# 11th Class Physics Chapter 3 Motion And Force Short Question Answers

## 11th Class Physics Chapter 3 Motion And Force Short Question Answers Below

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1.What is the difference between uniform and variable velocity?
Uniform velocity:If a body covers equal displacements in equal intervals of time however small may be the body is said to have uniform velocity.Its motion is said to be uniform.In other words the velocity of a body is said to be uniform if both its magnitude and direction do not change.Variable velocity:If a body covers unequal displacements in equal intervals of time it is said to be moving with variable velocity and its motion is said to be non-uniform.The velocity of the body may be variable due to change in magnitude,or direction or both m agnitude and direction.
2.An object is thrown vertically upward.Discuss the sign of acceleration due to gravity relative to velocity while the object is in air?
When an object is thrown vertically upward it is moving up against the direction of gravity.The sign of acceleration dur to gravity relative to velocity,is taken as negative because the direction of ‘g’ is opposite to the direction of velocity of the object moving upward in air.If the object is moving downward then the sign of acceleration relative to velocity will be taken as positive because velocity of the object and acceleration ‘g’ are in the sane direction.
3.Can the velocity of an object reverse direction when acceleration is constant?
Yes,the velocity of an object can reverse direction with constant acceleration.
4. A man standing on the t op of a tower a ball straight up with initial velocity vi and at the same time throws a second ball straight downward with the same speed which ball will have larger speed when it strikes the ground ignore air friction?
Both the balls will have the same speed on striking the ground.We have already read that when a body is projected vertically upward with certain initial velocity then it will hit the ground with the same velocity.Thus when the ball is thrown straight up with the initial velocity vi it will have the same velocity vi when it returns back.Hence this ball will strike the ground with the same speed.When a second ball is thrown vertically downward with initial velocity vi it will also strike the ground with the same speed.It results that in both the cases the balls will hit the ground with the same speed.But the difference is that both the balls will hit the ground at different times due to different heights.
5.Explain the circumstances in which the velocity v→ and acceleration a→ of a car are: (i)Parallel (ii) Anti-parallel (iii) Perpendicular to one another?
(i) When the velocity of the car is increasing along a straight line then vand  a are parallel to each other.
(ii) When the velocity of the car is decreasing along a straight line or whenever the brakes are applied on the moving car than velocity vand acceleration a are anti- parallel.
(iii)When the car moves along a circular path acceleration a is along radius and velocity v is along tangent.Thus they become perpendicular to each other  as shown in the fig.
6. Motion with constant velocity is a special case of motion with constant acceleration.Is this statement true?
Yes,this statement is true,When a body moves with constant velocity its acceleration is zero because in this case the velocity changes at the same rate throughout the motion.Hence the acceleration of the body will always remain constant during its motion,that is equal to zero.As zero is also a constant quantity therefore this is a special case motion.
7.State the law of conservation of linear momentum pointing out the importance of isolated system?
Statement: It states that the total linear momentum of an isolated system always remains constant. Isolated system: A system which is not acted upon by external forces is called an isolated system.In this system the bodies may interact with another and can exert a force on another but no agency can exert a force on them. Its importance:Its importance can be understood by many examples of isolated system which conserve momentum. (i)Molecules of a gas enclosed in a vessel at constant temperature. (ii)Rocket and its fuel. (iii)Elementary particles of an atom such as proton electron and neutron when they suffer collision. (iv) Gun and bullet.
8.Explain the difference between elastic and inelastic collisions.Explain how wold a bouncing ball behave in each case?
Elastic Collision: An elastic is the one in which both linear momentum and kinetic energy are conserved before and after the collision. Inelastic collision:A collision during which the total momentum is conserved but total k genetic energy before and after collision is not conserved is called inelastic collision.
9.At what point or points in its path does a projectile have its minimum speed its maximum speed?
The speed of projectile is minimum at the maximum height because at this height vertical component of velocity Vy becomes Zero.
The speed of a projectile is maximum at the point of projection and also just before it strikes the ground because the vertical component of velocity  Vy is maximum at these points.
10.Define motion with a suitable example?
A body is said to be in motion if it changes its position with respect to its surroundings.
11.Sketch a graph showing positive and negative accelerations in a particular moving object?
When the velocity of the object increases with time the object is said to be moving with positive acceleration from O to A as shown in fig.When the velocity of the object increases at a constant rate the object is said to be moving with uniform positive acceleration from A to E as shown in fig.When the velocity of the object decreases with time from E to F as shown in fig the acceleration of the object will be negative because its velocity decreases with time.
`12.Write any two properties of an inertial frame of reference?
Newton’s first law of motion is valid in the inertial frame of reference.It is the property of this frame to have zero acceleration.Secondly a free falling object will be weightless in such a frame of reference.
13.A 20g ball hits the wall of a squash court with a large force of 50 N.If of impact of force is 0.5 sec.Find the impulse?
Solution:Using the formula for impulse
Impulse =Force × time
Or          I=F×T………………. (1)
Here       F= 50N,          t=0.5sec.
Putting the values in equ(1) we get
I=50×0.5 = 25N –s
Hence         I = 25N –s .
14.Define isolated system with suitable example?
When the number of bodies are such that they can exert force upon another and no external agency exerts a force on them they are said to form an isolated system of interacting bodies. Example:The molecules of a gas enclosed in a vessel at constant temperature form an isolated system if interacting bodies.The enclosed molecules collide with one another due to their random motion but no external force acts upon them.
15.Why do you keep your lags far apart when you have to stand on the floor of a moving train?
We keep our legs far apart in order to produce the state of stability.Thus the center of gravity will lie in the middle of the body to keep it in stable equilibrium.
16.In the expression of Newton’s second law F= ma,Let f =0 give your comments about new situation?
The formula F=ma also leads to the law of inertia.As we know F=ma if F=0 in a zero force environment then putting its value in equat on
F = ma,we have
0=ma
or   a =0
The zero acceleration will not change the state of the body i.e. the body is at rest or moving with uniform velocity.Observers is a such force environment are called inertial observes and such a frame of reference which is not being accelerated
17.Is it possible for a body to have acceleration when moving with. (a) constant velocity (b) constant speed
(a) No,when the body is moving with constant velocity the body will have no acceleration.This is because neither the magnitude of the velocity nor direction will change. (b)Yes,when the body moves with constant speed the body can have acceleration due to the change in direction.For example a body moving with contestant speed in a circle has centripetal acceleration
18.A body is moving along a circle with a constant speed,is it moving with uniform velocity?
(a) It is not moving with uniform velocity because the direction of its velocity changes from point to point and it has centripetal acceleration (b) Its centripetal acceleration is always directed towards the center of the circle.
19.Show that the weight of a freely falling body is zero?
The weight of a body in ascertain frame of reference is a force which is equal and opposite to the forces required to prevent it from accelerating in that frame.Therefore it will be weightless.
20.Can there be an acceleration when a body is moving with constant speed?
Yes,when a body is moving with constant speed the body can have acceleration if its direction changes.For example if the body is moving along a circle with constant speed.It will have acceleration due to the change of direction at every instant.
21.Explain the significance of mass of a moving body?
According to Newton’s second law of motion when a force acts on body the body is accelerated in the direction of the force.The magnitude of the acceleration is directly proportional to the force and inversely proportional to the mass of the body.The relation between the mass force and acceleration is F= ma or m=F/a Thus the mass of a moving body is equal to the ratio of the force applied on it to the acceleration produced by that forces.It means that mass is the characteristic of a body which determines the magnitude of the acceleration produced when a certain force acts upon it.
22.Why is the first law of motion also called law of inertia?
According to first law of motion a body remains at rest or continues to move with uniform velocity unless acted upon by some unbalanced force.It can be stated as a body opposes any change in its state of rest or of uniform motion.This property of the body is known as inertia.This first law of motion states the same thing as stated by inertia.Hence first law of motion can also be called by inertia.
23.What is the physical signification of momentum?
Physical signification of momentum:- It is our common experience that a moving object possesses quality by which it exerts a force upon a body who tries to stop it.This quality of the moving object depends upon two things. (i)Its velocity;- The faster the object is moving the more force we require to stop it and (ii)Its Mass: The more massive the objective is the more difficult to stop it.
24.Define elastic collision?
A collision in which both linear moment and kinetic energy of the system are conserves is called an elastic collision. In case of elastic no kinetic energy is lost.For example when a hard ball is dropped on a hard smooth marble floor it rebounds very nearly the same height.It loses negligible amount of energy in the collision with the floor.
25.Define Momentum?
The momentum of a body is defined as the product of mass and velocity of the body.It is denoted by p.Let m be  the moving body with velocity v,then linear momentum is given by
p=mv It is a vector due to velocity.
26.State law of conservation of momentum?
The total linear momentum of an isolated system remains constant.Mathematically it is written as:If the system consists of two masses m1 and m2 with velocities v1 and v2 respectively then total momentum is given by
Ptotal= P+P2
=mivi+m2v2=constant
27.Under what condition average velocity and instantaneous velocity are equal?
When the body moves with a uniform velocity,its average and instantaneous velocity are equal.
28.Distinguish between elastic and inelastic collision?
Elastic Collision:An collision is the one in which both linear momentum and kinetic energy are conserved before and after the collision. Inelastic Collision:A collision during which the total momentum is conserved but total kinetic energy before and after collision is not conserved is called inelastic collision.
29.State Newton’s second law of motion in terms of momentum?
Statement:The rate of change of momentum of a body is equal to the applied force and the change of momentum takes place in the direction of force.
30.Explain impulse?
Definition:When a force acts on a body for a very short time the product of the force and the time for which the force acts is called impulse.
31.Define projectile motion?
Definition:- Projectile motion is the motion of a body that is thrown with an initial velocity and then moves under the action of gravity.The body thrown upward making a certain angle 0 with the horizontal is called a projectile,which is usually a parabola.
32.What happens when a very heavy ball collides with light stationary ball?
The velocity of the heavy ball remains unchanged and the lighter ball will move in the same forward direction with nearly twice the velocity of first ball.
33.Find the dimensional formula for impulse?
Impulse =Force× time
=N×S
=kgms-2 × s
=kgms-1
[I]=[MLT-1]
34.When light body collides with massive body at rest what will happen to the light body?
When light body collides with massive body at rest the lighter body bounces back with the same velocity.
35.If two balls of equal masses one at rest and other ball is moving with velocity v.if the moving ball collide with the ball at rest what will happen?
The colliding ball becomes stationary and the target ball starts moving with initial velocity of colliding ball.
36.What is the distance covered in the velocity-time graph?
The area between the velocity-time graph and the time axis is numerically equal to the distance covered by the object.
37.What is the average acceleration in the velocity time graph?
The average acceleration of an object is the slope of its velocity-time graph.
38.What is ballistic missile?Define its trajectory?
An un -powered and un – guided missile is called ballistic missile.The path followed by the ballistic missile is called ballistic trajectory.
39.What is the ballistic flight?
A flight in which a projectile is given an initial push and is then allowed to move freely due to inertia and under the action of gravity is called ballistic flight.
40.Describe two uses of ballistic missiles?
The ballistic missiles are useful only for short ranges for which the initial velocity is not very much large. For long ranges and greater accuracy powered and remote control guided missiles are used.
41.What is the difference between inertial frame of reference and non-inertial frame of reference?
Inertial Frame of Reference:-A frame of reference which is not being accelerated audit may be at rest or moving with uniform velocity is called inertial frame of reference.Newton’s first law of motion is valid in inertial frame of reference.It is the property of this farm to have zero acceleration. Non – inertial Frame of reference: A frame of reference which has non-uniform motion but certain acceleration is called non-inertial frame of reiteration.Newton’s first law of motion is not valid in non-inertial frame of reference.It is the property of this frame to have some acceleration.
42.Why does a rifle move backward when bullet is fired discuss it with reference to momentum?
When a bullet is fired from a rifle the total momentum of system remains constant that is zero.When a bullet is fired its momentum increases.In order to conserve the momentum of bullet but opposite.Thus the total momentum of system remains conserved.
43.What is the principle of rocket motion.How can it be applied on its motion to gain the escape velocity?
The motion of rocket is an application of law of conservation of linear momentum.The whole rocket consists of several stages of fuel which behave like individual rocket.When fuel of one stage is finished it is and thus acceleration increases.The rocket gains escape velocity in order to escape from the earth’s gravitational field.
44.What is horizontal range?
45.What do you understand by the time of flight?
46.What is the maximum height of projectile?
47.Discuss the importance of the velocity-time graph?
The distance covered by the object can be determined by calculating the area under the velocity-time graph.The average acceleration of an object can be found with the help of slope of velocity-time graph.

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1. Mishi Khan says:

please ans this question that “initial and final velocities remain same along which dimension and why?”

2. Abu Bakr Shahid says:

Also highlight important questions

3. junaid khan says:

a boy moving with horizental velocity of 15 mrter per second it changes dirrction upwards making an angle 60 with horizental and having same velocity what will be its acceleration.?