Force and Pressure - Exemplar Solutions 3

CBSE Class 8 Science
NCERT Exemplar Solutions
Force and Pressure

18. A chapati-maker is a machine which converts balls of dough into chapatis. What effect of force comes into play in this process?
Ans: The force applied with chapati-maker changes the shape of the dough and expands the ball of dough into a chapati. The applied force is distributed over a large surface area by the chapati-maker.

19. Fig. 11.7 shows a man with a parachute. Name the force which is responsible for his downward motion. Will he come down with the same speed without the parachute?
Ans: The force of gravity is responsible for the downward motion of the man with the parachute. He comes down slowly with the parachute since the air trapped beneath the open parachute provides an opposing force.
He will not come down with the same speed without the parachute. Without the parachute, the opposing upward force against the force of gravity will be missing and the speed of downward motion of the man will be higher.

20. Two persons are applying forces on two opposite sides of a moving cart. The cart still moves with the same speed in the same direction. What do you infer about the magnitudes and direction of the forces applied?
Ans: The cart moves with the same speed in the same direction when two persons are applying forces on two opposite sides of a moving cart. This implies that both the forces are of equal magnitude and are being applied in opposite directions (perpendicular to the direction of motion of the cart).

21. Two thermocol balls held close to each other move away from each other. When they are released, name the force which might be responsible for this phenomenon. Explain.
Ans: Electrostatic force is responsible for this phenomenon. When thermocol balls having similar charges are released, they move away from each other due to the repulsive electrostatic forces between the similar kinds of charges.
Explanation: Like charges repel each other whereas unlike charges attract each other.

22. Fruits detached from a tree fall down due to force of gravity. We know that a force arises due to interaction between two objects. Name the objects interacting in this case.
Ans: In this case, the force of gravitation exists between:-
1. The fruit that gets detached from the tree, and
2. The Earth
The Earth exerts a force of gravity on the fruit. The force of gravity pulls the fruit down.

23. A man is pushing a cart down a slope. Suddenly the cart starts moving faster and he wants to slow it down. What should he do?
Ans: When the man is pushing the cart down the slope, the force of gravity also acts on the cart. The net force acting on the cart increases and hence, the cart starts moving faster down the slope.
The man should apply an opposing force to decease the net force acting on the cart. Instead of pushing the cart, he should try to pull the cart up the slope so that the cart slows down.

24. Fig. 11.8 shows a car sticking to an electromagnet. Name the forces acting on the car? Which one of them is larger?
Ans: The forces acting on the car are:
1. Magnetic force (in the upward direction), and
2. The force of gravity or the weight of the car (in the downward direction).
The magnetic force is larger than the force of gravity (or the weight of the car).