The ground pushes forward on the horse (reaction), which then pulls the cart via the harness.
2. In empty space, there is no ground or any other object to provide that reaction force. The horse’s legs push against nothing — so no forward force acts on the horse-cart system.
3. By Newton’s first law, the system remains at rest (or continues with constant velocity if already moving). The horse cannot accelerate the cart.

(b) Passengers are thrown forward when a speeding bus stops suddenly

Concept: Newton’s first law — inertia. An object in motion stays in motion unless acted upon by an external force.

1. Before the bus stops, both the bus and the passengers are moving forward at the same speed.
2. When the brakes are applied, the bus decelerates rapidly due to friction between the tyres and the road. This is an external force acting on the bus.
3. The passengers, however, are not rigidly attached to the bus. Their bodies tend to continue moving forward at the original speed because no significant horizontal force acts on them yet (the seats may provide some friction, but it’s small).
4. So relative to the bus, the passengers lurch forward — they are “thrown forward.”

(c) It is easier to pull a lawn mower than to push it

Concept: The force of friction depends on the normal reaction. By pulling, you reduce the normal force; by pushing, you increase it.

1. Consider a lawn mower of weight $W$. When you push it with a force $F$ at an angle $\theta$ below the horizontal, the vertical component $F \sin\theta$ adds to the weight. So the normal reaction $N = W + F \sin\theta$.
2. Friction $f = \mu N = \mu (W + F \sin\theta)$. This is larger than when the mower is simply resting.
3. When you pull it with the same force $F$ at an angle $\theta$ above the horizontal, the vertical component $F \sin\theta$ now acts upward, reducing the normal force: $N = W - F \sin\theta$.
4. Hence friction $f = \mu (W - F \sin\theta)$ is smaller. Less force is needed to overcome friction and move the mower. …

Concept: Applications of Newton's Three Laws and Friction

Step 1 (a): Horse pulling a cart in empty space — Newton's third law

To move forward, the horse must push backward against the ground and receive a forward

reaction. In empty space there is nothing to push against, so no reaction force exists —

the horse-cart system cannot accelerate.

Step 2 (b): Passengers thrown forward when a bus stops — Newton's first law

The bus decelerates due to the brakes (an external force on the bus), but the

passengers' bodies are not rigidly fixed to it; by inertia they continue moving forward

at the original speed, so relative to the (now slower) bus they lurch forward.

Step 3 (c): Pulling vs. pushing a lawn mower — friction depends on normal force

Pushing at angle $\theta$ below horizontal adds a downward force component, increasing

the normal reaction: $N=W+F\sin\theta$, so friction $f=\mu N$ is larger. Pulling at

angle $\theta$ above horizontal reduces the normal reaction: $N=W-F\sin\theta$, so

friction is smaller — pulling needs less force to overcome friction. …