NCERT solutions for Science Exploration [English] Class 9 chapter 6 - How Forces Affect Motion [Latest edition]

Why does a canoe move forward when the canoeist pushes water backwards with their paddle and why does it move faster when they push harder?

Suppose the same canoeist uses the same paddle force in two different canoes, one empty and one carrying another passenger. In which case will the canoe move faster?

A weightlifter lifts a barbell (Fig.). List two forces that are acting on the barbell. Are these forces balanced if the weightlifter keeps the barbell steady?

Two players R and S are participating in an arm-wrestling match (Fig.). At the instant, when the arms tilt to the front direction (out of the page towards you), are the forces exerted by the players balanced? If not, which player exerted the larger force?

What if the force of friction disappears in the world? How will the motion of objects be impacted?

Now, conduct a thought experiment. We do a thought experiment when the conditions required for the experiment are difficult to recreate in the real world. Suppose, you find an object and a horizontal floor having such smooth surfaces that the force of friction between them is zero. Imagine, what will happen if you repeat steps 3 and 4 of Activity 6.1 with such an object and a horizontal floor? Will the velocity of the object decrease? Will the object ever come to rest or continue moving forever?

An object is moving with a constant velocity. Is there a net force acting upon it?

Suppose, no net force is acting on an object. Which of the following situations are possible?

1. Object remains at rest if at rest.
2. Object keeps moving with a constant velocity if already moving.
3. Object is moving with a constant acceleration.

In the real world, it is difficult to find a situation where no forces are acting on an object. But by applying additional forces, a condition can be achieved where the net force on the object is zero. Explain with the help of an example.

How much does a force of 1 N feel? If you hold a 100 g mass in your palm, the upward force your palm applies on the mass is around 1 N.

A toy car of mass 100 g is moving with a constant velocity of 0.5 m s^–1. What is the net force acting on the toy car?

Two children of different masses are sitting on identical swings. To impart identical initial acceleration, for which child would you require to apply a larger force? Explain why.

How are glass items packed for transportation using a bubble wrap or hay protected from damage?

Why does a fireperson sometimes struggle when holding the pipe issuing water?

Suppose a spacecraft is moving in a region of space where the gravitational force acting upon it is negligible. Suggest how can it change its velocity.

Using a horizontal force F, a table is moved across the floor at a constant velocity. How much is the frictional force exerted by the floor on the table?

For a ball moving on a smooth frictionless surface, choose the appropriate option that will make the following statement physically correct.

If no net force is applied on the ball, the velocity of the ball will remain the same/increase/decrease.

For a ball moving on a smooth frictionless surface, choose the appropriate option that will make the following statement physically correct.

If a net force is applied on the ball in the direction of its motion, the magnitude of the velocity of the ball will remain the same/increase/decrease.

For a ball moving on a smooth frictionless surface, choose the appropriate option that will make the following statement physically correct.

If a net force is applied on the ball in a direction opposite to the direction of its motion, the magnitude of the velocity of the ball will remain the same/increase/decrease.

Two blocks P and Q on a smooth horizontal surface are shown in Fig. a and Fig. b. Two forces of magnitudes 4 N and 5 N are acting in opposite directions on block P, while block Q is moving with a constant velocity.

Which of the following statement is correct?

While practising for the snake boat race (Vallum kalli in Kerala), 100 oarsmen are rowing a boat together. Out of these, 95 row backwards to propel the boat forward. But by mistake, 5 oarsmen row in the opposite direction. If each oarsman applies a horizontal force of 200 N, what is the net force on the snake boat? (Ignore drag forces, air friction, etc.)

When a net force acts on an object, we observe that the object accelerates ______

The position-time graph for four objects A, B, C and D moving along a straight line are given in the Fig. A net force acts on:

A sailor jumps out from a small boat to the shore (Fig.). As the sailor jumps forward, will the boat move? If yes, in which direction and why.

During a high jump event, a landing mat or sand bed is placed for the athlete to fall upon (Fig.). Explain the reason behind it.

A hand cart loaded with vegetables collides with an identical but empty hand cart. During the collision ______

The acceleration-mass graph for the acceleration produced by a force on objects of different masses is plotted in the Fig. Plot the force-mass graph for this case.

The velocity-time graph of an object of mass 10 kg moving along a straight line is shown in Fig. Calculate the force acting on the object by using the graph.

A bullet of mass 50 g moving with a speed of 100 m s^–1 enters a heavy stationary wooden block and stops after penetrating a distance of 50 cm. Estimate the stopping force acting on the bullet (assume that the bullet undergoes constant acceleration within the block).

An ace footballer converted a penalty shot by kicking the football with a speed of 108 km h^–1. The estimated force they imparted was 800 N. The mass of the football was 0.4 kg. Calculate the time of contact between their foot and the ball.

An object of mass 2 kg moving with a constant velocity of 10 m s^–1 encounters a rough patch where the force of friction on the object is 7 N. At the same time, an additional constant force of 3 N opposing the motion is applied on the object. After entering the rough patch, how much distance does the object travel before coming to rest?

A tractor pulls a harrow (a ploughing tool) of mass m1 with a net force F resulting in an acceleration of a₁. The same tractor pulls a trolley of mass m₂ with a force F producing an acceleration of a₂. If the tractor now pulls the trolley with the harrow placed on it (with the same force F), then obtain an expression for the resulting acceleration in terms of a₁ and a₂. Ignore friction.

When the pole of a bar magnet is brought close to a magnetic compass, the bar magnet and the compass needle (which is also a magnet) exert a magnetic force on each other. As per Newton’s third law of motion, both the forces are equal in magnitude and opposite in direction. However, the compass needle moves, whereas the bar magnet does not move (Fig.). Explain why.

You know that the force of friction depends on the nature of the surfaces in contact. Does it also depend on how hard the surfaces press each other? Is the friction acting on an object that is about to move larger than the friction after motion begins? Is the friction which acts on a rolling object less than that on a sliding object? Find answers to these questions and create an infographic. Such observations help explain why the invention of the wheel was a major milestone in human history.

Take two toy cars of equal mass and stick a bar magnet on top of each (Fig.). Fix a metre scale on a smooth surface. Place the cars near the midpoint of the metre scale with the like poles touching. Release the cars and record the time taken (using two stopwatches), and distance travelled by each before coming to a rest. Repeat the experiment after adding equal masses to both cars. Did the cars travel equal distances in opposite directions? Plot a graph of distance travelled versus mass. Analyse and discuss your findings.