Study Guide for Science Miniquiz #2

Force

A push or a pull on an object.

Contact force

A force that one object applies to another by touching it.

Noncontact force

A force that one object applies to another without touching it (e.g. gravity or magnetism).

Gravity

An attractive force that exists between all objects that have mass.

Mass

The amount of matter in an object, measured in kilograms (kg).

Weight

The gravitational force exerted on an object, measured in newtons (N).

Friction

A force that resists the motion of two surfaces that are touching.

Net force

The combination of all the forces acting on an object.

Balanced forces

Forces acting on an object that combine to give a net force of zero.

Unbalanced forces

Forces acting on an object that combine to give a net force that is not zero.

Newton's first law of motion

If the net force on an object is zero, the motion of the object does not change — an object at rest stays at rest, and a moving object keeps moving at the same velocity.

Inertia

The tendency of an object to resist a change in its motion.

Newton's second law of motion

The acceleration of an object equals the net force on the object divided by its mass. The acceleration is in the same direction as the net force.

Newton (N)

The SI unit of force. 1 N = 1 kg·m/s².

Acceleration

The rate at which velocity changes, measured in m/s².

Circular motion

Any motion in which an object moves along a curved path.

Centripetal force

A net force that acts toward the center of a circular path and keeps an object moving in a circle.

Newton's third law of motion

When one object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first object.

Force pair

Two forces that are equal in strength and opposite in direction and that act on two different objects.

Action force

The force that one object applies to a second object.

Reaction force

The equal and opposite force that the second object applies back on the first object.

Momentum

A measure of how hard it is to stop a moving object. Calculated as mass times velocity, in the same direction as the velocity.

Law of conservation of momentum

The total momentum of a group of objects stays the same unless outside forces such as friction act on the objects.

What is the law of universal gravitation?

All objects attract each other gravitationally. The strength depends on the mass of each object and the distance between them.

What happens to gravitational force when mass increases?

Gravitational force increases.

What happens to gravitational force when distance increases?

Gravitational force decreases.

What is the difference between mass and weight?

Mass is the amount of matter in an object (kg) and does not change. Weight is the gravitational force on an object (N) and depends on gravity.

What are the three types of friction?

Static (prevents motion), sliding (opposes sliding motion), and fluid/air resistance (between a surface and a fluid).

What is static friction?

Friction that prevents surfaces from sliding past each other.

What is sliding friction?

Friction that opposes the motion of surfaces sliding past each other.

What is air resistance?

Fluid friction between an object and air.

What is terminal velocity?

The constant velocity reached when air resistance equals the force of gravity on a falling object.

What causes friction at the microscopic level?

Microscopic bumps and dips on surfaces catch on each other. Also, weak electrical charges between surface particles attract each other.

How do lubricants reduce friction?

They separate surfaces, reducing contact between microscopic bumps and electrical charges.

What is the SI unit of force?

The newton (N).

What is the SI unit of mass?

The kilogram (kg).

What is the SI unit of momentum?

kg·m/s

What is the SI unit of acceleration?

m/s²

How do you find net force when forces act in the same direction?

Add them together.

How do you find net force when forces act in opposite directions?

Choose one direction as positive and the other as negative, then add the forces.

What does Newton's first law say about an object at rest?

It stays at rest unless an unbalanced force acts on it.

What does Newton's first law say about a moving object?

It keeps moving at the same velocity unless an unbalanced force acts on it.

What is the formula for weight near Earth's surface?

Weight (N) = mass (kg) x 10 m/s²

A 45 kg person is on Earth (gravity = 10 m/s²). What is their weight?

450 N

An astronaut has a mass of 55 kg on Earth. What is her mass on the space station?

55 kg

An astronaut weighs 540 N on Earth. What is her approximate weight on the space station 350 km up?

About 500 N.

If the net force on an object is zero, what is its acceleration?

Zero. Its motion does not change.

What direction does acceleration point relative to net force?

The same direction as the net force.

What happens to acceleration if mass increases but net force stays the same?

Acceleration decreases.

What happens to acceleration if net force increases but mass stays the same?

Acceleration increases.

Two forces: 200 N right and 150 N left. What is the net force?

50 N to the right.

Two forces: 120 N right and 90 N left. What is the net force?

30 N to the right.

Two forces: 300 N left and 250 N right. What is the net force?

50 N to the left.

Three forces: 130 N right, 150 N left, 165 N right. What is the net force?

145 N to the right.

A child pushes down 25 N, a friend pushes down 30 N, and a spring pushes up 60 N. Can they close the box?

No

A car travels at constant speed with engine force 1500 N. What is the friction force?

1500 N backward

What provides centripetal force for the Moon orbiting Earth?

Earth's gravity.

What provides centripetal force for Earth orbiting the Sun?

The Sun's gravity.

What is a satellite?

Any object in space that orbits a larger object.

A tablecloth is yanked from under dishes and the dishes barely move. Which law?

Newton's 1st law

A bat pushes forward on a ball and the ball pushes back on the bat with equal force. Which law?

Newton's 3rd law

An empty shopping cart accelerates more than a full one given the same push. Which law?

Newton's 2nd law

A balloon is released and air rushes out the back as the balloon zips forward. Which law?

Newton's 3rd law

A hockey puck glides across smooth ice in a straight line at nearly the same speed. Which law?

Newton's 1st law

A person jumps from a rowboat toward a dock and the rowboat slides backward. Which law?

Newton's 3rd law

A loaded delivery truck needs a bigger engine force than an empty truck to accelerate at the same rate. Which law?

Newton's 2nd law

A rug is beaten with a stick and the dust stays in place for an instant then falls out. Which law?

Newton's 1st law

A student on a skateboard pushes against a wall and rolls backward. Which law?

Newton's 3rd law

A sprinter pushes harder against starting blocks and leaves with greater acceleration. Which law?

Newton's 2nd law

When a bus suddenly moves forward, standing passengers lurch backward. Which law?

Newton's 1st law

A tennis player swings faster and the ball leaves with much greater acceleration. Which law?

Newton's 2nd law

What are the three forms of Newton's second law equation?

a = F divided by m, F = m x a, and m = F divided by a

A car has a mass of 1200 kg and accelerates at 2.5 m/s². What net force acts on it?

3000 N

A cart has a mass of 3 kg and a net force of 15 N. What is its acceleration?

5 m/s²

A sled experiences a net force of 90 N and accelerates at 3 m/s². What is its mass?

30 kg

A rocket has a mass of 500 kg and a net force of 4000 N. What is its acceleration?

8 m/s²

A 1000 kg car has an engine force of 2500 N and friction of 1500 N. What is its acceleration?

1 m/s²

Skater A (50 kg) pushes Skater B (75 kg) with 150 N. What is each skater's acceleration?

Skater A: 3 m/s². Skater B: 2 m/s². They move in opposite directions.

A 4 kg cart experiences a 12 N net force. What is its acceleration?

3 m/s²

A 50 kg skater accelerates at 2 m/s². What net force acts on the skater?

100 N

A 36 N net force causes an object to accelerate at 4 m/s². What is its mass?

9 kg

What is the formula for momentum?

p = m * v

What is the momentum of a 60 kg runner moving at 5 m/s?

300 kg·m/s

What is the momentum of a 12 kg bicycle moving at 5.5 m/s?

66 kg·m/s

What is the momentum of a 1.5 kg ball moving at 3.0 m/s?

4.5 kg·m/s

What is the momentum of a 6 kg bowling ball moving at 3 m/s?

18 kg·m/s

What is the momentum of a 0.15 kg baseball moving at 40 m/s?

6 kg·m/s

What is the momentum of a 1000 kg car moving at 20 m/s east?

20000 kg·m/s east

A 2 kg cart moving at 3 m/s hits a 2 kg cart at rest. The first cart stops. How fast does the second cart move?

3 m/s.

Why do force pairs NOT cancel each other out?

Because they act on two different objects, not the same object.

What is the action force when you jump?

You push down on the ground.

What is the reaction force when you jump?

The ground pushes up on you.

What is the action force when a rocket launches?

The engine pushes hot gas downward.

What is the reaction force when a rocket launches?

The gas pushes the rocket upward.

What is the action force when swimming?

You push water backward with your arms.

What is the reaction force when swimming?

The water pushes you forward.

What is an elastic collision?

A collision where objects bounce off each other.

What is an inelastic collision?

A collision where objects stick together.

Is momentum conserved in both elastic and inelastic collisions?

Yes, total momentum is conserved in both types unless outside forces act.

A bat hits a baseball with 800 N. How much force does the ball exert on the bat?

800 N