Force And Laws Of Motion Class 9 Science Worksheet Pdf
Class 9 Science students should refer to the following printable worksheet in Pdf for Force And Laws Of Motion in standard 9. This test paper with questions and answers for Grade 9 Science will be very useful for exams and help you to score good marks
Class 9 Science Worksheet for Force And Laws Of Motion
First Law of Motion
The force that opposes relative motion is called friction. It arises between the surfaces in contact.
Example: When we try to push a table and it does not move is because it is balanced by the frictional force.
• He observed the motion of objects on an inclined plane.
• When a marble is rolled down an inclined plane its velocity increases.
• When a marble is rolled down from the left – It will go up on the opposite side up to the same height at which it is dropped down.
• If the inclination of planes is equal – The marble would travel equal distances while climbing up as travelled while rolling down.
• If we decrease the angle of inclination of the right plane – The marble would travel further until it reaches its original height.
• If the right side plane is made flat – Marble would travel forever to achieve the same height.
• We need an unbalanced force to change the motion of the marble but no force is required when the marble is moving uniformly. In other words, objects move at a constant speed if no force acts upon them
First Law of Motion
A body continues to be in the state of rest or uniform motion in a straight line unless acted upon by an external unbalanced force. First Law is also called the Law of Inertia.
Basically, all objects have a tendency to resist the change in the state of motion or rest. This tendency is called inertia. All bodies do not have the same inertia. Inertia depends on the mass of a body. Mass of an object is the measure of its inertia. More the mass → more inertia and vice versa.
Types of Inertia
It is of three types:
▪ Inertia of Rest
▪ Inertia of Motion
▪ Inertia of direction
Inertia of Rest: Is the inability of body to change the state of rest on its own.
Inertia of Motion: Is the inability of body to resist the change in state of motion on its own.
Inertia of direction: Is the inability of body to resist the change in direction of body on its own. Examples to understand more about Inertia
1. Have you noticed that passengers in a bus fall forward when bus stops suddenly.
REASON:- When the bus is moving, the bus and the passengers sitting in it are in motion. But when force is applied on the brakes, it stops, but due to inertia of motion the passengers remain in the state of motion, hence, fall forward.
2. Have you experienced that when carpet is beaten with a stick, the dust separates out
REASON:- Initially the carpet and dust are in state of rest. But when carpet is beaten with a stick, it comes in motion and moves backward. But due to inertia of rest, the dust particles remain in the state of rest hence, get separated.
3. Have noticed that leaves and fruits falls down when branch of tree is shaken vigorously
REASON:- Initially, the branch of a tree and its leaves and fruits are in state of rest. But when force is applied on the branch of a tree, the tree comes in motion but due to inertia of rest, the leaves and fruits remain in state of rest hence, falls down.
Exam Questions NCERT Class 9 Science Chapter 9 Force and Laws of Motion
Question. Mention the factors on which the force of friction depends?
Ans : (a) The force of friction is directly proportional to the weight of the body sliding over the surface.
(b) The force of friction depends on the surfaces in contact.
Question. What does first law of motion indicates when an unbalanced external force acts on an object?
Ans : First law of motion indicates that when an unbalanced external force acts on an object, its velocity changes i.e. the object gets acceleration.
Question. What happens to the velocity of an object when the force acting on the object are balanced?
Ans : An object can move with uniform velocity only when the forces (pushing force and friction force) acting on the object are balanced.
Question. What is resultant force?
Ans : The resultant force of several forces acting simultaneously on a body is that single force which produces the same effect on a body as all these forces produce together.
Question. How is inertia measured quantitatively?
Ans : Quantitatively the inertia of an object is measured by its mass.
Question. Why are roads on mountains inclined inwards at turns?
Ans : A vehicle moving on mountains is in the inertia of motion. At a sudden turn there is a tendency of vehicle to fall off the road due to sudden change in the line of motion hence the roads are inclined inwards so that the vehicle does not fall down the mountain.
Question. If a person jumps from a height on a concrete surface he gets hurt. Explain.
When a person jumps from a height he is in state of inertia of motion. When he suddenly touches the ground he comes to rest in a very short time and hence the force exerted by the hard concrete surface on his body is very high, and the person gets hurt.
Question. When a force of 40 N is applied on a body it moves with an acceleration of 5 ms2. Calculate the mass of the body.
Let m be the mass of the body.
Given : F = 40 N, a = 5 ms2
From the relation F = m a, we have
40 = m × 5
m= 40/5 = 8 kg
Question. Which will have more inertia a body of mass 10 kg or a body of mass 20 kg?
Ans : A body of mass 20 kg will have more inertia, as inertia is measure of mass.
Question. Name the factor on which the inertia of the body depends.
Ans : Inertia of a body depends upon the mass of the body.
Question. If a man jumps out from a boat, the boat moves backwards. Why?
Ans : When a man jumps out of a boat to the bank of the river, he supplies a force in the forward direction. Due to the reaction of this, the boat moves backwards.
Question. It is easier to push an empty box than to push the box full of books. Give reason.
Ans : This is because the empty box has less inertia of rest than the box full of books.
Question. Define SI unit of force.
What is one Newton force?
Ans : A Newton is that force which act on a body of mass of 1 kg to produce an acceleration of 1 ms–2 in it.
Question. Why do passengers tend to fall sideways when the bus takes a sharp turn?
Question. Why are road accidents at high speed very much worse than accidents at low speed?
Question. Name the action and reaction forces acting during rocket propulsion?
Question. Why do we have to run in the direction of the moving bus while getting down from the bus?
Question. Why does an electric fan continue to rotate for sometime after the current is switched off?
Question. A person hit harder when he falls on a concrete floor than when he falls on a heap of sand from the same height. Why?
Question. Two identical bullets are fired one by a light rifle and another by a heavy vehicle with same force. Which rifle will hurt more and why?
Question. Define impulse. Show that impulse of a force is equal to the change in momentum?
Question. It is difficult to balance our body when we accidently step on a peel of banana. Explain based on Newton’s third law of motion?
Question. A bullet fired against a glass window pane make a clear hole in it, but a stone smashes the glass pane. Why?
Question. Define inertia. How does it depend on mass of the object? Explain (a)Dusting of a carpet by beating it with a stick. (b)Removal of water from wet cloth?
Question. A ball is suspended by a cord from the ceiling of a car. What will be the effect on the position of the ball if
(i) The car is moving with constant velocity?
(ii) The car is in accelerated motion?
(iii) The car is turning towards right?
Question. Give reason and give the law related to these statements
(a) It is easier to push an empty box than a box full of books.
(b) It is difficult for a fireman to hold a hose which ejects large amount of water with high velocity.
Question. State and prove the law of conservation of momentum?
Question. In the diagram below, a 1 kg mass on a rough horizontal surface is joined to a 2 kg mass by a light, inextensible string running over a frictionless pulley. Will the 1 kg mass move at a lower, higher or zero acceleration?
Explain based on Newton’s laws of motion and state the law.
SECTION-B NUMERICAL PROBLEMS
Question. A bullet leaves a rifle with a velocity of 100m/s and the rifle of mass 2.5 kg recoils with a velocity of 1m/s. Find the mass of the bullet?
Question. A cricket ball of mass 0.15kg is moving with a velocity of 1.2m/s. Find the impulse on the ball and force applied by the player if he is able to stop the ball in 0.18s?
Question. Two bodies of mass 1kg and 2kg moving in the direction opposite to each other collide with a speed of 5m/s. Calculate the total momentum of the system before collision.
Question. A speedboat has a mass of 500kg. It starts from rest and travels 200m in 12seconds. The boat undergoes constant acceleration during 12seconds. Find the magnitude of unbalanced force acting on the boat?
Question. A motor car of mass 200kg is moving with a certain velocity. It is brought to rest by the application of brakes, within a distance of 20m when the average resistance being offered to it is 500N. What was the velocity of the motor car?
Question. A man throws a ball weighing 500g vertically upwards with the speed of 10m/s, find
(a) Initial momentum.
(b) Its momentum at the highest point.
Question. A cricket ball of mass 0.15 kg is moving with a velocity of 1.2m/s . Find the impulse on the ball and average force applied by the player if he is able to stop the ball in 0.18s?
Question. The velocity of a body of mass 10kg increases from 4m/s to 8m/s when a force acts on it for 5s. Find (a) the momentum before the action of force (b) the momentum after the action of force (c) the magnitude of force.
Question. Two blocks of mass m1= 1kg and m2= 2kg are placed in contact on a frictionless horizontal surface. A force of 10N is acting on m1. What is the acceleration of m1 and m2. What is the magnitude of action and reaction forces?
Question. A force 40N acting on a body of mass 10 kg changes its velocity from 5 m/s to 25 m/s Determine : (i) Acceleration (ii)Time for which force acts (iii) Distance covered.
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