NEWTON’S LAWS OF MOTION SUTHERLAND HIGH SCHOOL- GRADE 11 -

NEWTON’S LAWS OF MOTION • SUTHERLAND HIGH SCHOOL- • GRADE 11 -

A QUICK SUMMARY OF NEWTONS LAWS 04 01 02 NEWTONS SECOND LAW OF MOTION If you push or pull an object in a certain direction, it will move in that very direction. NEWTONS FIRST LAW OF MOTION An object will continue doing what it was originally doing, unless you do something about it. NEWTONS LAW OF UNIVERSAL GRAVITATION There is always a force of gravity between any two objects that have mass. NEWTONS THIRD LAW OF MOTION If you exert a force on a box, the box will exert an equal and opposite force on you. 03

NEWTONS’S FIRST LAW OF MOTION Because of inertia objects tend to remain at rest or continue at uniform velocity. An object will stay at rest or continue to move at a constant velocity, unless acted upon by an external force. Inertia is the resistance of an object to a change in its state of motion or rest. This law is also known as the law of inertia.

INERTIA • The mass of an object is a measure of it’s inertia. • The larger the object, the more difficult it is to move it from rest. No wonder a large truck causes so much damage when its brakes fail. • It is not necessary to have a net force on an object to ensure that it moves continuously at a constant speed in a straight line. Instead it is rather a net force that is needed to stop a moving object! • Inertia of a resting object makes it difficult to get it moving, and the inertia of a moving object makes it difficult to change its velocity in either magnitude or direction.

Examples of INERTIA

YOUR TURN… EXERCISE 5 Pg. 63-64

NEWTONS’S SECOND LAW OF MOTION Gr. 10 revision Acceleration is the rate of change of velocity of an object. In Newton’s first law: acceleration is 0 because the object moves at a constant velocity or doesn’t move at all. There is no external force that causes it to accelerate in a direction. Newton’s second law is where we now introduce and consider this net force that will cause an object to accelerate…

NEWTONS’S SECOND LAW OF MOTION If a net force acts on an object, the object will accelerate in the direction of the net force. The acceleration is directly proportional to the net force and inversely proportional to the mass of the object. TB. pg 70

One newton is the force that accelerates a mass of at in the direction of the force. • Since we are now working with forces in 2 dimensions, we need to consider the net force separately in the x and y dimension. • If an object accelerates horizontally, then the resultant force is calculated by finding the vector sum of all the forces in the x dimension. • If an object accelerates vertically, then the resultant force is calculated by finding the vector sum of all the forces in the y dimension. TB. pg 70

Let’s clarify… In which direction is this object moving? If an object accelerates horizontally, then the resultant force is calculated by finding the vector sum of all the forces in the x dimension. Use Newton’s 2d law of motion to calculate the net force on the object. What forces act in the horizontal direction? What actual situation could this force diagram be representing? Horizontally : . What forces act in the vertical direction? Label the forces in the diagram. Vertically : .

Let’s clarify… In which direction is this object moving? • If an object accelerates vertically, then the resultant force is calculated by finding the vector sum of all the forces in the y dimension. Use Newton’s 2d law of motion to calculate the net force on the object. What forces act in the horizontal direction? What actual situation could this force diagram be representing? Horizontally : . What forces act in the vertical direction? Label the forces in the diagram. Vertically : .

Let’s clarify… In which direction is this object moving? • If an object accelerates horizontally, then the resultant force is calculated by finding the vector sum of all the forces in the x dimension. • If an object accelerates vertically, then the resultant force is calculated by finding the vector sum of all the forces in the y dimension. Use Newton’s 2d law of motion to calculate the net force on the object. What forces act in the horizontal direction? What actual situation could this force diagram be representing? Horizontally : . What forces act in the vertical direction? Label the forces in the diagram. Vertically : .

Examples - worksheet

NEWTONS’S THIRD LAW OF MOTION If object A exerts a force on object B, object B exerts an equal but opposite force on object A.

You are all sitting on your chairs. • You exert and force on the chair and the chair exerts a force on you. • The forces are equal in magnitude. (How do you know this?). • The forces act in the opposite direction but in the same line. • The forces are the same type (contact forces / magnetic / friction etc.). • The forces act simultaneously on two objects. (you and the chair).

A man stands on a scale in a lift. The 4 third law force pairs are: • The force of the scale on the floor and the force of the floor on the scale. • The force of the man on the scale and the force of the scale on the man. • The force of the cable on the lift and the force of the lift on the cable. • The force of the earth on the lift and the force of the lift on the earth.

NEWTON’S LAW OF UNIVERSAL GRAVITATION WHY DO OBJECTS FALL TO THE GROUND? Legend has it that a young Isaac Newton was sitting under an apple tree when he was bonked on the head by a falling piece of fruit, a 17th-century “aha moment” that prompted him to suddenly come up with his law of gravity. In reality, things didn’t go down quite like that. Newton, the son of a farmer, was born in 1642 near Grantham, England, and entered Cambridge University in 1661. Four years later, following an outbreak of the bubonic plague, the school temporarily closed, forcing Newton to move back to his childhood home, Woolsthorpe Manor. It was during this period at Woolsthorpe (Newton returned to Cambridge in 1667) that he was in the orchard there and witnessed an apple drop from a tree. There’s no evidence to suggest the fruit actually landed on his head, but Newton’s observation caused him to ponder why apples always fall straight to the ground (rather than sideways or upward) and helped inspired him to eventually develop his law of universal gravitation. In 1687, Newton first published this principle, which states that every body in the universe is attracted to every other body with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them, in his landmark work the “Principia,” which also features his three laws of motion.

NEWTON’S LAW OF UNIVERSAL GRAVITATION SOUNDS LIKE? A force of gravitational acceleration exists between any two objects in the universe that have mass. This force of attraction is directly proportional to the product of the masses of the objects and inversely proportional to the squared distance between their centers of gravity.

Gravitational attractive force Universal gravitational constant Mass of objects Distance between centres The force that object A exerts on object B is exactly the same as the force that object B exerts on object A but in the opposite direction, EVEN IF the masses are different. Greater mass – greater force of attraction and vice versa. Greater distance between objects – smaller force of attraction and vice versa.

using: You have 6 seconds to answer each question… The force of attraction of one object on another is F. What will the force be if a. One mass is doubled? b. Both masses are doubled? c. The distance between their centres are doubled? d. Both masses are doubled and distance between centres are halved?

using: You have 60 seconds to answer this question Derive the unit of the constant G in the above equation.

The difference between mass and weight.

Weightlessness • Weight is the product of mass and gravitational acceleration. W=mg • The feeling of weight is indirectly caused by the gravitational force but is one only feels it because of the other forces resisting the gravitational force. Without these objects, you won’t “feel” the gravity that is present – even though it is still there. You will feel “weightless”. • Example – you can feel that something is pulling down because you are able to sit on your chair and stay there. If the chair suddenly disappears, you will feel “weightless” for a moment before you hit the ground.

Weightlessness • When is weightlessness experienced? • Free-fall in a lift or vacuum. • Free-fall in a plane or spacecraft. • Objects far away from stars or planets.

Relationship between g and G • In physics, there are 3 kinds of fields that exert non-contact forces: • Gravitational fields • Electric fields • Magnetic fields • G is the a constant value that represents the relationship between the mass of objects and the forces that exist between them. • G is constant throughout the Universe. That’s why it’s called “universal.” • Some astronomers believe that if the universe is expanding as the popular Big Bang theory suggests, the value of G is slowly decreasing. • Weight, is the gravitational attractive force between a planet and an object. • On the surface of the earth, gravitational acceleration is On another planet, this value varies. • A gravitational field exists around the earth and around any object with mass. • Gravitational acceleration (g) decreases as the distance from the earth increases.

Proving that gravitational acceleration is the same everywhere on a planet and is independent of mass… 2 objects: One is a planet (M) and the other is a person (m). The attractive force that pulls them down is called . weight m and M

NEWTON’S LAWS OF MOTION SUTHERLAND HIGH SCHOOL- GRADE 11 -